ICAM Photo

31st ICAM Aviemore, Scotland, United Kingdom, 23-27 May 2011

Oral abstracts

Session Number: K1
Presenting Author: Evelyne Richard
Author Email: rice@aero.obs-mip.fr
Author Affiliation: Laboratoire d'Aérologie,
Toulouse, France
Co-Author: B. Vié, O., Nuissier, B. Vincendon, V. Ducrocq
Co-Author Affiliation: Laboratoire d'Aérologie, CNRS and University of Toulouse, Toulouse
Abstract Title:
Towards a mesoscale Ensemble Hydro-Meteorological Prediction System for the northwestern Mediterranean
During the last 15 years, ensemble weather forecasting has made substantial progress and has proved its skill in forecasting the probabilities of relevant weather events. More recently, the development and growing use of high-resolution, convection-permitting models has significantly increased the potential of atmospheric modelling. However, this opens new questions regarding the representation of the initial state, boundary conditions, and model uncertainties. In particular, the relative weight of each contribution remains to be investigated.
This talk focuses on model uncertainties (and especially those related with the cloud physics and turbulence parameterizations) which, so far, have been much less studied than initial state uncertainties. The sensitivity of the precipitation forecasts to the details of the parametrisation is assessed i) by varying the tunable parameters of the model microphysical and turbulence schemes within the range of admitted values and ii) by introducing random perturbations on the time tendencies that govern hydrometeor and/or turbulent kinetic energy evolution. Here, this methodology is applied to past cases of heavy precipitation over south-eastern France. It is found that the second set of perturbations induces a wider spread of the results and that low-skill forecasts show much higher sensitivity to the microphysical scheme perturbations than high-skill forecasts. Furthermore, the spread amplitude is found to be quite comparable to the one obtained with initial state perturbations. These results indicate that the model uncertainties associated with physical parametrisations should be taken into consideration in convective scale ensemble prediction systems.

Session Number: K2
Presenting Author: Vanda Grubišić
Author Email: vanda.grubisic@univie.ac.at
Author Affiliation: Department of Meteorology and Geophysics, University of Vienna, Vienna, Austria
Co-Author: Stefano Serafin, Lukas Strauss
Co-Author Affiliation: Department of Meteorology and Geophysics, University of Vienna, Vienna, Austria
Abstract Title:
Wave-Induced Boundary-Layer Separation and Turbulence
The wave-induced boundary-layer separation in flow over orography has received significant attention in recent years, especially in relation to formation of atmospheric rotors. Rotors–traditionally depicted as horizontal eddies in the lee of mountain ranges–are characterized by intense turbulence and as such pose a known threat to aviation.
This study focuses on the first observationally documented case of wave-induced boundary-layer separation. On Jan 26 2006, the University of Wyoming King Air (UWKA) aircraft, highly instrumented for collecting both in situ and remote sensing measurements, the latter with the airborne Wyoming Cloud Radar WCR), flew repeated passes over the Medicine Bow Mountains in SE Wyoming at a range of altitudes as low as 700 m above the highest terrain. The Dual-Doppler synthesis of WCR data and the corresponding retrieval of the two-dimensional flow field in the vertical plane beneath the aircraft indicate strong wave activity, downslope winds in excess of 30 m s^-1 within 200 m above the ground and severe turbulence in the lee of the mountains.
High-resolution numerical simulations of this event with the NRL COAMPS model, using multiple nested domains to achieve a horizontal resolution of ~300 m over the area of observations, show presence of rapidly evolving terrain-generated lee waves and boundary-layer separation occurring over the lee slopes. At this horizontal resolution the simulated flow is in good agreement with the overall features of the retrieved flow from the dual-Doppler analysis. However, the much finer resolution of WCR data (of the order of 40x40 m2 for two-dimensional velocity fields), allows one to discern fine-scale coherent vortical structures within the rotor zone, where more turbulent dynamics and smaller-scale vortices appear to be prevalent. These flow features cannot be explicitly resolved by a mesoscale numerical model; their simulation requires a higher-resolution grid and the use of a large-eddy simulation (LES) approach. Presented herein will be the results of LES simulations of wave-induced boundary-layer separation and breakdown of flow into turbulence at the leading edge of the rotor. The simulation results are discussed within the context of the WCR observations and other recent related studies.

Session Number: 1a
Presenting Author: Richard Werner
Author Email: richard.werner@vorarlberg.at
Author Affiliation: Institute for environment and food safety of the state of Vorarlberg; Bregenz
Co-Author: Erwin POLREICH
Co-Author Affiliation: Central institute for meteorology and geodynamics; Vienna
Abstract Title:
Course of streaming situations over the Alps from 1961 to 2003 for 850 hPa
In this work the 10 types of streamingsituations (sts) in 850 hPa over the Alps from Steinacker (2010) are analysed in a statistical manner. The frequency from year to year is taken to define a middle value of each type and to calculate the standard deviation over 30 years (1961 to 1990). For 8 directions of mean flow over the Alps we found a middle frequency for Western flow of 34.4 days per year. In the normal period also a middle frequency of 13.1 days per year is found for southeast flow. In about 94 days a system with high pressure dominates the region of research.
The variation-coefficient for each type is calculated within a range from 14 % to 49 %. The lowest value was found for the type “variable” which means a changing sts in the day. The highest value is connected with northern flow.
In the third step we define also seldom cases, when in one year the frequency of the type is higher than 2SD or lower than -2SD. From this criterion it is possible to detect the chronic of seldom cases over the whole period of 43 years. We found years with two seldom cases also as periods longer than five year without any seldom case.

Session Number: 1b
Presenting Author: Ian Renfrew
Author Email: i.renfrew@uea.ac.uk
Author Affiliation: School of Environmental Sciences, University of East Anglia, Norwich, NR4 7TJ, UK
Co-Author: Ben Harden, G Nina Petersen
Co-Author Affiliation: UEA and Icelandic Met Service
Abstract Title:
A climatology of wintertime barrier winds off southeast Greenland
A climatology of barrier winds along the southeast coast of Greenland is presented based on 20 years of winter months (1989-2008) from the ECMWF reanalysis product ERA-Interim. Barrier wind events occur predominantly at two locations: Denmark Strait North (67.7N,25.3W) and Denmark Strait South (64.9N, 35.9W). Events stronger than 20 m/s occur on average once per week during winter with considerable inter-annual variability, from 7 to 20 events per winter. The monthly frequency of barrier wind events correlates with the monthly NAO index with a correlation coefficient of 0.57 (0.31) at DSN (DSS). The associated total turbulent heat fluxes for barrier wind events (area averaged) were typically about 200 W/m^2 with peak values of 400 W/m^2 common in smaller regions. Area averaged surface stresses were typically between 0.5 and 1 N/m^2. Total precipitation rates were larger at DSS than DSN, both typically less than 1 mm/h^1. The total turbulent heat fluxes were shown to have a large range as a result of a large range in two meter air temperature.
Two classes of barrier winds, warm and cold, were investigated and found to develop in different synoptic-scale situations. Warm barrier winds developed when there was a blocking high pressure over the Nordic Seas, while cold barrier winds owed their presence to a train of cyclones through the region.

Session Number: 1c
Presenting Author: Andrew Elvidge
Author Email: a.elvidge@uea.ac.uk
Author Affiliation: University of East Anglia, Earlham Road, Norwich NR4 7TJ
Co-Author: Ian Renfrew, John King, Tom Lachlan-Cope, Amélie Kirchgasessner, Russ Ladkin, Victoria Smith
Co-Author Affiliation: University of East Anglia, British Antarctic Survey, British Antarctic Survey, British Antarctic Survey, British Antarctic Survey, University of Leeds
Abstract Title:
Boundary layer evolution during a westerly föhn warming event across the Antarctic Peninsula, as observed during the Orographic Flows and Climate of the Antarctic Peninsula (OFCAP) field season.
Enhanced circumpolar westerly winds associated with the recent positive phase of the Southern Annual Mode are hypothesized to be incurring more frequent cases of föhn warming felt at low levels to the East of the Antarctic Peninsula (AP). Over a 5 week period during the 2011 austral summer, as part of the Orographic Flows and Climate of the Antarctic Peninsula (OFCAP) project, an instrumented twin otter aircraft, recently deployed automatic weather stations and radiosondes were used to observe westerly föhn events. Aided by high resolution (4km and 1.5km) numerical model simulations using the UK Met Office Unified Model, one of these cases is presented here. Plunging lee side flow associated with upwind flow characterised by non-dimensional mountain height >1 has been observed to incur mixing of the stable boundary layer above the Larsen Ice Shelf to the East of the AP, allowing föhn warming to extend to surface or near-surface levels.

Session Number: 1d
Presenting Author: Lavinia Laiti
Author Email: laitil@ing.unitn.it
Author Affiliation: Atmospheric Physics Group, Department of Civil and Environmental Engineering, University of Trento, Trento, Italy
Co-Author: Dino Zardi, Massimiliano de Franceschi
Co-Author Affiliation: Atmospheric Physics Group, Department of Civil and Environmental Engineering, University of Trento, Trento, Italy
Abstract Title:
Analysis of the thermal structure of the “Ora del Garda” wind from airborne and surface measurements
Systems of daily-periodic valley winds typically develop in the Alps, driven by the interaction between the thermally forced motion of air masses and the complex orographic configuration. The occurrence of large lakes can mark these phenomena with local peculiarities.
This study investigates a well known valley/lake breeze phenomenon, the so-called Ora del Garda. The latter is a diurnal wind originating in the late morning on the northern shores of Lake Garda, channelling into the Sarca River Valley and the Lakes Valley nearby, and reaching, on days of greater intensity, the Adige River Valley, where it gets mixed with the local up-valley winds and produces a strong and gusty local flow. The Ora blows very regularly on sunny days under fair weather conditions, from late spring to early autumn, and marks local weather conditions in the area.
In order to explore how the development of this wind affects BL processes in the valleys, four measurements campaigns were performed in 1998-2001, including flights of an instrumented light airplane. Each flight trajectory explored three or four specific sections of the valley atmosphere (namely over the lake coast, at half valley, at the end of the valley). At the same time, data from surface weather stations located both on the valley floor and on the sidewall slopes were collected and analysed.
For each single section the dominating vertical profile of temperature and humidity and the characteristic spatial scales of variability of local deviations from these mean structures were inferred. Then temperature and humidity fields on the entire explored sections were reconstructed by means of a geostatistical interpolation technique (kriging).
This allowed an evaluation of the influence of the Ora del Garda on the temperature and moisture structure of the valley atmosphere and of the connections between the breeze flow depth and the CBL height. Features common to most diurnal valley winds are displayed. In particular the CBL height appears to be rather shallow and surmounted by a deep stable layer. However close to the lake the atmosphere tends to stabilize throughout the BL depth, due to the advection of colder air from above the lake surface.
Moreover some local small scale features of the investigated circulation, chiefly due to morphological irregularities and surface coverage inhomogeneities, were recognized and interpreted, such as the presence of the small Lake of Cavedine far up-valley, and the local contrasts between valley-bottom areas covered with bare rocks and densely vegetated areas.

Session Number: 1e
Presenting Author: Ivana Stiperski
Author Email: stiperski@cirus.dhz.hr
Author Affiliation: Croatian Meteorological and Hydrological Service of Croatia, Zagreb, Croatia
Co-Author: Vanda Grubišić
Co-Author Affiliation: University of Vienna, Austria
Abstract Title:
Influence of secondary orography on boundary-layer separation and rotors
Boundary-layer separation in the lee of mesoscale orography is important for the formation of mountain-wave induced rotors. Rotors, turbulent horizontal eddies forming in the lee of orography, pose a significant threat to aviation but are also important for air-pollution due to fluid recirculation and mixing. Boundary-layer separation and rotors are therefore particularly important in valleys. Results of a limited number of previous studies suggest that, apart from surface roughness, terrain slope and the flow regime, boundary layer separation might be facilitated by the existence of downstream orography.
In this study we examine how secondary orography, such as islands or downstream ridges, influences boundary layer separation and rotor formation under different flow regimes, including trapped waves and undular hydraulic jumps. Here were present the results of idealized and real-data high-resolution numerical simulations of flow over double orography performed with the NRL COAMPS model.
The results show that certain types of gravity wave response are particularly sensitive to secondary orography, even when the secondary orography is significantly lower than the primary one. In the trapped lee wave regime the onset of boundary layer separation within the valley, in between the two ridges, appears almost unaffected by the secondary ridge; however, for higher mountains downstream orography is responsible for the decrease in rotor intensity within the valley. Even downstream of the secondary ridge, where trapped lee wave interference determines the wave amplitude, boundary layer separation is not facilitated. On the other hand, for hydraulic-type flows the secondary ridge is able to induce boundary layer separation for conditions under which it would not occur in the lee of a single ridge. In this case, the point at which flow separates from the surface as well as the intensity of rotor circulations are both affected by the downstream mountain.

Session Number: 2a
Presenting Author: Dale R. Durran
Author Email: drdee@uw.edu
Author Affiliation: Atmospheric Sciences, Box 351640, Univ. of Washington
Seattle, WA 98195
Co-Author: Matt O.G. Hills
Co-Author Affiliation:
Abstract Title:
Trapped waves and low-level orographic drag in a slowly evolving flow
Current parameterizations of mountain wave drag tend to focus on momentum transport by vertically propagating waves and assume a steady large-scale flow. Drag due to trapped waves is often neglected, and the behavior of trapped waves in slowly varying synoptic-scale flows is largely unexplored. One unanswered question is: what is the fate of trapped waves after their generation? Do they simply decay through the slow leakage of energy into the stratosphere or are they removed by some dissipative process? How does their dissipation mechanism influence the wave drag?
We report on 3D simulations of trapped waves triggered by a translating barotropic jet passing over an isolated ridge in an atmosphere with a two-layer static stability structure. The background flow accelerates from rest to 20 m/s and drops back to zero over a period of 1.5 days. In our simulation, a long train of lee waves is generated by the cross-mountain flow. A large part of this wave train ultimately becomes untrapped, and then rapidly propagates out of the lower troposphere. This behavior is explained using ray tracing to analyze individual wave packets after they are generated above the mountain. The time dependent momentum fluxes are also discussed, along with the impact of the waves on the larger-scale flow.

Session Number: 2b
Presenting Author: Miguel A. C. Teixeira
Author Email: mateixeira@fc.ul.pt
Author Affiliation: CGUL, IDL, University of Lisbon,
Edificio C8, Campo Grande, 1749-016 Lisbon, Portugal
Co-Author: Jose L. Argain and Pedro M. A. Miranda
Co-Author Affiliation: Department of Physics, University of Algarve, Faro, Portugal, and CGUL, IDL, University of Lisbon, Lisbon, Portugal
Abstract Title:
The importance of friction in mountain wave drag enhancement by parametric resonance
Wells and Vosper (2010) recently identified a mechanism for the amplification of mountain wave drag, which requires the existence of a vertically oscillating Scorer parameter profile. They attributed this drag amplification to a resonance associated with intrinsically nonlinear wave-wave interactions. We show here that substantial drag amplification with the same characteristics can be produced by a mechanism akin to parametric resonance, which can be explained qualitatively in a linear framework.
An atmosphere with a Scorer parameter that varies sinusoidally in the vertical is considered. A linear mountain wave model is developed, where the flow perturbations are expanded in power series of a small parameter proportional to the amplitude of the Scorer parameter oscillation. This model accounts for friction in the simplest possible way, representing it through a Rayleigh damping coefficient. Numerical simulations of the same situation are also carried out using a nonlinear, nonhydrostatic model.
The mountain wave drag is found to display extrema in the vicinity of the parameter space region where the wavelength of the dominant mountain waves is twice the wavelength of the Scorer parameter oscillation. These extrema can be maxima or minima depending on the phase of the oscillation, and their amplitude can be considerable (say, corresponding to a drag amplification by a factor of two), even when the amplitude of corresponding Scorer parameter oscillation is relatively small. Friction tends to attenuate the drag extrema, but for certain values of the phase of the Scorer parameter oscillation (including those considered by Wells and Vosper), the drag extrema vanish altogether in inviscid conditions. This shows that friction is essential for producing these drag modulations.
In numerical simulations, friction always exists, either due to numerical dissipation inherent to the discretization scheme, or to the adopted turbulence closure. Hence, an accurate representation of the present drag amplification mechanism in those simulations should be quite sensitive to the formulation of friction. The drag behaviour produced by the numerical model in nominally inviscid conditions is shown to be

optimally reproduced by the linear model for a relatively small but non-zero value of the friction coefficient. The drag modulation is also shown to be attenuated by nonhydrostatic effects, since these lead to increased wave dispersion, and enhanced downstream propagation of the waves.

Session Number: 2c
Presenting Author: Victoria Smith
Author Email: v.smith@see.leeds.ac.uk
Author Affiliation: Institute for Climate and Atmospheric Science,
University of Leeds, Leeds, LS2 9JT, UK
Co-Author: Stephen Mobbs, Simon Vosper, Andrew Ross
Co-Author Affiliation: National Centre for Atmospheric Science; UK Meteorological Office; Institute for Climate and Atmospheric Science, University of Leeds
Abstract Title:
A hypothesis to demonstrate the limitation of modelling rotors and sub-rotors in two dimensions
We propose a hypothesis for the limitation of modelling subrotor vortices in two-dimensions. Using the Boundary Layer Above Stationary Inhomogeneous Uneven Surfaces (BLASIUS) numerical model, we demonstrate that both resolved and unresolved turbulence can be an order of magnitude greater when simulated in three dimensions compared to a two-dimensional run that is identical except for the absence of a y-direction dimension. The differences are attributed to the fact that in three-dimensional flow, energy will cascade to smaller time and length scales (Tennekes, 1978). In two-dimensional flow however, the energy will preferentially cascade to longer scales (Tripoli and Cotton, 1989) , transferring the energy back to the main rotor. Radar wind profiler observations from the Terrain-induced Rotor Experiment (T-REX) suggest that the spatial scale of subrotors to be of the order 500 m – 1 km (Doyle et al., 2009). High resolution is therefore required if hoping to resolve the main features of these phenomena and a horizontal resolution of 122 m has therefore been used in this study. In the three-dimensional simulation, the two-dimensional nature of the rotor-generating orography is maintained so that differences in the turbulence cannot be attributed to complex terrain. In addition to the differing magnitude of the turbulence, it will also be shown that a significant proportion of the vorticity acts in the x and z-direction, reflecting the true three-dimensional nature of subrotors.

Session Number: 2d
Presenting Author: Jože Rakovec
Author Email: joze.rakovec@fmf.uni-lj.si
Author Affiliation: University of Ljubljana, Faculty of Mathematics and Physic, Chair of meteorology
Co-Author: Rahela Žabkar, Mark Žagar
Co-Author Affiliation: University of Ljubljana, Faculty of Mathematics and Physic & Vestas, Danemark
Abstract Title:
The inner structure of the Bora flow
The case of bora wind in December 2001 which exhibited strong pulsations on one day, and no pulsations on the second day (already studied by Belusić at al. 2007), has been simulated using the non-hydrostatic model WRF.
Separate terms of equation in balance form that determine local changes of kinetic energy per unit volume were computed from the fields of the simulated meteorological variables in 1 s temporal, 333 m horizontal resolution, and 62 vertical levels. The spatial and temporal characteristics of the two advection terms and of the source/sink terms of kinetic energy are investigated in a region where Bora gusts are strongest for the case studied. Several approaches are used to estimate the relative contributions of the separate terms with the goal of finding the physical origin of the gusts: their time courses at different locations within the flow, their spatial distributions (in horizontal and vertical cross-sections), as well as using their structure functions (being represented by spatial cross-correlations between the time course of local change of kinetic energy in some selected points and the time courses of terms of the right-hand side of the equation in all other sub-domain points), etc. The along-slope pulsations of kinetic energy up to approx. 800 J/m3 are developing quasi-periodically with approx. 7 to 8 minutes period ending with a hydraulic jump-like development over the sea. There are noticeable differences in how the local changes of kinetic energy and the advection, and sources/sinks terms that are causing them, are correlated on the slope, and above the sea. The shapes of spatial structure functions of different terms are different: some show the well expressed, well localized and clearly moving patches of stronger and weaker cross-correlations (e.g. the work of gravity term), while some appear more filamented and move less (e.g. the non-convective advection term or the work of eapansion term). It appears that although the local tendency of kinetic energy per unit volume is not very large (slightly exceeding the +/- a few W/m3) the highest values of some contributing terms may reach a few hundreds W/m3. As these terms partly cancel, the remaining ones are essentially contributing to local change of kinetic energy advection, of pressure gradient force and of gravity force: the first one\'s spatial and temporal variability exceeds +/- few tens W/m3, while the latter two exceed +/- +/- a

few hundreds W/m3. These are of the opposite sign, in particular along the slope and in the hydraulic jump region and hence the hydrostatic part of the pressure gradient force contribution cancels the gravity force contribution: the remaining contribution of these two effects is as far as the amplitude is concerned, similar to the contribution of advection, but in general of the opposite sign.

Session Number: 2e
Presenting Author: Ólafur Rögnvaldsson
Author Email: or@belgingur.is
Author Affiliation: Institute for Meteorological Research & University of Bergen
Co-Author: Haraldur Ólafsson and Hálfdán Ágústsson, Marius Opsanger Jonassen
Co-Author Affiliation:
Abstract Title:
Extreme southwesterly windstorms in Borgarfjörður in 2011 and in the 10th Century
A novel, open source, software system has been developed that greatly simplifies running the WRF atmospheric model in Large Eddy Simulation (LES) mode. The work was funded by the University of Bergen, Norway, and carried out by the Institute for Meteorological Research in Iceland. Running the WRF model in LES has mode can be both time consuming and confusing. Typically, one runs the model with a regular planetary boundary layer (PBL) scheme down to a horizontal resolution of few kilometers using the model's nesting option. Using the output data from the innermost (i.e. highest resolution) PBL domain, one can create initial and boundary data for the WRF model in LES mode. This is done by using a component of the WRF modeling suite called NDOWN (short for Nest DOWN). Once that is done, one can finally run the WRF model in LES mode for the chosen area. Care must be taken when editing the WRF model's control files (called namelist.wps and namelist.input) during this procedure. As the user defines the domain setup "top down", it can be very time consuming getting the exact location of the innermost domain correct. The new software package solves this by allowing the user to define the exact location, and extent, of the innermost domain. Either by defining two corner points, or by setting the domains center latitude and longitude as well as a radius. From these information the system than sets up the necessary control files in such a way that the innermost domain is as the user wishes for. The system further sets up a unique directory structure for each simulation, copies or links relevant input data and creates the necessary runtime scripts that make it straight forward to run the WRF model through all the necessary steps.Additionally, the system includes methods to use the (near global) 1 second ASTER topography data and the high resolution Corrine land use data, that are available for large parts of Europe. The system has been tested for a number of locations in Norway, Denmark and Iceland. Main usability is foreseen in research of wind energy for regions with relatively complex terrain. Here, the system is tested on a southwesterly windstorm in the Borgarnes area in Iceland in 2011. The windstorm is of the same type as described as a stone-mover in the saga of Egill Skallagrimsson and is influenced by the mountains south of the town of Borgarnes, located in the fjord of Borgarfjörður in W-Iceland, although the static stability in the lower troposphere is very low.

Session Number: 2f
Presenting Author: Georg J. Mayr
Author Email: georg.mayr@uibk.ac.at
Author Affiliation: Institute of Meteorology and Geophysics, University of Innsbruck, Austria
Co-Author: Laurence Armi
Co-Author Affiliation: Scripps Institution of Oceanography, University of California, USA
Abstract Title:
What comes first: downslope wind (foehn) or the wave aloft? A case study from T-REX.
ross-barrier density differences and a westerly cross-barrier flow component aloft established a descending stratified flow across the Sierra Nevada (USA) on 9-10 April 2006 during T-REX. Downslope flow and an internal hydraulic jump occurred only when the potential temperature of the westerly descending flow was at least as cold as the existing up-valley flowing valley air mass. A sequence of visible satellite images and weather stations time series describe the onset. The University of Wyoming King Air flew through the stratified flow and imaged the structure of the internal hydraulic jump with its cloud radar. Shear layer instabilities, which first developed near the jump face, grew and paired downstream, mixing the internal hydraulic jump layer. Aloft a wave formed as response to the downslope flow and internal hydraulic jump

Session Number: 3a
Presenting Author: Samantha Smith
Author Email: samantha.smith@metoffice.gov.uk
Author Affiliation: Met Office, UK
Abstract Title:
Sensitivity of orographic enhancement of precipitation to horizontal resolution in the operational Met Office NWP model
Using archived operational Met Office Unified Model forecasts of precipitation within warm sectors identified over the most hilly regions of the UK (i) during 2009 and (ii) between 17 June 2010 and 2 Feb 2011, it was found that the mean precipitation patterns are dominated by orographic variations, with the magnitude of the precipitation rate maxima increasing with horizontal resolution.
At 40km and 12km resolution the precipitation maxima occur over the windward slopes but at higher resolutions (4km and 1.5km) the smaller scale precipitation maxima occur near to the local peaks.
The coarse resolution simulations underestimate the area averaged precipitation rate over orography by 29 to 36% at 40km resolution (2009 global model) and by 6 to 16% at 12km resolution (NAE). The 4km resolution simulations (UK4) seem to predict too much precipitation over orography, particularly over the Cumbrian mountains where it produces 17% more precipitation than the UKV. The UKV reduces the precipitation rate over small scale valleys which were unresolved by the 4km grid.
These results suggest that a parameterisation of the orographic precipitation enhancement by sub-grid scale orography would be beneficial, and that the scheme must also be able to reduce the precipitation rate due to sub-grid scale valleys.

Session Number: 3b
Presenting Author: Dirk Cannon
Author Email: d.j.cannon@pgr.reading.ac.uk
Author Affiliation: Department of Meteorology, University of Reading
Reading, RG6 6BB, United Kingdom
Co-Author: Dan Kirshbaum, Suzanne Gray
Co-Author Affiliation: Department of Meteorology, University of Reading, Reading, RG6 6BB, UK
Abstract Title:
A scaling of orographic precipitation for both stratiform and convective clouds
Although convective orographic clouds generally produce larger local precipitation rates than stable laminar clouds, the impact of this convection on area-averaged precipitation amounts, and hence the large-scale flow, is still not fully understood. A simple scaling is proposed to quantitatively predict the condensation rate and precipitation efficiency of orographic precipitation, for both stratiform and convective orographic clouds. In order to capture the dominant environmental and terrain-related controls on convective vigor, condensation amounts and precipitation efficiency, a wide range of parameter space is considered. These results are compared to a large number of idealised, cloud-resolving numerical simulations of moist flow over Gaussian orography. Embedded convection is shown to yield a significant precipitation enhancement only in a well-defined region of parameter space and this can be quantitatively understood through the scaling.

Session Number: 3c
Presenting Author: Daniel J. Kirshbaum
Author Email: d.kirshbaum@reading.ac.uk
Author Affiliation: University of Reading
Co-Author: Alan L. M. Grant
Co-Author Affiliation: University of Reading
Abstract Title:
Cumulus broadening and precipitation enhancement over steep terrain
The dynamical and microphysical properties of cumulus clouds sharply change during rapid forced ascent over steep terrain. This is owing to two distinct processes: (1) a buoyancy enhancement in cloudy parcels as they are lifted alongside unsaturated parcels with different adiabatic lapse rates and (2) a widening of cloud cores, which reduces the dilution and evaporative cooling associated with entrainment. This study closely examines the second mechanism through large-eddy simulations of precipitating shallow cumuli and simple scaling ideas. The simulations show a clear upscale shift of cumulus size spectra during the rapid ascent, which is collocated with a strong decrease in the fractional entrainment rate within the cloud cores. As a result, protected areas within the cores rise virtually undiluted through the cloud layer, leading to enhanced liquid-water concentrations. This enhances precipitation initiation and efficiency, allowing for a denser population of precipitating cells and substantially larger rainfall rates. To interpret these numerical results, a similarity theory based on the turbulent kinetic energy budget is used. The theory successfully predicts not only the simulated cloud broadening and reduction in core dilution, but also the enhanced buoyancy fluxes and turbulent dissipation over the high terrain.

Session Number: 4a
Presenting Author: Ronald B. Smith
Author Email: Ronald.smith@yale.edu
Author Affiliation: Dept of Geology and Geophysics, Yale University, New Haven, Connecticut, 06520
Co-Author: Daniel Kirshbaum, Alison Nugent, Justin Minder
Co-Author Affiliation: University of Reading; Yale University
Abstract Title:
Orographic Precipitation in the Tropics: The Dominica Experiment
Mountainous Dominica in the eastern Caribbean is a natural laboratory for orographic precipitation in the tropics. With little diurnal cycle, the forced ascent of conditionally stable air triggers frequent shallow convection and precipitation, totaling nearly seven meters per year. We hypothesize that incoming humidity fluctuations contribute to the triggering. After a three-year preliminary research phase with radar, satellite and rain gauges, a more focused phase with the Wyoming King Air took place from April 4 to May 8, 2011. In the observations and modeling, particular emphasis was placed on 1) upstream control parameters and predictability and 2) mountain airflow deformation and convective triggering. We will report the preliminary findings from this project. (project website: http://domex2011.com/)

Session Number: 4b
Presenting Author: Faisal Boudala
Author Email: faisal.boudala@ec.gc.ca
Author Affiliation: Cloud Physics and Severe Weather Research Section, Environment Canada, Toronto, Ontario, Canada
Co-Author: George A. Isaac, Ismail Gultepe, Stewart G. Cober , and Ivan Heckman
Co-Author Affiliation: Cloud Physics and Severe Weather Research Section, Environment Canada, Toronto, Ontario, Canada
Abstract Title:
Observation of Precipitation and Precipitation Type in Complex Terrain during the 2010 Winter Olympics in Vancouver and Its Impact on Visibility: Implication for Numerical Weather Prediction Models
Precipitation plays a significant role in our planet by modulating the hydrological cycle and also by affecting daily human activities including air and ground transportation. For example, during winter precipitation can reduce visibility and also contributes to other severe weather phenomena such as blowing snow. Thus, accurate determination and prediction of precipitation, particularly over a complex terrain is very important. In principle, precipitation can be measured using a weighting gauge which is as simple as an open container on the ground that can collect falling rain drops, snow and hail particles etc.. However, it is usually more complex, because of many factors such as wind effects; accuracy and the representativeness of the measurements in spatial and time scales. There are other kinds of instruments that can measure precipitation such as vertically pointing radars, distrometers, hot plates, and scattering probes. Some of these instruments also suffer from wind effects and also need calibrations using weighing gauges. Considerable efforts have been made towards measuring precipitation and visibility using several of the instruments mentioned above during the Vancouver 2010 Olympic and Paralympic Winter Games as part of the Science of Nowcasting Olympic Weather for Vancouver 2010 (SNOW-V10) project. This presentation will include the following: a) Several snowfall rate measuring instruments such as a vertically pointing X-band radar, Parsivel distrometer, Yankee HotPlate, Viasala FD12P, and Pluvio and Geonor gauges that were located at several heights over the Whistler mountain and Vancouver International Airport will be compared under various atmospheric conditions b) Several parameterizations for retrieving precipitation intensity from radar reflectivity will be compared c) Precipitation type and visibility measured using the FD12P and Parsivel will be compared and the effect of precipitation type on visibility will be discussed, d) Finally the data will be used for validation of the current Canadian GEM Regional (GEM-REG) and Limited Area (GEM-LAM) models.

Session Number: 4c
Presenting Author: Fanny Duffourg
Author Email: fanny.duffourg@meteo.fr
Author Affiliation: CNRM-GAME / Météo-France & CNRS, Toulouse, France
Co-Author: Véronique Ducrocq
Co-Author Affiliation: CNRM-GAME / Météo-France & CNRS
Abstract Title:
Origin and transport of the moisture supply to the Mediterranean Heavy Precipitating Events
Mediterranean regions are regularly affected by Heavy Precipitating Events (HPE) triggered by the steep orography of the area. The heavy precipitating systems are fed by a marine low-level flow providing the moisture required for the precipitating systems to produce huge amounts of precipitation. Previous studies – Ducrocq et al. (2002), Bresson et al. (2009) – have shown that the location, intensity and dynamics of the precipitating systems are very sensitive to the characteristics of this moist inflow. In this study, we examine the origins and the features of the moisture supply feeding the Mediterranean heavy precipitating systems.
All the HPEs that occurred during two successive falls (in 2008 and 2009) over a particular Mediterranean region (Southeastern France) are systematically studied. The origins of their moisture supply and the pathways of moisture transport are diagnosed thanks to multi-scale numerical simulations. Convective-scale simulations provide a fine description of the precipitating systems and their short-range moisture supply, while larger-scale simulations downscaling synoptic-scale reanalyses enable to describe the moisture transport over a longer period (4 days before each HPE). The analysis of the HPE moisture supply is performed with two complementary tools: lagrangian backward trajectories and water budgets.
The moisture feeding the heavy precipitating systems appeared to be provided mainly by both the evaporation of the Mediterranean Sea within the last two days before the HPE, and the transport over more than 3-4 days of remote moisture coming from the Atlantic Ocean or tropical Africa. Moisture from remote sources is transported within the lower half of the troposphere. The remote moisture pathways descend in the low levels when arriving above the Mediterranean Sea. Moisture is then conveyed northwards over the sea towards the French coast following two main channels, along the eastern Spanish coast and west to Sardinia. All along this low-level marine transport, the evaporation of the Mediterranean Sea contributes to moisten even more the air mass in the boundary layer.
This contribution of the Mediterranean Sea is particularly important when anticyclonic conditions prevail during the last 4 days before the HPE. On the contrary, when cyclonic

conditions prevail before the HPE, less moisture is gained from the sea and the contribution of remote sources gets more important.
These results help for the definition of the HyMeX SOP (Special Observing Period) observation strategy.

Session Number: 4d
Presenting Author: Silvia Terzago
Author Email: silvia.terzago@unito.it
Author Affiliation: Dipartimento di Scienze della Terra, Università di Torino
Via Valperga Caluso 35, 10125 Torino, Italia
Co-Author: Simona Fratianni, Fiorella Acquatto, Roberto Cremonini
Co-Author Affiliation: Università di Torino, Dipartimento di Scienze della Terra; Agenzia Regionale per la Protezione Ambientale (ARPA Piemonte)
Abstract Title:
Solid and liquid precipitation variability in the Western Italian Alps and links to large scale forcings
The evaluation of climate change and its environmental and socio-economical effects in the Alpine region can not prescind from considering variability and trends of solid and liquid precipitation over long periods.
In the frame of the interregional project STRADA (ADAptation STRategy to climate change) aiming at the long term time series data rescue, daily rainfall and snow precipitation (fresh snow and snow depth) time series recorded in Western Italian Alps manual stations have been recovered and considered for the climatological analysis.
The time series are almost continuous except for short gaps which have been filled on monthly and seasonal basis using the Singular Spectrum Analysis technique based on temporal correlation of the time series (Ghil et al., 2001, Kondrashov et al., 2006). The so-obtained time series have been used to determinate and compare the climatological rain and snow indices over different time periods and to investigate on the presence of trends in snow (rain) precipitation amount, number of snowy (wet) days and mean snow depth.
The presence of significant oscillatory modes embedded in the seasonal snow (rain) precipitation time series has been investigated using the Singular Spectrum Analysis. The identification of inter-annual and inter-decadal cycles allows to put in relation the snow (rain) precipitation over Western Alps and large scale forcings represented by the North Atlantic Oscillation (NAO) index, the Western Mediterranean Oscillation (WMO) index and the El Nino Southern Oscillation (ENSO) index. The aim is to explore the precipitation variability over different temporal scales in relation to large scale climate modes.

Session Number: 4e
Presenting Author: Birthe Marie Steensen
Author Email: birthesteensen@gmail.com
Author Affiliation: Norwegian Meteorological Institute, Oslo, Norway
Co-Author: Haraldur Ólafsson & Marius Opsanger Jonassen
Co-Author Affiliation:
Abstract Title:
The role of mountains in an extreme precipitation event in Central-Norway
At the end of January and beginning of February 2006, an extreme precipitation event occurred over Central Norway. The precipitation in addition to warm temperatures produced flooding and landslides that caused considerable damage to infrastructure. The event is explored with conventional data, data from remote sensing and numerical simulations. It is shown that there was very little quasi-geostrophic forcing during the event and that the extreme precipitation is locally generated by strong and persistent winds impinging the mountains. The mountains in the southwestern part of Norway, far away from the precipitation, contributed significantly to the extreme, by blocking, deflection and enhancement of the low-level flow.

Session Number: 5a
Presenting Author: Andreas Behrendt
Author Email: andreas.behrendt@uni-hohenheim.de
Author Affiliation: University of Hohenheim, Institute of Physics and Meteorology, Garbenstr. 30, Stuttgart, Germany
Co-Author: Volker Wulfmeyer (1), Christoph Kottmeier (2), Ulrich Corsmeier (2), Hans-Stefan Bauer (1) , Christian Barthlott (2), Norbert Kalthoff (2)
Co-Author Affiliation: (1) University of Hohenheim, Institute of Physics and Meteorology, Garbenstr. 30, Stuttgart, Germany; (2) Karlsruhe Institute of Technology (KIT), Germany
Abstract Title:
COPS science questions revisited: What have we learned from COPS?
The Convective and Orographically-induced Precipitation Study (COPS) was an international field campaign carried out in summer 2007 with the overall goal to advance the quality of forecasts of orographically-induced convective precipitation by 4-dimensional observations and modelling of its life cycle. The pre-convective environment, the formation of clouds and the onset and development of precipitation were observed in a low-mountain area in south-western Germany and eastern France covering the Vosges mountains, the Rhine Valley, and the Black Forest mountains during 18 Intensive Observations Periods from June 1 to August 31, 2007, under different forcing conditions. Meanwhile, in the nearly four years since the COPS field phase, a large number of results on analyses of selected COPS IOPs and of continuous measurements during the COPS period have been published; recently in January 2011 in a special issue of the Quarterly Journal of the Royal Meteorological Society alone 21 papers appeared. In this contribution, we will revisit the original science questions formulated in the preparatory phase of COPS, summarize the results gained so far from COPS, and discuss questions which remain open.

Session Number: 5b
Presenting Author: Christophe Merlet
Author Email: christophe.merlet@latmos.ipsl.fr
Author Affiliation: Laboratoire Atmosphère, Milieux, Observations Spatiales, CNRS/UVSQ/UPMC, Paris, France
Co-Author: Laurent LABBOUZ (2), Cyrille FLAMANT (1), Sophie BASTIN (1), Cédric CHAMPOLLION (3), Paolo De GIROLAMO (4), Jean-Pierre CHABOUREAU (5), Juan CUESTA (5), Grégoire PIGEON (6), Volker WULFMEYER (7)
Co-Author Affiliation: (1) Laboratoire Atmosphère, Milieux, Observations Spatiales, CNRS/UVSQ/UPMC, Paris, France; (2) Laboratoire de Météorologie Physique, Université Blaise Pascal, Clermont-Ferrand, France; (3) Géosciences, Montpellier, France; (4) Dipartimento di Ingegneria
Abstract Title:
Convective inhibition over the Vosges and water vapor variability during COPS IOP 8b
For operational forecasts anticipate convective inhibition processes is as important than convective initiation processes. Based on the 15th July 2007 case study this work shows how the morning shallow convection over the Vosges is inhibited while it develops over the Black Forest. This case occurs during IOP 8b of the Convective and Orographically-Induced Precipitation Study (COPS). This international field campaign took place during the summer of 2007 over the Vosges, the Rhine Valley and the Black-Forest. The aim of this intensive observation campaign was to study various phases of convection and convective environment and improve quantitative precipitation forecasts.
Thanks to GPS tomography data the study show the differences of low-level water-vapor variability between Vosges and Black-Forest. The analysis of ground based and airborne lidar data suggests the importance of dry air layers advected from the south and the influence of the mountains and diurnal cycle of mountain breeze which interfere strongly with these layers preventing the dry air to reach the eastern side of the mountain and the Black Forest. This dynamics is confirmed by models, wind profiler and radiosounding data.
So we show how the inhibition of the convection over the Vosges during this case study can be explained with the water vapor variability due to advection and local boundary layer processes.

Session Number: 5c
Presenting Author: Kersten Schmidt
Author Email: kersten.schmidt@dlr.de
Author Affiliation: Deutsches Zentrum für Luft- und Raumfahrt (DLR)
Institut für Physik der Atmosphäre (IPA)
Co-Author: Hagen, Martin; Hoeller, Hartmut; Volkert, Hans
Co-Author Affiliation: DLR, IPA
Abstract Title:
Detailed flow, hydrometeor and lightning characteristics of an isolated, hail producing thunderstorm during COPS
A detailed analysis of air flow, hydrometeor characteristics, cloud-top height, and lightning is presented for the 15/07/2007 case during the Convection and Orographically Induced Precipitation Study (COPS) field campaign. In a synergetic manner, the study makes use of multiple-Doppler analysis from three operational radars, POLDIRAD\'s polarimetric capabilities, data from different channels of Meteosat Second Generation (MSG) satellite, as well as lightning data to document the life cycle of deep convection.
The cloud-top height derived from MSG infrared channel and radar volume scans increased rapidly during 14:20 and 14:35 UTC. The most active phase was identified during 14:35 and 14:45 UTC when cloud tops reached the tropopause, lightning occured with a maximum rate of 15 strokes per minute, and hail was detected via polarimetric radar products.
During the cell\'s lifetime horizontal and vertical flow were derived from four consistent triple-Doppler calculations with 500 m and 15 min space and time resolution, respectively. A main updraft was identified to originate from the more humid air masses to the east of the convergennce line responsible for the cell initiation. Consistency checks of the calculated wind fields with the observed radial velocities were performed. Areas of enhanced discrepancies between the observing systems (during the early phase of mature state) match with areas of higher turbulence marked by polarimetric radar parameters.
Moreover, lightning locations correspond well with volumes of distinct vertical motion, particularly within regions of extended updrafts. By interpreting hydrometeor types from polarimetric radar data the evolution of the fraction of different types per volume is investigated. While during the early phase of thunderstorm the fraction of ice (snow, graupel, hail) in the upper cloud part became enhanced, the fraction of rain increased during the decay phase.

Session Number: 5d
Presenting Author: Alan Blyth
Author Email: blyth@env.leeds.ac.uk
Author Affiliation: National Centre for Atmospheric Science, University of Leeds, UK
Co-Author: 1. Lindsay J Bennett, 2. Tammy M Weckwerth, 3. Ralph R Burton, 4. Alan M Gadian
Co-Author Affiliation: 1. Institute for Climate and Atmospheric Science, University of Leeds, UK. 2. Earth Observing Laboratory, National Center for Atmospheric Research, Boulder, USA. 3. National Centre for Atmospheric Science, University of Leeds, UK. 4. National Centre for
Abstract Title:
A Case Study of Convection Initiation on 6 August 2007 during COPS
Deep convection was triggered ahead of a cold front along the slopes of the Vosges Mountains in southwest France on 6 August 2007 during the Convective and orographically-induced Precipitation Study (COPS). As the system moved off the slopes it produced a large outflow and then quickly dissipated, although the outflow propagated across the Rhine Valley and initiated secondary convection. Observations of the horizontal wind field prior to and during the development of the storms were obtained by the Doppler On Wheels (DOW) mobile radars located in the centre of the Rhine Valley. Vertical profiles of temperature, humidity and winds were obtained
at the Achern Supersite on the eastern side of the valley. There were limited observations over the Vosges Mountains themselves. High-resolution simulations of the Weather Research and Forecasting (WRF) model have been performed to investigate the mechanisms responsible for the initiation of the storms. The model correctly
simulates the timing and location of the convection, but underestimates its size and strength. The convection was triggered by the development of a deep, moist convective boundary layer (CBL) over the Vosges and surface convergence between easterly upslope flow and westerly flow ahead of the cold front.

Session Number: 5e
Presenting Author: Christian Barthlott
Author Email: christian.barthlott@kit.edu
Author Affiliation: Institute for Meteorology and Climate Research, Karlsruhe Institute of Technology (KIT)
Co-Author: Norbert Kalthoff
Co-Author Affiliation: Institute for Meteorology and Climate Research, Karlsruhe Institute of Technology (KIT)
Abstract Title:
Impact of soil moisture on convection-related parameters and convective precipitation over complex terrain: A numerical case study for COPS IOP 15a
The impact of soil moisture on convection-related parameters and convective precipitation over complex terrain is studied by numerical experiments using the non-hydrostatic COSMO model. For one day of the Convective and orographically induced Precipitation Study (COPS) conducted in summer 2007 in southwestern Germany and eastern France, initial soil moisture is varied from -50% to +50% of the reference run in steps of 5%. As synoptic-scale forcing is not relevant on the day under investigation, the triggering of convection is mainly due to soil-atmosphere interactions and boundary-layer processes.
Whereas a systematic relationship exists for a number of variables (e.g. latent and sensible fluxes at the ground, near-surface temperature, humidity), a systematic increase of 24 h-accumulated precipitation with initial soil oisture
is only present in the simulations drier than the reference run. The time evolution of convective precipitation can be divided into two regimes with different conditions to initiate convection. In general, increasing the soil moisture had a much smaller effect on convection-related parameters than decreasing it. The results demonstrate the high sensitivity of numerical weather prediction to initial soil moisture fields.

Session Number: 6a
Presenting Author: Norbert Kalthoff
Author Email: norbert.kalthoff@kit.edu
Author Affiliation: Institut für Meteorologie und Klimaforschung (IMK), Karlsruher Institut für Technologie (KIT), Karlsruhe, Germany
Co-Author: Stephan Späth1, Katja Träumner1, Jan Handwerker1, Fabio Madonna2, Andreas Behrendt3
Co-Author Affiliation: 1Institut für Meteorologie und Klimaforschung (IMK), Karlsruher Institut für Technologie (KIT), Karlsruhe, Germany
2Instituto di Metodologie per l’Analisi Ambientale, CNR-IMAA, Potenza, Italy
3Universität Hohenheim, Institut für Physik und Meteorologie
Abstract Title:
Analysis of boundary layers with dry and moist convection over a mountain top observations from the COPS campaign
During the COPS experiment performed in 2007 in southwestern Germany and eastern France, several in situ and remote sensing systems were operated at supersite Hornisgrinde - a summit of the northern Black Forest. The collected data allow investigating convective cells in the convective boundary layer and exchange processes between the boundary layer and the free troposphere.
For this case study, data from the surface flux station, radiosoundings, cloud camera, cloud radar, wind lidar, differential absorption lidar, and infrared radiometer were applied to analyse two periods, during which dry and moist convective cells were observed over the Hornisgrinde mountain top. The simultaneous measurements of vertical wind speed and humidity were used to calculate the turbulent moisture flux in the boundary layer and entrainment zone. The fluxes were compared with the surface flux values. In both cases (dry and moist convection) a strong transport of moisture from the boundary layer to the free atmosphere could be observed. The vertical wind speed measurements also show that downdrafts were present in the surrounding of the convective cells. Finally, some features of the dry and moist convective cells are compared. The study shows that the combination of different observation systems can provide additional insight into processes in the dry and moist boundary layer.

Session Number: 6b
Presenting Author: Romi Sasse
Author Email: romi.sasse@kit.edu
Author Affiliation: Institute for Meteorology and Climate Research Karlsruhe Institute of Technology Karlsruhe, Germany
Co-Author: (1) Gerd SCHÄDLER, (2) Michael BENDER
Co-Author Affiliation: (1) Institute for Meteorology and Climate Research, Karlsruhe Institute of Technology, Karlsruhe, Germany
(2) Department Geodesy and Remote Sensing, German Research Centre for Geosciences, Potsdam, Germany
Abstract Title:
Impact of synoptic situations and topography on the regional atmospheric water budget: A balance related approach using COSMO-CLM simulations and COPS observations
The atmospheric water budget, mainly consisting of atmospheric water vapour, moisture advection, precipitation and evapotranspiration, is closely connected to the climate system. On the global, but even more on the regional scale, the partitioning and transfer rates of the water budget components are highly variable and not routinely known from observations. Research in the context of climate change and water management necessarily requires reliable data in a high spatial and temporal resolution for any region. The balance related approach advances a comprehensive understanding of the effect of weather conditions and topography on the atmospheric water budget.
In this study the budget components are quantified using the high-resolution regional climate model COSMO-CLM. The COPS campaign (Convective and Orographically-induced Precipitation Study), which took place in Southwest Germany and East France in summer 2007, provides the opportunity to validate the simulated components using precipitation and energy balance measurements as well as modern GPS observations. The latter offers a powerful tool to determine the integrated water vapour content. The budget components are calculated for control volumes comprising various regions with dimensions of about 10^3 to 10^4 km^2 within the COPS area. The dependency of the atmospheric water budget on air mass characteristics and inflow directions is discussed. To explore the impact of the topography control volumes are set up in the Rhine Valley and the low mountainous region of the Black Forest and the Swabian Jura. Water budget calculations and detailed statistical analyses are realised for the summer months from 2005 to 2009.
To encourage the use of COSMO-CLM simulations for water budget analyses, comparisons indicating good agreements between simulations and observations are shown. The high variability of the components makes a clear characterisation of the atmospheric water budget for a specific synoptic situation difficult. By introducing a classification of the daily water budgets based on the inflow direction and examining the distribution of the components, we obtain statistically significant differentiations. They are caused by air mass characteristics and the occurrence of high or low pressure systems. In contrast to the Rhine Valley we notice an increase of moisture convergence, precipitation and evapotranspiration in the low mountainous region of the Black Forest and the Swabian Jura. This enhanced water transport is controlled by factors closely connected to the topography like wind systems, convective processes, vegetation and land use. Hence, this study suggests an intensification of the atmospheric water budget processes in low mountainous regions.

Session Number: 6c
Presenting Author: Hans Volkert
Author Email: Hans.Volkert@DLR.de
Author Affiliation: Deutsches Zentrum fuer Luft- und Raumfahrt (DLR)
Institut fuer Physik der Atmosphaere (IPA)
Co-Author Affiliation:
Abstract Title:
Lessons learnt from field experiments: Inspection of the ALPEX-PYREX-MAP-COPS chain
In atmospheric sciences controlled experiments are difficult to impossible. Instead more data are collected during \'field experiments\' than is possible on a routine basis, be it at more locations, more frequently or with novel instrumentation. The efforts are typically justified by general reference to topics like \'model validation\', \'detailed study of important physical processes\', \'enhancing physical understanding\', \'more detailed assessment of model skill\'.
For mountain meteorological experiments in Europe the chain of large international experiments from ALPEX (special observation period: March - April 1982) via PYREX (SOP: Oct. - Nov. 1990) and MAP (SOP: Sep.- Nov. 1999) to COPS (SOP: June - Aug. 2007) spanned a quarter of a century in nearly equals intervals. A combination of routine and specialized surface and aerological networks with aircraft mission and satellite retrievals was common to all four enterprises.
The presentation compares primary findings with the original plans, regards the gradual concentration to smaller scales, inspects the growing importance of modelling studies for planning, conduct and analysis of the SOP as a whole and some of the imbedded intense observation periods. Interactions with steering bodies of WMO as GARP or WWRP are reflected to underpin the empirical finding: Field experiments (continue to) form an important pillar for progress in (mountain) meteorology.

Session Number: 6d
Presenting Author: Véronique Ducrocq
Author Email: veronique.ducrocq@meteo.fr
Author Affiliation: CNRM-GAME Météo-France & CNRS
Co-Author: HyMeX International Scientific Steering Committee
Co-Author Affiliation:
Abstract Title:
HyMeX - the Mediterranean coastal orographic heavy precipitation field campaing (SOP1 -fall 2012)
HyMeX (HYdrological cycle in the Mediterranean Experiment, http://www.hymex.org/) is an international program aiming at a better quantification and understanding of the water cycle in Mediterranean - with emphases on intense events - by monitoring and modelling the Mediterranean coupled system (atmosphere-land-ocean), its variability (from the event scale, to the seasonal and interannual scales) and characteristics over one decade in the context of global change. In particular, HyMeX aims at addressing key issues related to (1) the water budget of the Mediterranean basin, (2) the continental hydrological cycle and related water resources, (3) heavy precipitation and flash-flooding and (4) intense air-sea exchanges produced by severe regional winds and cyclogenesis. HyMeX aims also at monitoring vulnerability factors and adaptation strategies developed by different societies to accommodate the impacts of climate change and intense events.
The aim of the talk is to present an update of the program implementation regarding the observation and modelling strategy for heavy precipitation over the mountainous Mediterranean coastal regions. The general observation strategy is based on a three-level nested observation scheme: (1) a Long-term Observation Period (LOP, 2010-2020) to gather and provide observations on the whole coupled system in order to analyze the seasonal-to-interannual variability of the water cycle and to estimate the water budget, (2) Enhanced Observation Periods (EOP) for both budget and process studies lasting several years and (3) Special Observation Periods (SOP) lasting several months. The observation strategy of the EOP/SOP dedicated to heavy precipitation and flash-flooding will be presented as well as the associated modelling activities. HyMeX will constitute a unique test-bed for new-generation convection-permitting ensemble prediction and data assimilation systems in order to advance the predictability of these high-impact weather events.

Session Number: 7a
Presenting Author: Roy Rasmussen
Author Email: rasmus@ucar.edu
Author Affiliation: National Center for Atmospheric Research, 3450 Mitchell Lane, Boulder, CO 80301
Co-Author: David Gochis, Kyoko Ikeda, Changhai Liu, Fei Chen, Jimy Dudhia, Greg Thompson, Vanda Grubišić, Ethan Gutmann, Mukul Tewari, Michael Barlage
Co-Author Affiliation: NCAR, University of Vienna, Vienna
Abstract Title:
High-resolution simulation of seasonal snowfall over the Colorado Rockies Headwaters region and some impacts of climate change
Snowpack is the most important water source in the western U.S., and thus it is critical that water managers be provided with as accurate as possible estimate of the likely changes expected of this resource in the future. Previous climate studies have shown that the head waters region of the Colorado river seems to be a particularly difficult area for climate model to handle, with inconsistent snowpack trends in this region from both the 3rd and 4th IPCC reports (2001, 2007), despite consistent prediction of temperature increases in this region from all climate models. In this study WRF regional model simulations of snowfall between 1 November and 1 May were performed over the Colorado Headwaters region for: (1) retrospective years (2001-02, 2003-04, 2005-06, and 2007-8) at a horizontal grid resolution of 2 km using North American Regional Re-analysis (NARR) data, (2) 2007-08 season at coarser resolution of 6, 18, and 36 km using NARR data, and (3) future climate scenario at 2 km grid resolution using NARR data for the retrospective years perturbed with the CCSM3 model output for an A1B simulation to initialize the simulation using a Pseudo Global Warming approach (temperature and moisture perturbation, but no change in storm tracks). Key questions explored in this study are, (1) how well can we simulate seasonal snowfall in the Colorado Headwaters region (comparison to SNOTEL data), (2) how will snowfall in the Colorado Headwaters region be impacted by a warmer and moisture climate, (3) how well do we simulate snowpack in the Colorado Headwaters region (comparison to SNOTEL data), (4) how will snowpack amount and timing change in a warmer and moister climate, and (5) how high a resolution of the regional climate model do we need to answer these questions. Results from on-going analyses will be presented.

Session Number: 7b
Presenting Author: Guðrún Nína Petersen
Author Email: gnp@vedur.is
Author Affiliation: Icelandic Meteorological Office, Reykjavík, Iceland
Co-Author: Þórður Arason, Halldór Björnsson, Sibylle von Löwis
Co-Author Affiliation: Icelandic Meteorological Office, Reykjavík, Iceland
Abstract Title:
The impact of the atmospheric flow on the Eyjafjallajökull eruption plume and resuspened ash during and after the eruption
The volcanic plume of the Eyjafjallajökull eruption in 2010 was monitored using a C-band weather radar situated 154 km from the volcano and by web cameras near the volcano. In addition, airborne observations allowed for detailed examination of the plume, and pilot reports and on-site visual observations were useful in verifying the radar data.
The eruption had two explosive phases, 14-18 April and 3-20 May and the eruption ceased on 23 May. During these phases the eruption plume reached altitudes of 5 to 9 km. Between the two explosive phases a mixed phase occurred, with lava effusion and mild magmatic explosive activity of lower intensity. The volcanic plume was much weaker, with altitude range from below radar detection limit to about 5 km.
Due to the length of the eruption and the variations in the plume activity the two data series of the plume altitude as observed by the weather radar and a web camera situated 34 km from the volcano, together with atmospheric soundings and numerical modelling provide unique opportunity to study the interaction of the ambient atmosphere with the small to medium size volcanic plume.
For example, a diurnal oscillation is observed in the volcanic plume during the less explosive phase of the eruption when the plume was rather weak. This behaviour in the plume is most readily explained by a diurnal oscillation in the stability of the atmosphere above the volcano, with a nocturnal inversion acting to cap the weak plume and the plume only reaching above this altitude when the inversion weakens or dissipates.
During and after the eruption there have been cases of significant resuspension of ash in the vicinity of the volcano. Dispersion of resuspended ash is highly dependent on wind speed and wind direction and consequently dependent on distortion of the airflow by the orography .

Session Number: 7c
Presenting Author: Reinhold Steinacker
Author Email: reinhold.steinacker@univie.ac.at
Author Affiliation: University of Vienna, Department of Meteorology and Geophysics, Althanstrasse 14, 1090 Vienna, Austria
Co-Author Affiliation:
Abstract Title:
A low cost device for using tipping bucket ombrometers in winter
Tipping bucket ombrometers are widely used today, as they measure with a high degree of reliability and allow a sufficient temporal and rain amount resolution. When using such devices on a year round basis in climate regions with solid precipitation, a heating of the ombrometer is necessary. Solar energy use for the heating is problematic, as the power consumption is especially high, when little or no solar radiation is available. This means that such stations are restricted to locations, where electric power supply is provided.
For some field studies, especially over complex terrain, where a dense array of precipitation stations or a raster of stations is requested, electricity is hardly available at all locations. Then, a low budget device may be applied, which operates without additional electric power (electric power for the data logger is easily provided by batteries). It consists of a container on top of the reception area, filled with salty water and covered with a layer of oil, similar to a totalisator. If the rain gauge collecting area of the device is chosen the same as of the tipping bucket ombrometer itself, no recalibration of the solid precipitation amount is necessary. One of the advantages of such a device , besides the operability without electricity, is the fact, that evaporation on the heated collector surface is not occurring.
The presentation will describe the setup of such a device and present some results of parallel measurements during the last winter. Although the results are very promising, the lessons learned during this test phase will lead to an improved design, which will be tested on several Alpine locations in the next winter.

Session Number: 8a
Presenting Author: C. David Whiteman
Author Email: dave.whiteman@utah.edu
Author Affiliation: Department of Atmospheric Sciences, University of Utah
Salt Lake City, UT 84112 USA
Co-Author: John Horel, Erik Crosman, Neil Lareau
Co-Author Affiliation: University of Utah
Abstract Title:
Overview of the Persistent Cold-Air Pool Study (PCAPS) Field Program
The Persistent Cold-Air Pool Study (PCAPS) is a National Science Foundation-supported investigation of persistent, multi-day, wintertime temperature inversions that form in valley and basin topography. Persistent wintertime inversions are known throughout the world\'s mountain areas and are a key factor leading to wintertime air pollution episodes, periods of freezing rain and drizzle, extreme temperature minima, and attendant transportation and other weather-related societal problems. As part of PCAPS, a major field study was conducted in Utah\'s Salt Lake Basin during the period from 1 December 2010 to 7 February 2011 to document the wintertime events. Synoptic conditions were unusually conducive for the formation of cold pools this winter, leading to 10 Intensive Observational Periods (IOPs) of up to 8 days in duration. The cold pools were intensively documented using a large array of surface-based, airborne and remote sensing instruments, with support from many scientific collaborators and over 40 students.
Sub-experiments were conducted under specified conditions during some of the IOPs to investigate the roles of different meteorological phenomena on persistent inversion buildup, maintenance and destruction, including:
1. synoptic-scale warm and cold air advection
2. diurnal cycling of cold air and pollutant storage over the Great Salt Lake
3. warm air subsidence at the top of the basin
4. diurnal changes in turbulent sensible heat flux at the basin floor,
5. strong southerly low-level flows in advance of approaching lows, and
6. inflows from tributaries of the Salt Lake Basin
This paper will present an overview of this major mountain meteorology field program, its objectives, the basin topography, the meteorological intrumentation, the synoptic setting of the winter, the IOPs, and the initial findings.

Session Number: 8b
Presenting Author: John Horel
Author Email: john.horel@utah.edu
Author Affiliation: Department of Atmospheric Sciences University of Utah
Co-Author: Chris Ander, C. David Whiteman
Co-Author Affiliation: University of Utah
Abstract Title:
Estimating the Intensity of Persistent Cold-Air Pools
Ten distinct cold-air pools in the Salt Lake Valley during the 2010-2011 winter were studied as part of the Persistent Cold-Air Pool Study (PCAPS). Objective techniques to estimate the intensity of these events relative to others during the past six years are compared using GPS soundings available twice daily from the Salt Lake City airport as well as PCAPS research soundings. Common thermodynamic approaches (mean lapse rate or mean temperature deficit relative to an assumed potential temperature profile) are contrasted to a “Cold Pool Inhibition (CPIN)” index similar to Convective INhibition (CIN), which is used to assess the work required to overcome the negative buoyancy in a stable boundary layer and lift parcels to the level of free convection. CPIN is defined as the work required to lift the lowest 500 m layer upwards 500 m (the depth of the layer and vertical extent of the lifting are arbitrary). The 500 m layer is subdivided into 10 m “parcels” and each of those parcels is lifted adiabatically assuming they cool at the saturated adiabatic lapse rate above their lifting condensation level. The CPIN index can be weighted by the basin volume within each 10 m layer to estimate more realistically the work required to lift the layer. CPIN can also be expressed in terms of the mean vertical velocity required to lift the entire layer.
It is not difficult to examine soundings subjectively and assess the presence and intensity of cold-air pools. For example, many cold-air pools in the Salt Lake Valley during PCAPS began after a period characterized by a deep well-mixed layer followed by a subsidence inversion aloft that descends over the next several days. Commonly applied thermodynamic sounding indices tend to depict this evolution as a weakening of cold-air pool intensity as the mixed layer becomes shallower and the subsidence inversion lowers. However, pollutants tend to become more firmly entrenched with time during cold-air pools as the surface-based mixed layer decreases in vertical extent. High CPIN values provide a very good indication of the presence of strong cold-air pools and the temporal evolution of CPIN during such events is more consistent with physical reasoning. The various indices of cold-air pool intensity are related to carbon dioxide concentrations during the past six winters. The forecast skill of the operational NCEP NAM model to predict the temporal evolution of CPIN during the PCAPS field campaign is examined. 

Session Number: 8c
Presenting Author: Peter Sheridan
Author Email: peter.sheridan@metoffice.gov.uk
Author Affiliation: Met Office, UK
Co-Author: Simon Vosper
Co-Author Affiliation: Met Office
Abstract Title:
Relating cold air pool formation to synoptic conditions during the COLPEX field campaign
The accurate prediction of cold-air pools which form in valleys during clear, still nights represents a major challenge to NWP models for two reasons: (1) They often occur on scales too small to be resolved by current operational models (2) The underlying processes which govern their formation are not well understood.
The field phase of the COLd-air Pooling Experiment (COLPEX) was carried out in the small-scale, rolling terrain of the Clun valley, Shropshire, UK for 15 months during 2009-2010. The experiment involved in-situ measurements with an array of instruments both in and outside the valley, along with radiosonde releases, lidar measurements and an instrumented car.
In this study wind and temperature data from 30-50m turbulence towers, an array of AWS and HOBO stations, radiosondes, and operational model forecast data are used to determine the influence of synoptic conditions on cold pool formation.
Relationships are developed which have implications for NWP downscaling techniques used to enhance NWP forecasts in complex terrain.

Session Number: 8d
Presenting Author: Simon Vosper
Author Email: simon.vosper@metoffice.gov.uk
Author Affiliation: Met Office, FitzRoy Road, Exeter EX1 3PB, UK
Co-Author: Emilie Carter(1), Aurore Porson(1), Adrian Lock(1), Humphrey Lean(1), Peter Sheridan(1), Peter Clark(2), Bradley Jemmet-Smith(3), Andrew Ross(3)
Co-Author Affiliation: (1) Met Office, UK; (2) University of Surrey; (3) University of Leeds
Abstract Title:
Very high resolution model simulations of cold-air pooling in complex terrain during COLPEX
The COLd-air Pooling EXperiment (COLPEX) is a research programme concerned with understanding and improving predictions of cold-air pools which form in valleys during stable night time conditions. Cold pools are associated with hazardous conditions such as localised fog, icy road surfaces and poor air quality. Accurate forecasting of such phenomena is important. However, the small-scale nature of cold pools means they are often poorly represented in even the highest resolution operational numerical weather prediction models.
As part of the COLPEX project very high resolution simulations of cold pooling in the Clun valley (the field site), in Shropshire, UK, have been conducted using nested versions of the Met Office Unified Model. The inner-most model domain has a variable horizontal grid spacing, with a finest resolution of 100 metres. Simulations have been conducted for several cold-pool and valley-fog episodes that were observed during the field phase of COLPEX.
This presentation will consider how well the 100m-resolution model simulates the observed flows for clear-sky and foggy cases. Sensitivity studies have been conducted, examining the dependence of results on model properties such as the vertical resolution, time step, sub-grid mixing schemes and soil moisture content. The model results will be used to identify the key physical processes involved in cold pool formation and to develop relationships between the cold pool properties and the synoptic conditions.

Session Number: 8e
Presenting Author: Volker Horlacher
Author Email: volker.horlacher@metoffice.gov.uk
Author Affiliation: Met Office Field Site, Cardington Airfield, Shortstown
Bedfordshire, MK42 0SY, UK
Co-Author: Jeremy D. Price
Co-Author Affiliation: Met Office
Abstract Title:
Turbulence spectra during cold pooling events within complex terrain
Predicting the occurrence of cold pooling within complex terrain is a challenge for numerical weather forecasting models. The grid spacing is still too coarse to resolve small scale features such as local temperature variations. Consequences can be locally icy roads in regions with an average temperature above freezing, which are not predicted accurately by numerical models. In addition there are still uncertainties about the key processes involved in the formation of cold pooling, e.g. the influence of turbulence, surface heterogeneity, drainage currents, and topographically-induced effects.
In order to improve the knowledge about small scale valley flows for stable conditions the COLd-air Pooling Experiment (COLPEX) was carried out in a region of small hills on the border between England and Wales, UK. The duration was 15 months.
In this study turbulence data at three sites from masts up to 50m high are analysed and spectra of velocity perturbations in u’, v’ and w’ during the formation and evolution of cold-air pools, is examined. In particular, it is investigated whether cold pools show a characteristic signature in the turbulent spectra (including the nature of the inertial subrange), and also turbulent structure at different sites within the region

Session Number: 9a
Presenting Author: Dino Zardi
Author Email: Dino.Zardi@ing.unitn.it
Author Affiliation: Atmospheric Physics Group, Department of Civil and Environmental Engineering, University of Trento, Trento, Italy
Co-Author: Stefano Serafin
Co-Author Affiliation: Institute for Meteorology and Geophysics, University of Vienna, Vienna, Austria
Abstract Title:
An analytic solution for periodic thermally driven flows on an infinite slope - Defant (1949) revisited.
The flow generated as a consequence of a periodically varying surface temperature along an infinite slope is studied.
The set of equations derived by Defant (1949) - extending Prandtl’s (1942) theory to include nonstationary surface temperature conditions - is adopted. In particular the flow properties are assumed to be invariant along the slope, and turbulent fluxes of momentum and heat are parameterized with a K-closure, with constant eddy viscosity Km and eddy diffusivity Kh and assuming a Prandtl number Pr = Km/Kh = 1. Differently form Defant (1949), the initial problem of a flow starting from rest is evaluated. Solutions for both the transient and the subsequent steady periodic regime are found. Accordingly, slope-normal profiles of the potential temperature anomaly with respect to the basic state, and of the along-slope wind velocity can be evaluated at any time t.
The transient part can only be expressed in an integral form. Instead the solution for the steady periodic state can be explicitly expressed as a combination of exponential and sine/cosine functions of time and slope-normal coordinate n.
The resulting expressions are different from those found by Defant (1949). Let alpha be the slope angle, T the time period of driving surface temperature cycle, w=2 pi/T the related angular frequency and N_alpha=N/sin(alpha), where N is the Brunt-Vaisala frequency of the stably stratified unperturbed atmosphere. It can be shown that three different regimes may occur, namely subcritical (N_alpha

Session Number: 9b
Presenting Author: Manuela Lehner
Author Email: manuela.lehner@utah.edu
Author Email: manuela.lehner@utah.edu
Author Affiliation: University of Utah, Department of Atmospheric Sciences
Salt Lake City, UT, USA
Co-Author: C. David Whiteman, Sebastian W. Hoch
Co-Author Affiliation: University of Utah, University of Utah
Abstract Title:
Large-eddy simulations of thermally driven cross-basin winds using WRF
Asymmetric irradiation on two opposing sidewalls in a valley or basin can cause the formation of thermally driven flows across the valley/basin from one sidewall to the other. Stronger heating of the sun-facing sidewall compared to the less sunlit or shaded sidewall produces a horizontal temperature and pressure gradient across the valley/basin. The pressure gradient then leads to a flow from the shaded to the more sunlit sidewall. Observations in Arizona’s Meteor Crater have shown the presence of cross-basin winds near the crater floor on quiescent, clear-sky days, which were driven by asymmetric heating of the crater sidewalls.
Based on these observations we performed WRF LES simulations for an idealized crater topography. The idealized simulations produced a flow pattern near the surface similar to the observations in the Meteor Crater. In this presentation we investigate vertical profiles of cross-basin flow to determine the depth, strength, and spatial distribution of the cross-basin flow and the opposing return flow aloft. Different background wind speeds and wind directions above the crater rim are discussed with respect to their impact on the thermally driven cross-basin circulation within the basin.

Session Number: 9c
Presenting Author: Branko Grisogono
Author Email: bgrisog@gfz.hr
Author Affiliation: Department of Geophysics Faculty of Science, University of Zagreb, 10000 Zagreb, Croatia
Co-Author: Simon Axelsen
Co-Author Affiliation: Utrecht, the Netherlands
Abstract Title:
An analytic and LES study of the pure katabatic low-level jet
The evolution of the stably-stratified atmospheric boundary layers (SABL) is still understood inadequately, especially for airflows over inclined (mountainous) areas. A special class of the SABL flows consists of katabatic winds, which sub-class makes the glacier wind. The latter is important for glacier mass balance estimates, and it is poorly (or not at all) represented in NWP and climate models; at the same time, glaciers behave as climate „barometers“ and are important for many reasons. Overall, katabatic flows are still treated with insufficient level of knowledge and accuracy in majority of models used today.
One basic aspect of simple (pure) katabatic flows is addressed in this study: the dependency of the katabatic low-level jet on the value of the constant slope angle. The maximum of pure katabatic wind over moderate slopes, the inclination angle varying between 3 to 6 degrees, is studied using large-eddy simulation (LES) and compared to the classical Prandtl model. The numerical results show that both the maximum katabatic wind speed and its height, i.e., the overall low-level jet, decrease with increasing slope angle, while in the analytic solution only the wind maximum elevation is affected by the slope angle. For the range of slope inclinations given, a linear fit between the magnitude and the height of the low-level jet is obtained. The results are corroborated qualitatively with the PASTEX field campaign (Pasterze glacier, Austria, 1994).
The pure katabatic steady-state flow is approached earlier for the larger angles than for the smaller angles of inclination. Therefore, the larger-angle flow accelerates for the relatively shorter period of time and thus accumulates less momentum that is spread through relatively thinner layers. In other words, for the relatively small range of inclination angles studied here, the more inclined katabatic flow exhibits relatively weaker low-level jet than its less inclined (and thus more gradually evolving) counterpart.

Session Number: 9d
Presenting Author: Haraldur Ólafsson
Author Email: haraldur68@gmail.com
Author Affiliation: University of Iceland, Icelandic Meteorological Office and Institute for Meteorological Research
Co-Author: Hálfdán Ágústsson
Co-Author Affiliation:
Abstract Title:
The sea breeze and the upslope wind
Numerical simulations of thermally driven flows reveal that the presence of slope winds accelerates the sea breeze as its front moves towards the slope, while at a later stage the slope contributes to a deceleration of the sea breeze. The sea breeze also gives the slope wind an extra push. All the above interactions can be explained by the impact the vertical circulation by one of the systems has on the pressure field driving the other system.

Session Number: 10x
Presenting Author: Mathias W Rotach
Author Email: Mathias.Rotach@uibk.ac.at

Author Affiliation: University of Innsbruck, Austria
Abstract Title:
On the spatial (in)homogeneity of boundary layers in complex terrain
Boundary Layer parameterizations as applied in essentially all applications use concepts that are based on the assumption of horizontally homogeneous conditions. Clearly, this pre-requisite cannot strictly be fulfilled in complex terrain. It has not, however, been established so far at what spatial scales this limitation is serious and what could be done about it. In the light of ever increasing spatial resolution in numerical modelling in particular, the boundary layer treatment starts to emerge from a mere ‘tuning knob’ to an important ingredient of the modelling systems. In this contribution therefore the problem is first outlined on the example of available evidence from experimental campaigns and numerical modelling. It is shown that horizontal inhomogeneity is serious (i.e., dominant) for spatial scales of a typical topographic feature. Unfortunately these are the scales that are often not (or poorly) resolved (e.g., in numerical models). Possible approaches on how to systematically address the problem will be presented and ongoing initiatives are discussed that are likely to contribute to further progress.

Session Number: 10a
Presenting Author: Stefano Serafin
Author Email: stefano.serafin@univie.ac.at
Author Affiliation: University of Vienna, Department of Meteorology and Geophysics
Co-Author: Dino ZardiI
Co-Author Affiliation: University of Trento, Department of Civil and Environmental Engineering, Atmospheric Physics Group
Abstract Title:
Daytime development of the boundary layer over a plain and in a valley under fair weather conditions: A comparison by means of idealized numerical simulations
The daytime thermal structures of the valley boundary layer (VBL) and of the convective boundary layer above a plain (CBL), as revealed by Large Eddy Simulations, are compared. Simulations in the two environments consider similar thermal forcing, thus allowing to analyze the atmospheric heating processes in the VBL and CBL in light of the volume-effect theory, traditionally invoked to explain the larger diurnal temperature ranges observed in valleys.
It is found that, after an equal input of thermal energy, the atmospheric volumes affected by thermal perturbations in the CBL and in the VBL are comparable. Although the boundary-layer top is higher in the VBL than in the CBL, the average VBL depth is approximately equal to the CBL depth, since the ground elevation is non-uniform in the valley. Accordingly, the volume-averaged potential temperature increments in the CBL and VBL are comparable. Nevertheless, surface air temperature variations are larger in the VBL, while differences in the thermal structures of the CBL and the VBL are found to be larger at elevated levels. These effects are related to the heat and mass transfer processes associated with upslope flows and mid-valley subsidence.
As far as the simulated CBL and VBL cases are representative of two asymptotic regions (respectively far up-valley and far over the plain) of a plain-valley system with a horizontal floor, their comparison provides insight in the mechanisms responsible for the generation of the pressure contrasts driving a daytime plain-to-valley wind at lower levels and possibly a valley-to-plain upper flow.

Session Number: 10b
Presenting Author: Sebastian W. Hoch
Author Email: sebastian.hoch@utah.edu
Author Affiliation: Atmospheric Sciences, University of Utah, Salt Lake City, Utah 84112, USA
Co-Author: C. David Whiteman, Joe Young
Co-Author Affiliation: University of Utah
Abstract Title:
Evolution of wintertime temperature, humidity and wind profiles in a deep open-pit copper mine and in the adjacent Salt Lake Valley
The 1200-m deep, 4000-m diameter Bingham Canyon open-pit copper mine is located in the Qquirrh Mountains that form the west sidewall of Utah\'s Salt Lake Valley. A low pass 570 m above the mine floor connects the mine to the adjacent Salt Lake Valley. The elevation of the valley floor is only 50 m lower than the floor of the mine.
In the winter of 2010-2011 an automatic weather station was placed in the pass and a line of temperature and humidity sensors was run from the floor of the cone-shaped mine, up the mine sidewall, over the east ridge of the mine and down its outer sidewall to the floor of the Salt Lake Valley. These observations provided synchronous pseudo-vertical temperature and humidity profiles on the inside and outside slopes of the mine. A scanning Doppler wind lidar was also deployed near the floor of the mine to measure wind and aerosol backscatter profiles within and above the mine. Further, a radar wind profiler at the floor of the Salt Lake valley measured wind profiles in the valley atmosphere.
The goals of this experimental program were to investigate the differences in boundary layer evolution between the mine and valley atmospheres and to determine how the differences are influenced by overlying winds.
Temperature differences between the mine and valley atmospheres reached 10°C, with the greatest differences occurring at heights near the mine bottom. Over the entire experimental period, the mine atmosphere generally remained warmer than the adjacent valley atmosphere at the same elevation. This finding is in contrast to other high altitude basins, such as Austria\'s Gruenloch Basin, Utah\'s Peter Sinks Basin, and Colorado\'s Sinbad Basin where temperatures near the basin floors can reach extreme temperature minimums.
This presentation will state the goals and objectives of the experimental program, describe the topography and the meteorological instrumentation, and summarize the observations of the measurement program, which ended on 30 April 2011. Initial results from the data analysis will be presented, including case studies of cold-air pool episodes, their breakup, and the effects of ambient winds on boundary layer evolution.

Session Number: 10c
Presenting Author: Bianca Adler
Author Email: bianca.adler@utah.edu
Author Affiliation: Atmospheric Sciences, University of Utah, Salt Lake City, UT
Co-Author: C. David Whiteman, Manuela Lehner, Sebastian W. Hoch
Co-Author Affiliation: Atmospheric Sciences, University of Utah, Salt Lake City, UT
Abstract Title:
Warm air intrusions in Arizona’s Meteor Crater – evidence for hydraulic jumps?
Episodic intrusions of warm air, accompanied by strong winds, enter the enclosed near-circular Barringer Meteorite Crater basin during clear, synoptically undisturbed nights. These warm air intrusions remain confined in the immediate lee of the crater\'s upwind rim. Using data from the 2006 METCRAX (Meteor Crater Experiment) field study we determined the spatial and temporal characteristics of the warm air intrusions, their effects on the atmospheric structure inside the crater, and their relationship to larger-scale flows and atmospheric stability. The warm air intrusions appear to be produced by small-scale atmospheric hydraulic jumps.
The intermittent warm air intrusions are associated with near-continuous shallow cold-air intrusions that come over the upwind crater rim during synoptically undisturbed nights, descend into the crater on the upwind inner sidewall, and destabilize the crater atmosphere. The cold air inflow is a result of the evolution of a stable surface layer and a mesoscale (~50 km) southwesterly drainage flow on the inclined plain outside the crater. Occasional instabilities occur in the flow coming over the crater rim, which extends 50 m above the surrounding plain, causing warm air from above the inversion on the surrounding plain to descend up to 130 m into the 170-m-deep crater. This warm air intrusion does not destroy the shallow cold air inflow on the slope below it, does not reach the crater floor, and does not extend into the center of the crater, where the nighttime stable layer in the remainder of the crater volume remains largely undisturbed. New field experiments and model simulations are suggested to test the hypothesis that the warm air intrusions are hydraulic flows.

Session Number: 10d
Presenting Author: Željko Večenaj
Author Email: zvecenaj@gfz.hr
Author Affiliation: Department of Geophysics Faculty of Science, University of Zagreb, 10000 Zagreb, Croatia
Co-Author: Danijel Belušić, Branko Grisogono
Co-Author Affiliation: Univ. of Zagreb, Department of Geophysics, Faculty of Science, 10000 Zagreb, Croatia
Abstract Title:
Recent findings about bora turbulence at the eastern Adriatic coast
Downslope windstorms, due to the frequent severity, represent a great threat to traffic, infrastructure, tourism, agriculture and lives, as well as great potential in wind energy resources. Therefore, they are a subject of intense and continuous research in the meteorological community. In the lee of Dinaric Alps at the eastern Adriatic coast, a downslope windstorm called bora often occurs. Over the past ~ 40 years or so, many scientists addressed bora in the terms of bora-wind statistics, synoptic and dynamical patterns but only few assessed its turbulence characteristics.
This is an overview of the recent work on the bora-related turbulence during the past several years. Different data sets of horizontal and vertical wind are used. Primary sets are 4 Hz single point measurements in the town of Senj (44.99°N, 14.90°E, 2 m above MSL, 13 m above the ground) and Vratnik Pass (44.98°N, 14.98°E, 600 m above MSL, 10 m above the ground) which are in the period from 2004 to 2006 continuously performed for more than two years and almost a year, respectively. Turbulence analysis and correspondent algorithms developed on these two data sets are then expanded to the 25 Hz aircraft measurements taken above the north-eastern Adriatic Sea during the MAP campaign in 1999 and the 5 Hz tower measurements with three levels on Pometeno Brdo (43.62°N, 16.47°E, 600 m above MSL, 10, 20 and 40 m above the ground) near the city of Split.
Some of the most important turbulence variables are investigated (e.g. turbulent kinetic energy and its dissipation rate); these are also used for testing the similarity theory and are compared with realistic numerical simulations. As an outcome, some interesting features of bora turbulence are revealed. These features crucially depend on the averaging procedure deployed.

Session Number: 11a
Presenting Author: Temple R. Lee
Author Email: trl2y@virginia.edu
Author Affiliation: University of Virginia
Co-Author: Stephan F. J. De Wekker
Co-Author Affiliation: University of Virginia
Abstract Title:
Temperature variations along slope and ridge locations in the Blue Ridge Mountains and their relationship to temperatures in the free atmosphere
The downscaling of weather forecast and climate models in mountainous regions relies on a good understanding of surface temperature variability in space and time and its relationship to temperatures in the adjacent atmosphere. In an effort to improve this understanding for the Blue Ridge Mountains in the eastern United States, we are deploying 60+ temperature and humidity sensors on slope and ridge locations in this region. These measurements are supported by long-term meteorological observations from two nearby mountain ridge sites, Pinnacles and Big Meadows. The two sites are located at about the same elevation but 15 km apart and characterized by different vegetation. Temperatures are measured 2 m agl at Big Meadows and at 2 m, 5 m, 10 m, and 17 m agl at Pinnacles. Free atmospheric temperatures are obtained from radiosonde and reanalysis data.
In this presentation, we will discuss the initial analysis of surface temperatures from Pinnacles and Big Meadows and their comparison with adjacent free atmospheric temperatures. Significant differences in the minimum and maximum surface temperatures are found between the sites with a larger diurnal variability at Pinnacles than at Big Meadows at 2 m agl which cannot be explained by vegetation differences between the two sites. We also find that differences between the surface and free atmospheric temperatures are larger during the day than at night. We will investigate the factors and meteorological processes that explain these differences and discuss how these results can be used to allow for a better assessment of current and projected surface climate conditions at high spatial resolution in the Blue Ridge Mountains of Virginia.

Session Number: 11b
Presenting Author: Lorenzo Giovannini
Author Email: lorenzo.giovannini@ing.unitn.it
Author Affiliation: Atmospheric Physics Group, Department of Civil and Environmental Engineering, University of Trento. Via Mesiano 77, 38123 Trento, Italy.
Co-Author: Dino Zardi, Massimiliano de Franceschi
Co-Author Affiliation: Atmospheric Physics Group, Department of Civil and Environmental Engineering, University of Trento. Via Mesiano 77, 38123 Trento, Italy.
Abstract Title:
Analysis of the urban thermal fingerprint of the city of Trento in the Alps
The temperature contrasts typically marking Urban Heat Island (UHI) effects in the city of Trento, located in an Alpine valley and inhabited in its inner urban area by a population of about 56 000 citizens, are investigated. Time series of air temperature data, collected at an urban weather station, in the city center, and at five extra-urban stations are compared. The latter are representative of rural and suburban areas, both on the valley floor and on the valley sidewalls.
It is found that the extra-urban weather stations, being affected by different local-scale climatic conditions, display different temperature contrasts with the urban site. However the diurnal cycle of the UHI is characterized by similar behaviors at all the extra-urban weather stations: the UHI intensity is stronger at night, while during the central hours of the day an “urban cool island” is likely to occur. The diurnal maximum UHI intensity turns out to be typically of order 3°C, but under particularly favorable conditions it may be higher than 6°C. An urban cool island effect is also detected, which is probably caused by the compactness of the inner urban area, and displays typical diurnal maximum intensities of order 1.5°C.
As to the seasonal dependence, at the extra-urban weather stations on the valley floor the UHI intensity tends to be slightly stronger during dry months, while on the valley sidewalls it is mainly influenced by the seasonal lapse rate changes. Further weather factors, such as wind speed and cloud cover, also affect urbanization effects, making them weaker with stronger winds and cloudier skies.

Session Number: 11c
Presenting Author: Megan Daniels
Author Email: megan.daniels@epfl.ch
Author Affiliation: Ecole Polytechnique Federale de Lausanne
Co-Author: Eric Pardyjak, Wilfried Brutsaert, Daniel Nadeau, Gillermo Barrenetxea, Raphael Mutzner, Marc Parlange
Co-Author Affiliation: Univ. Utah USA, Cornell Univ. USA, EPFL Switzerland
Abstract Title:
Evaporation versus heating: links between soil moisture and surface fluxes in an alpine valley
Soil moisture affects flow in the atmospheric boundary layer through the relative partitioning of energy into the surface sensible and latent heat fluxes. While this effect has been well-studied over flat terrain through both field experiments and simulations, in complex terrain it has previously only been investigated through simulations. In the current study, we use observations to investigate effects of surface soil moisture on the strength and onset of buoyancy-driven slope winds following the diurnal cycle in an alpine valley. In the summers of 2009 and 2010, the Val Ferret catchment ( 20 km2) in southern Switzerland was instrumented with surface weather stations measuring wind speed and direction (2 m), soil moisture, surface skin temperature, air temperature (2 m), humidity (2 m), incoming solar radiation, and precipitation. Results indicate that on calm, cloudless days, increased soil moisture leads to weaker up-slope and up-valley winds. This is corroborated through the calculation of surface sensible and latent heat fluxes using Monin-Obukov similarity theory, which shows that increased soil moisture leads to increased latent heat fluxes and correspondingly decreased sensible heat fluxes at stations within the valley. Calculated fluxes are compared to measured fluxes from an eddy covariance station. The summer 2011 field campaign begins mid-May and will include additional instrumentation to aid in this study.

Session Number: 11e
Presenting Author: Samiro Khodayer
Author Email: samiro.khodayar@kit.edu
Author Affiliation: Institute for Meteorology and Climate Research, Karlsruhe Institute of Technology, Karlsruhe, Germany
Co-Author: G. Schaedler, and N. Kalthoff
Co-Author Affiliation: Institute for Meteorology and Climate Research, Karlsruhe Institute of Technology, Karlsruhe, Germany
Abstract Title:
The impact of soil moisture on seasonal climate simulations of precipitating convection over a complex terrain
In order to gain confidence in statements about the present and future changes in weather and climate, it is necessary to asses how well and with what uncertainties the components of the regional water cycle such as soil moisture, evapotranspiration and precipitation can be modelled, especially in complex orographic areas. The availability of moisture for precipitation is controlled by a number of processes including land surface processes, which in turn are strongly influenced by spatially variable fields of soil moisture (SM). In global, regional and weather forecast models multi-layer soil-vegetation models (SVAT) predict this soil-atmosphere exchange. Therefore, an improved representation of the SM fields in regional model systems can be expected to produce better agreement between modelled and measured surface energy fluxes, boundary layer structure and precipitation. Usually there is a lack of observations which could be used for the validation and improvement of our soil and atmospheric models, especially the SM is currently one of the least assessed quantities with almost no data from operational monitoring networks available. In summer 2007, the field campaign ‘Convective and Orographically-induced Precipitation Study’ (COPS) was performed in south-western Germany and eastern France. During COPS an innovative measurement approach using a very high number of different SM sensors was introduced. Each station was equipped with sensors at three different depths (5, 20 and 50cm) simultaneously measuring SM and soil temperature. In this investigation, a strategy to study the effects of SM, evapotranspiration and water vapour in the PBL on convective precipitation is applied on different scales, from local to regional. The SM measurements together with other PBL observations from the COPS campaign will be used for validation and analysis of the sensitivity of the COSMO-CLM model results, to establish a reference simulation to test several modifications in the initialisation and representation of soil and land surface processes in the model. The impact of model initialization with SM measurements from COPS on convective precipitation will be discussed. Additionally, the impact of SVAT models, coupled on-line with the COSMO-CLM model, and the prescribed soil type distributions on seasonal climate simulations will be investigated. Finally, the impact of local and streamflow soil moisture on convective precipitation and the simulated summer climatology of the COPS’ orographic complex domain will be presented.The combination of dense observations with COSMO-CLM simulations permitted a rigorous analysis of the water transfer process chain from SM and fluxes to convective initiation and pre

Session Number: 12a
Presenting Author: George A. Isaac
Author Email: george.isaac@ec.gc.ca
Author Affiliation: Cloud Physics and Severe Weather Research Section, Environment Canada, 4905 Dufferin Street, Toronto, Ontario, Canada, M3H5T4
Co-Author: George A. Isaac1, Paul Joe1, Jocelyn Mailhot2, Monika Bailey1, Stephane Bélair2, Faisal Boudala1, Mindy Brugman4, Edwin Campos1, Richard Carpenter3, Stewart Cober1, Bertrand Denis4, Chris Doyle4, D. Forsyth5, I. Gultepe 1, Thomas Haiden6, Laura Huang1, Jason Milbrandt2, Ruping Mo4, Roy Rasmussen7, Trevor Smith4, Ron Stewart 8, and Donghai Wang9
Co-Author Affiliation: 1 Cloud Physics and Severe Weather Research Section, Environment Canada, 2 Recherche en prévision numérique, Environment Canada, 3Weather Decision Technologies, Oklahoma, USA, 4 Meteorological Service of Canada, Environment Canada. 5National Severe Storms Laboratory, Norman, Oklahoma, USA, 6Central Institute for Meteorology and Geodynamics (ZAMG), Austria, 7 National Center for Atmospheric Research, Colorado, USA, 8 Department of Environment and Geography, University of Manitoba, 9Chinese Academy of Meteorological Science, China
Abstract Title:
Improving our Ability to Nowcast Winter Weather in Complex Terrain
The field phase of a project entitled the Science of Nowcasting Olympic Weather for Vancouver 2010 (SNOW-V10) was conducted in British Columbia, Canada, during Jan-Mar 2010. SNOW-V10 is a project endorsed by the World Meteorological Organization’s World Weather Research Programme (WWRP). Short term weather forecasting or Nowcasting, which concentrates on 0-6 hr predictions, had been the focus of several WWRP projects associated with the Sydney-2000 and the Beijing-2008 Summer Olympic Games. SNOW–V10 is the first similar project to look at winter weather. It is designed to produce better techniques to nowcast cloud, fog, visibility, precipitation type and amount, and wind and turbulence in mountainous terrain. This was done by using state-of-the-art numerical modeling systems, new on-site surface and remote sensing observing systems, as well as Nowcasting systems which blend observations and model predictions into improved short term forecasts. Results of the field measurements are now available. This paper will discuss some of the measurement difficulties, some interesting physical processes that were documented, and introduce some of the Nowcast products developed and tested. It will also present some statistics on the accuracy of forecasting variables like visibility, winds and wind gusts, precipitation rate and type in complex terrain.

Session Number: 12b
Presenting Author: Georg Pistotnik
Author Email: georg.pistotnik@zamg.ac.at
Author Affiliation: Central Institute for Meteorology and Geodynamics (ZAMG)
Hohe Warte 38, 1190 Vienna, Austria
Co-Author: Stefan Schneider
Co-Author Affiliation: ZAMG
Abstract Title:
Including a life cycle of convective cells in precipitation nowcasting
Especially during convective situations, “conventional” nowcasting of precipitation (i.e., an extrapolation of the latest analysis into the near future) regularly reaches its limits due to its inabilities to account for initiation, intensification and weakening of convective cells. Within the framework of the high-resolution analysis and nowcasting system INCA (“Integrated Nowcasting through Comprehensive Analysis”) operated at ZAMG, it has been investigated whether the inclusion of information from convective analysis fields like Convective Available Potential Energy (CAPE) and Moisture Flux Convergence (MOCON) can improve the quality of nowcasting. For this purpose, a “convective nowcasting” algorithm has been developed and run in a testbed environment for a selected set of convection days in summer 2009 and 2010.
The INCA system provides analyses and short-term forecasts of numerous parameters like temperature, humidity, precipitation, wind and various convective indices. Operationally, the background fields for the 3-dimensional analyses (temperature, humidity and wind) are derived from forecasts of ALADIN numerical model and corrected by latest surface observations. Previous investigations have shown that the proper detection of areas of pronounced MOCON had been the limiting factor in developing a reliable “convective nowcasting” algorithm, since they are often neither fully resolved by the observational dataset nor by the numerical model background information.
For the current study, ALADIN model (with a horizontal resolution of 9.6 kilometres) has been replaced by AROME model (with a horizontal resolution of 2.5 kilometres) for providing the background information for the INCA analysis fields. While the AROME precipitation forecasts still suffer some problems also known from other high-resolution numerical models (like too early and too widespread initiation of convection), its forecasts of the wind field can – at least in pre-convective situations – be considered fairly skilful. In particular, the model can resolve individual mountain chains and valleys and therefore better represent diurnal up-valley and upslope winds. These local wind systems act as the main trigger for convection in Alpine terrain and make it exhibit a “semi-deterministic” appearance: Places of occurrence and especially places of initiation are regularly tied to orographic features.

In this presentation, case studies and verification series of the convective nowcasting algorithm with AROME background information are shown and are compared to conventional nowcasting as well as to a pilot study of convective nowcasting with ALADIN background information, which was conducted a few years ago. Furthermore, some general considerations on the predictability of convective initiation and sustenance are addressed.

Session Number: 12c
Presenting Author: Hálfdán Ágústsson
Author Email: halfdana@hi.is
Author Affiliation: University of Iceland, Icelandic Meteorological Office and Institute for Meteorological Research
Co-Author Affiliation:
Abstract Title:
High-resolution simulations of mountain weather improved with observations from small unmanned aircraft
Novel observations aloft with the SUMO (small unmanned aircraft) are used to improve high resolution atmospheric simulations of weather in complex terrain in Iceland. During the summer of 2009 winds aloft at several locations in Southwest-Iceland were observed with the SUMO as a part of the international MOSO-project. For the case of 15 July 2009 presented here, high-resolution atmospheric simulations, observations of weather at the surface and aloft with the SUMO are used to explore orographic winds at Mt. Esja (900 m) in Southwest-Iceland, near Reykjavík. The observed winds at mountain top level were weak from the northeast and near to perpendicular to the mountain ridge. Nevertheless, the observed low-level winds in the lee were strong, gusty and oriented down the mountain slopes, with weaker winds further aloft and near mountain top level. The observed wind maximum does not reach the surface thus making only surface based observations inadequate in describing this situation correctly. In spite of the relatively weak winds at mountain top level the observations from the SUMO indicate gravity wave activity aloft. Unless the atmospheric model is forced with the observations of the vertical profiles of wind and temperature from the SUMO, and in spite of horizontal resolutions down to 500 m, the model fails to reproduce the observed winds aloft as well as at the surface in a relatively large region in the lee of the mountain. When nudged with the SUMO-observations, the atmospheric model reproduces a localized weak gravity-wave induced downslope flow with a maximum in wind speed at approx. 100 m in the lee of the mountain. This wind maximum is weaker and more diffuse a short distance further downstream. Other cases from the MOSO-project have used observations from the SUMO to improve simulations of a sea-breeze situation. With the system developed, in-situ observations from the SUMO may be feed in realtime via a 3G-link into an online database from which they may be ingested into the AR-WRF model for improved operational forecasting of local weather, e.g. in complex terrain.

Session Number: 13a
Presenting Author: Thomas Haiden
Author Email: thomas.haiden@ecmwf.int
Author Affiliation: ECMWF, Shinfield Park, Reading, UK
Abstract Title:
Analysis of global model precipitation forecast skill in mountain areas
Mountains both complicate and simplify the task of forecasting precipitation. They create additional small-scale variability in the precipitation field, which makes forecasting more challenging, but part of the variability takes the form of recurring spatial patterns, adding determinism to the process. This study presents an inter-model, inter-continent analysis of precipitation forecast skill in mountain areas versus lowland areas. Deterministic forecasts of 24-h precipitation totals from four global numerical weather prediction (NWP) models are verified against standard SYNOP observations disseminated via the Global Telecommunication System (GTS). Classical scores like the Equitable Threat Score (ETS), and the True Skill Score (TSS), as well as the new Stable Equitable Error in Probability Space (SEEPS) score are evaluated for lead times of 1 to 6 days. The SEEPS score is based on a 3x3 contingency table with thresholds depending on the climatological precipitation distribution. It allows direct comparison of climatologically diverse areas. The relative importance of systematic and non-systematic errors in mountain and lowland areas is investigated. Seasonal and latitudinal variations of forecast skill differences between mountain and lowland areas are discussed with regard to predominant precipitation processes. The significance of the differences is assessed using confidence intervals derived from a re-sampling method.

Session Number: 13b
Presenting Author: Theresa Gorgas
Author Email: theresa.gorgas@univie.ac.at
Author Affiliation: Department of Meteorology and Geophysics
University of Vienna
Althanstr 14, 1090 Vienna, Austria
Co-Author: Manfred Dorninger
Co-Author Affiliation: Department of Meteorology and Geophysics, University of Vienna
Abstract Title:
Quantifying verification measure uncertainty due to reference data selection
With the advent of high resolution non-hydrostatic NMP models the field of forecast verification has got an increased attention during the last years. New methods have been developed and the question of uncertainty estimates for verification measures has been raised. In this study a set of reference data (‘truth’) from different sources is used to investigate the influence of these data on the verification measures in a comparative way.
Three pairs of NWP-model runs generated during the WWRP-FDP-MAP D-PHASE in summer 2007 are evaluated for selected case studies These are: COSMO-7 and COSMO-2, COSMO-EU (former LME) and COSMO-DE (former LMK), CMC-GEML and CMC-GEMH. The reference data are provided by three different interpolation methodologies: VERA (Vienna Enhanced Resolution Analysis), Ordinary Kriging and an adapted Barnes interpolation scheme. Operated for horizontal grid resolutions of 4, 8 16 and 32km, and for 2 different observation networks (JDC – Joint D-PHASE and COPS and GTS networks) these analysis methodologies provide a poor man’s ensemble of ‘model independent’ analysis schemes, i.e. they do not require any other information than the observational data for interpolation.
The range of verification results spanned by the basic variation of analyses is compared with alternative approaches to quantify verification uncertainty: Bootstrapping, as a quite common re-sampling methodology, and two ensemble approaches, Sequential Gaussian Simulation and an analysis ensemble based on the perturbation of observations.
A number of basic verification statistics is applied on the matched forecast-analysis fields themselves as well as on scale-reduced field representations provided by 2-dimensional wavelet transforms. The latter shall give an insight in which scales are most affected by changes in verification data.
The presentation will take up and discuss following issues: The influence of verification data with different resolutions based on different sources of observation data on verification statistics, and their impact on different spatial scales. 

Session Number: 13c
Presenting Author: Sarah Umdasch
Author Email: sarah.umdasch@univie.ac.at
Author Affiliation: Department of Meteorology and Geophysics
University of Vienna, Althanstr 14, 1090 Vienna, Austria
Co-Author: Stefan Kiesenhofer, Theresa Gorgas, Manfred Dorninger
Co-Author Affiliation: Department of Meteorology and Geophysics University of Vienna, Althanstr 14, 1090 Vienna, Austria
Abstract Title:
An intercomparison of high resolution NWP models during MAP D-PHASE
In this study forecast performances of NWP models from different weather services during the WWRP projects COPS and D-PHASE in summer and autumn 2007 are evaluated and compared in terms of surface parameters (precipitation, wind, pressure, (equivalent) potential temperature). Special attention is paid to high resolution models and model chains and their performance over complex terrain. Reference data are provided by the NWP-model independent analysis scheme VERA.
Methodologically focus is on summarizing measures that can be used to directly compare various models. For that purpose, besides standard verification measures, skill scores which show the improvement over an independent reference forecast, like a long-term climatology (MESOCLIM = 35 year mesoscale climatology over the larger Alpine Region) or persistence (VERA), are employed. Additionally the different models’ abilities to reproduce precipitation objects are analyzed by means of the Method for Object-Based Diagnostic Evaluation (MODE) which is included in MET (Model Evaluation Tool). Convection-permitting models are verified as well as their coarser counterparts that include a parameterization scheme for convection.
Results of the model inter-comparison are presented on the one hand over the largest possible spatial domain over Europe that is covered by all used models, on the other hand over a small domain of complex terrain which was issue of the COPS project.

Session Number: 14a
Presenting Author: Kirsty Hanley
Author Email: k.e.hanley@reading.ac.uk
Author Affiliation: Department of Meteorology
University Of Reading
Earley Gate
PO Box 243
Reading, RG6 6BB.
Co-Author: Daniel Kirshbaum, Nigel Roberts, Giovanni Leoncini
Co-Author Affiliation: University Of Reading, Met Office, Met Office
Abstract Title:
Predictability of convection over orography: high-resolution ensemble forecasts from the Unified Model
Accurate forecasting of convective precipitation over complex terrain is important because of the substantial flooding that can be caused by the intense precipitation. In principle, the predictability of convection should be increased over orography, where localized vertical motions owing to specific topographic features can remove the convective inhibition and trigger cells. At present however, the forecasting skill for heavy convective showers and thunderstorms over orography is low. This is partly due to poor understanding of the thermally-driven orographic flow of moisture and aerosols which feed the storm. Additional uncertainties arise from parameterised sub-gridscale processes within the model as many of the small-scale features that initiate convection are either completely unresolved (e.g. boundary layer thermals) or highly uncertain (e.g. surface fluxes), even in high resolution models.
The availability of data collected during the Convective and Orographically-induced Precipitation Study (COPS) provides a unique opportunity to study the role of orography in determining the predictability of convection. Accurate prediction of the location of convective initiation requires high-resolution grids which can represent the fine-scale terrain details. In this study, we conduct high-resolution simulations using the UK Met Office Unified Model over the COPS region to quantify the predictability of severe convective storms encountered during COPS. An ensemble approach is taken using initial and boundary data from the Met Office Global and Regional Ensemble Prediction System (MOGREPS). Results are presented for IOP 9c (20th July 2007) where convection was triggered by a squall line produced in the outflow boundary of a weak Mesoscale Convective System (MCS). In this case, the high resolution ensemble is shown to be sensitive to errors on the low-resolution grid, which propagate in through the lateral boundaries. Ensemble sensitivity analysis is used to link the mesoscale response of the high-resolution ensemble members to the differences in the large-scale driving conditions.

Session Number: 14b
Presenting Author: Stuart Webster
Author Email: stuart.webster@metoffice.gov.uk
Author Affiliation: Met Office, Fitzroy Road, Exeter, EX1 3PB, United Kingdom
Co-Author: Simon Vosper, Mark Gallani and Clive Wilson
Co-Author Affiliation: Met Office
Abstract Title:
High-resolution wind resource modelling in complex terrain using the Met Office Unified Model.
The Met Office Unified Model (MetUM) is used operationally for both global and limited area numerical weather prediction. Forecasts which cover the United Kingdom are now run at horizontal resolutions of 25 km (the global model) and at 12 km, 4 km and 1.5 km (the limited area models). Furthermore, in research mode, recent improvements to the MetUM physics have enabled realistic simulations at sub-one-kilometre horizontal resolution to be performed.
The capability outlined above has enabled high resolution MetUM predictions of wind resource in complex terrain to be performed. Simulations of a week and longer have been run at horizontal resolutions as fine as 100 m. The boundary conditions for these limited area simulations are generated by multiply nesting the 100 m model within progressively coarser resolution limited area models
which are themselves nested within global model model simulations initialised using operational global analyses. This suite of nested models is therefore capable of making wind resource predictions for any area of the world.
In this presentation model predictions for a particular site, including verification of the predictions against wind mast observations, will be described and illustrated. Possible ways in which these relatively short model timeseries could be used to improve the current multi-year wind resource predictions provided by the Met Office to external customers will also be discussed.

Session Number: 14c
Presenting Author: Saša Gaberšek
Author Email: sasa.gabersek@nrlmry.navy.mil
Author Affiliation: NRL Monterey
Co-Author: James D, Doyle, Francis X. Giraldo
Co-Author Affiliation: NRL Monterey, NPS Monterey
Abstract Title:
Simulations of idealized mesoscale phenomena with a 2D spectral element model
Highly accurate numerical methods for solving partial differential equations that have been traditionally used in the computational fluid dynamics are being considered as a tool to study the mesoscale atmospheric phenomena as well. We are presenting results obtained with a two-dimensional, fully compressible, non-hydrostatic spectral element (Continuous Galerkin) model. Its accuracy is determined by the polynomial order (p) of basis functions and number of elements (h) into which the original domain is subdivided.
The computational domain is divided into equally sized elements (h in horizontal direction) and within each element the solution is expressed as a sum of basis functions (pth order Lagrange polynomials), using unevenly spaced nodal (Legendre-Gauss-Lobatto) points. By varying number of elements (h) from 6 to 120 and polynomial orders (p) from 4 to 10, the hp parameter space is mapped out, resulting in 91 sets of parameters.
For each set of parameters, we perform two types of idealized simulations: 1) a dry, linear, hydrostatic mountain wave and 2) a moist squall line. For the mountain wave, the results are assessed by comparing the solution of the momentum flux to the analytic solution. Results obtained by a finite difference model are also compared for assessment of computational efficiency. The method, experimental setup, error statistics, speed of convergence to the steady–state solution and spectral (hp) convergence will be presented. For the squall line with no analytic solution, we present integrated quantities (e.g. amount of rainfall) and compare solutions obtained with different values of parameters in the hp parameter space.

Session Number: 14d
Presenting Author: Günther Zängl
Author Email: guenther.zaengl@dwd.de
Author Affiliation: Deutscher Wetterdienst
Frankfurter Strasse 135
D-63067 Offenbach
Abstract Title:
Simulation of airflow over steep mountains: pushing the limits of terrain-following coordinates
With few exceptions, present day's mesoscale and global numerical models use terrain-following coordinates, which means that the coordinate surfaces may be strongly distorted over steep topography. This reduces the numerical accuracy of the dynamical core and transport algorithms, and tends to induce spurious circulations in weak-wind situations, e.g. due to pressure-gradient errors. At some vertical distance from the ground, these problems may be alleviated by choosing a coordinate specification in which the topography signal decays rapidly with height (e.g. the so-called SLEVE coordinate). However, at low levels, the limitations of terrain-following coordinates are more difficult to circumvent. One important limitation applies to the numerical stability of the dynamcial core, which is usually restricted to slope angles of about 45 degrees.
The word presented here has been conducted within the ICON project (ICOsahedral Nonhydrostatic model), in which the Deutscher Wetterdienst (DWD) and the Max-Planck-Institute for Meteorology (MPI-M) are jointly developing a new nonhydrostatic global model with a two-way nesting capability ad a limited-area mode. Since ICON is supposed to be usable over a wide range of scales, an effort has been made to extend the numerical stability limit over steep topography as far as possible. One important component is a pressure-gradient discretization that operates quasi-horizontally rather than along terrain-following coordinate surfaces, similar to what had already been proposed by Mahrer in the 1980\'s, but refined with higher-order corrections to improve the discretization accuracy for non-equidistant model layers. Moreover, a special hydrostatic approximation is used at the ground over steep slopes in order to circumvent downward extrapolation of vertical gradients evaluated over very thin layers. Idealized tests indicate that this allows for shifting the numerical stability limit to slope angles around 65 degrees, even though tests with an initially resting atmosphere indicate that one should not be too confident into the model results for slope angles larger than 56 or 57 degrees.

Session Number: 14e
Presenting Author: Craig Smith
Author Email: craig.smith@epfl.ch
Author Affiliation: EPFL
Co-Author: Fernando Porte-Agel
Co-Author Affiliation: EPFL
Abstract Title:
On subgrid model choice for eddy resolving simulations of katabatic flows
Recent increases in computing power have enabled simulation of previously unresolvable atmospheric phenomena, such as katabatic flows, in numerical weather prediction (NWP) models. Verification of forecasts of drainage flows, however, is still lacking. Herein we present findings from a series of numerical experiments designed to test the ability of large eddy simulation (LES) models to resolve and reproduce cold air drainage flows. Comparisons are made with historical drainage flow data taken on a simple slope is Southeast Washington, USA, and different subgrid models are tested. Preliminary findings suggest that the accuracy of the Smagorinksy model becomes suspect when the grid aspect ratio, defined as horizontal grid spacing divided by vertical grid spacing, becomes large. In particular the Smagorinsky models tends to over predict the downslope mass flux of cold air, which could lead to erroneous forecasts of cold air pool formation. The dynamic Smagorinsky and Lagrangrian scale dependent dynamic model perform better at moderate resolution and offer a forecast which does not necessitate tuning of the model coefficient and Prandtl number that the Smagorinsky requires.

Session Number: 15a
Presenting Author: Bruce Ingleby
Author Email: bruce.ingleby@metoffice.gov.uk
Author Affiliation: Met Office, Fitzroy Road, Exeter, EX4 8EB
Abstract Title:
Assimilation of surface data in mountainous areas
Assimilation of air temperature, humidity, wind and pressure from surface stations requires careful processing, monitoring and quality control. Adjustment for the difference between station height and model surface height is a major part of the processing, especially in mountainous areas. Pressure and temperature have long been adjusted using a standard lapse rate - a synoptic dependent lapse rate is now being investigated. Height adjustment for humidity and (for stations above model height)
wind speed have been introduced into the Met Office regional and global data assimilation systems. Exclusion of data with poor quality or that is unrepresentative of model scales is also important. The comparison of short-range forecasts with observations provides valuable information on errors. Temperature differences tend to be largest in wintry conditions - probably reflecting large errors in both the forecasts and observations. Currently the Met Office model tends to melt snow a few weeks early, and associated with this the model near-surface humidity becomes too high. The low level model humidity is slightly high anyway, due to precipitation being too large
especially just after the analysis - this is ameliorated by a forthcoming change.
Near surface air temperature, humidity and wind were assimilated in the Met Office global model from April 2008 and were found to have a positive impact on short range forecasts. The temperature and humidity had a clear benefit, the wind data showed much less benefit (consistent with previous work with the Met Office local forecasting system). Use of these variables is possible given ongoing improvements in model resolution, boundary layer and surface modelling. Screen temperature and humidity are also used in the update of model soil moisture fields which has benefits for both short and medium range forecasts.

Session Number: 15b
Presenting Author: Olivier Caumont
Author Email: olivier.caumont@meteo.fr
Author Affiliation: CNRM-GAME (Météo-France, CNRS), Toulouse, France
Co-Author: Anouck Foray (1), Lucas Besson (2), Chiraz Boudjabi (2), Jacques Parent du Châtelet (2)
Co-Author Affiliation: (1) National School of Meteorology (Météo-France), Toulouse, France, (2) Observing Systems Department (Météo-France), Trappes, France
Abstract Title:
Towards the assimilation of near-surface refractivity measurements from weather radars: meteorological value, data quality, and monitoring
Weather radars were originally designed to detect and quantify precipitation. In the late 90's, a technique to measure radio refractivity with these radars has been devised. This new technique takes advantage of ground clutter returns to estimate the propagation time of radio waves from the radar to well-defined ground echoes. It potentially provides information about pressure, temperature, and humidity in the lower layers of the atmosphere within a few tens of kilometers around the radar. This technique is thus of great meteorological interest, both for numerical weather prediction and process studies. The method, originally designed for klystrons, has been recently adapted to magnetrons, which equip most European radars. This recent work therefore paves the way to a use at the regional scale, and motivates studying the potential of the assimilation of weather radar refractivity data.
The presentation will cover different aspects related to the assimilation of these data in a broad sense. First, the usefulness of this new observation type will be demonstrated by highlighting the close relationships between near-surface refractivity, on the one hand, and temperature and humidity, on the other hand. These features will be shown for a high-resolution Meso-NH simulation of a high precipitation event that occurred in a mountainous area located in southeastern France.
Measuring refractivity with weather radars comes with specific issues. As for radial velocities, refractivity is estimated through phase measurements. This leads to ambiguities in the retrieved refractivity changes. This issue is also referred to as aliasing or folding. We will demonstrate how aliasing occurrence rates can be inferred from the Meso-NH simulation mentioned above and a multi-year high-resolution dataset from a station located in northwestern France, respectively. Aliasing dependence on seasonal and diurnal cycles will also be shown.
Further specific work is required to assimilate weather radar refractivity data. In particular, it is necessary to estimate measurement errors and dismiss spurious data. Results from a specific study on that issue will be presented. The assimilation of refractivity data also necessitates an observation operator. Such an obervation operator has been developed to simulate refractivity measurements from Arome, the French high-resolution limited-area numerical weather prediction system. Long-term comparisons between radar measurements, in-situ data, and 3-h Arome analyses show meaningful consistency. Finally, we will report on sensitivity experiments aimed at assessing the impact of simulated beam altitude on simulated refractivity change.

Session Number: 15c
Presenting Author: Tim Appelhans
Author Email: tim.appelhans@uni-bayreuth.de
Author Affiliation: University of Bayreuth, Climatology, Nuernberger Strasse 38, 95440 Bayreuth, Germany
Co-Author: Andreas Hemp, Thomas Nauss, Joerg Bendix
Co-Author Affiliation: University of Bayreuth, University of Bayreuth, Philipps -University Marburg
Abstract Title:
Climate dynamics of the Kilimanjaro region: A field measurement campaign to investigate climatological drivers of a tropical montane ecosystem.
Within the framework of the research unit “Kilimanjaro ecosystems under global change: Linking biodiversity, biotic interactions and biogeochemical ecosystem processes (KiLi)“, funded by the German Research Foundation (DFG), the project „Climate dynamics of the Kilimanjaro region“ aims to investigate and identify climatic processes and their role within the earth-atmosphere system in the area across a range of scales. The two main goals of this project are (i) the provision of high-resolution hourly meteorological datasets for the project period, and (ii) the generation of time series for cloud, fog and rainfall dynamics from 1979 until today. Additionally, meso-scale influences, such as potential regional land-cover (change) feedbacks, and climatological drivers, e.g. Indian-Ocean-Dipole dynamics will be investigated.
The paper presented here focuses on the first goal and gives a preliminary overview of the field measurement campaign. A brief summary of the intended observation network is given below:
Fieldwork commences in Feb 2011. Five automated weather stations (AWS) collecting observations of precipitation, fog water input, temperature, relative humidity, atmospheric pressure, wind speed and direction, radiation balance and soil temperature will be installed on the southern slopes of Kilimanjaro along an altitudinal gradient between approximately 1000 m asl and 4700 m asl. In addition, 60 micro-loggers that measure temperature and relative humidity will be distributed on the (biodiversity) sampling plots following both an altitudinal and a land-use gradient. A further 12 rain gauges, 8 of which will be accompanied by fog collectors, will be set up to try and disentangle their respective contributions to atmospheric moisture input, especially within the tropical montane forest belt. The spatial extent of the network will be limited to the southern slopes of the mountain. This is because of the close integration with other subprojects of the research unit, which follow a rather localised approach. However, an attempt to capture in situ information on meso-scale dynamics is made through installation of two AWS in the lowlands east and west of the mountain at similar altitudes that are separated by approximately 50 km.

Session Number: 15d
Presenting Author: Heidi Escher-Vetter
Author Email: Heidi.Escher@kfg.badw.de
Author Affiliation: Commission for Geodesy and Glaciology, Bavarian Academy of Sciences and Humanities
Co-Author: Matthias Siebers
Co-Author Affiliation: Commission for Geodesy and Glaciology, Bavarian Academy of Sciences and Humanities
Abstract Title:
Statistical analysis of air temperature, precipitation and discharge records since 1974 from a high Alpine Site (Pegelstation Vernagtbach, Oetztal Alps, Austria)
Meteorological and hydrological parameters have been recorded since 1974 at the site of the Pegelstation Vernagtbach, Oetztal, Austria. This gauging station monitors 11.44 km² of the Vernagtferner basin, which ranges from 2640 m a.s.l. to 3633 m a.s.l. and is approximately to 70% glacierized. In this study, we analyse the monthly sums of precipitation and averages of air temperature and discharge for the summer months (May to October) of the years 1974 to 2010 in order to identify trends caused by climate change in this high alpine basin. Special emphasis is laid on the identification of precipitation type changes. To this purpose, the number of days with snowfall and snowfall amounts are determined on the basis of air temperature, wet bulb temperature and daily photographs of the basin during the summer months. Thereafter, the relative share of snowfall from total precipitation amount as well as the number of snowfall days in respect to all days with precipitation is presented.
The main findings are: (1) The monthly means of air temperature for all summer months (excepting that of September) show positive trends, but the slopes are different for the individual months and highest for June (0.1 K/a). (2) Discharge increases in all months except October, where it remains practically constant. Similar to air temperature, the largest rise is found for June (9.5 mm/a). (3) The proportion of snowfall to the total amounts of precipitation decreases for May, June and October, but increases for July, August and September, with the most negative trend in June (-0.01%/a), the most positive for September (0.01 %/a). The ratio of snowfall days to all precipitation days decreases in May (-0.74%/a), June (-0.66%/a) and October (-0.63%/a), but increases in July and August. The rise is most pronounced in September, where the linear trend increases from 28% in 1975 to 65% in 2010. (4) For air temperature, the smallest scatter is determined in June (R²=0.38) and for discharge in May (R²= 0.43) and June (R²=0.42). The scatter is much larger for all analysed precipitation records, with the highest correlation determined for the trend of the snowfall fraction to total precipitation in June (R²= 0.16).

To sum up: Whereas air temperature averages show a positive trend for all but one month, the analysis of precipitation characteristics reveals a more ambiguous picture. The dominant increase in June indicates a seasonal shift of the annual discharge pattern towards spring.