Diurnal Valley Winds in a Deep Alpine Valley: Model Results

Research Square (Research Square), 2023

The advancement of computational resources has allowed researchers to conduct convection-permitting regional climate model (CPRCM) simulations. A pioneering effort promoting a multimodel ensemble of such simulations is the CORDEX Flagship Pilot Studies (FPS) on "Convective Phenomena over Europe and the Mediterranean" over an extended Alps region. In this study, the Distribution Added Value metric is used to determine the improvement of the representation of all available FPS hindcast simulations for the daily mean wind speed. The analysis is performed on normalized empirical probability distributions and considers station observation data as the reference. The use of a normalized metric allows for spatial comparison among the different regions (coast and inland), altitudes and seasons. This approach permits a direct assessment of the added value between the CPRCM simulations against their global driving reanalysis (ERA-Interim) and respective coarser resolution regional model counterparts. In general, the results show that CPRCMs add value to their global driving reanalysis or forcing regional model, due to better-resolved topography or through better representation of ocean-land contrasts. However, the nature and magnitude of the improvement in the wind speed representation vary depending on the model, the season, the altitude, or the region. Among seasons, the improvement is usually larger in summer than winter. CPRCMs generally display gains at low and medium range altitudes. In addition, despite some shortcomings in comparison to ERA-Interim, which can be attributed to the assimilation of wind observations on the coast, the CPRCMs outperform the coarser regional climate models, both along the coast and inland.

Monthly Weather Review, 2016

A comparison between anelastic and compressible convection-permitting weather forecasts for the Alpine region is presented. This involves mesoscale simulation of a typical westerly flow accompanied by a passage of frontal systems as well as intense airmass convection and orographic convection. The limited-area model employing a 2.2-km horizontal grid length is driven by time-dependent boundary conditions from a coarse-resolution model. The results obtained with the anelastic and the compressible model versions show good agreement. Validations of the 10-m wind, 2-m temperature, 2-m dewpoint temperature, total cloud cover, and surface precipitation against observations for a seven-member forecast ensemble reveal only minor differences between the two configurations. The sensitivity study demonstrates only a small impact of realistic pressure perturbations (about a reference profile) on the solutions. Overall, anelastic approximation proves remarkably accurate in simulating moist mesos...

Natural Hazards, 2007

An innovative methodology aimed at establishing a numerical model-based high-resolution climatology of extreme winds over Switzerland is described, that makes use of the Canadian Regional Climate Model where a new windgust parameterization has been implemented. Self-nesting procedures allow windstorms to be studied at resolution as high as 2-km. The analysis of ten major windstorms concludes that the average spatial pattern and magnitude of the simulated windspeeds are well captured, and the areas that experienced extreme winds correspond well with observations and to the location where forest damage was reported following the last two of these storms. This climatology would eventually serve to form risk assessment maps based on the exceedance of windspeed thresholds. There is, however, a need for further investigations to encompass the full range of potential extreme wind cases. The ultimate goal of this methodology is to assess the change in the behaviour of extreme winds for a climate forced by enhanced greenhouse gas concentrations, and the impact of future windstorms over the Alpine region at high resolution.

The purpose of this work is the analysis of the capabilities of the Limited Area Model COSMO-CLM in simulating the main features of the observed climate over an area characterized by complex orography. Two sets of simulations of the XX century climate have been carried out, at a spatial resolution of 14 km, in order to provide a detailed description of the climate variability on local scale, useful for impact studies. A first experimental set-up consists of a set of simulations driven by the boundary conditions provided by the global model SINTEX-G. Our aim is to test the capability of this experimental set in realistically reproducing the main characteristics of the Alpine region climate under present climate conditions in order to provide a reliable tool for future climate scenario investigations. A second set of simulations has been carried out, driven by ERA40 reanalysis, in order to test the influence of the global model boundary conditions. Results are shown in terms of 2-m temperature and total precipitation and they are compared with two observational datasets: CRU and ARPA Piedmont. A good capability of the model in reproducing both the mean field and the seasonal cycle of temperature is observed, while the precipitation is affected by accuracy problems, related to the resolution. Though the model resolution is fine enough to reproduce fairly well the intensity of winter precipitation, it often fails in its localization. As far as summer precipitation is concerned, the model resolution appears too coarse to simulate the localized convection, which characterizes summer precipitation over Alps.

Theoretical and Applied Climatology, 2012

This study reports on the ability of the Canadian Regional Climate Model to simulate the surface wind gusts of 24 severe mid-latitude storms in Switzerland during the period 1990-2010. A multiple self-nesting approach is used, reaching a final 2-km grid which is centred over Switzerland, a country characterised by complex topography. A physicallybased wind gust parameterization scheme is applied to simulate local surface gusts. Model performance is evaluated by comparing simulated wind speeds to time series at weather stations. While a number of simulated variables are reproduced in a realistic manner, the surface wind gusts show differences when compared to observed values. Results indicate that the performance of this parameterization scheme not only depends on the accuracy of the simulated planetary boundary layer, the vertical temperature, wind speed and atmospheric humidity profiles, but also on the accuracy of the reproduction of the surface fields such as temperature and moisture.

For generating highly resolved wind fields in the Alpine region the presented work focuses on the evaluation of a hybrid dynamicdiagnostic downscaling procedure. The diagnostic model CALMET is driven by the dynamic model MM5, which is nested into ECMWF's re-analysis ERA-40. Near surface winds are downscaled via multiple nesting from ~120 km to 200 m grid spacing without ingestion of any further observational data for the test period between 7 September to 15 November 1999 within a mountainous study area (140 km x 70 km) located in the eastern Alps (Hohe Tauern). Two MM5 grid spacings for driving the diagnostic model were evaluated, 5 km and 10 km, respectively. Detailed error statistics based on observations from surface stations show drastic improvements of modelled air-flows compared to the grossly deviating driving data (ERA-40). The overall bias relative to the station-averaged observed mean wind speed of 5.8 m/s is systematically reduced from -4.1 m/s to -0.7 m/s. In general, low wind speeds are slightly overestimated, while higher wind speeds are increasingly underestimated. A reduction of the finest horizontal grid resolution of the dynamic model from 5 km to 10 km, which would be computationally favourable, induces additional errors with respect to unresolved air flows, which the diagnostic model is unable to correct in most cases. Important wind climatologic characteristics (e.g., bimodal frequency distributions) disappear, which accentuates the importance of high-quality, high-resolution initial wind fields for diagnostic models operating in complex terrains.

Hydrological Processes, 2009

Wind-induced snow transport has remarkable effects on the snow cover spatial variability and on the temporal dynamics of snowmelt runoff. For accurate snow cover modelling, valid atmospheric forcing fields are essential. Since it is impossible to generate appropriate wind fields by a simple spatial interpolation of station data, a new approach was developed: A modified version of the Penn State University-National Centre for Atmospheric Research (MM5) model was used to generate wind fields with 200-m resolution. Because of the high computational costs of MM5, it was not practicable to include the wind field generation as an operational part of the snow cover modelling. Therefore, an archive consisting of 220 wind fields was generated prior to the simulation of the snow transport processes. These fields represent the most relevant synoptic situations for wind-induced snow transport occurring at our test site. The criteria to generate which wind field to use in a specific snow model time step are mean wind speed and direction at the 700 hPa level derived from German Weather Service/Deutscher Wetterdienst (DWD) Lokalmodell (LM) analysis data. The wind field library provided physically derived wind speed and direction fields that were used to drive a snow transport model (SnowTran-3D) in a high Alpine area in the Berchtesgaden National Park, Germany. In complex Alpine terrain, the procedure provides an alternative to simple interpolation methods, yielding improved physical realism at reasonable computational expense.

Sustainability, 2023

An OpenFOAM computational fluid dynamics model setup is proposed for simulating thermally driven winds in mountain–valley systems. As a first step, the choice of Reynolds Averaged Navier–Stokes k−ε turbulence model is validated on a 3D geometry by comparing its results vs. large-eddy simulations reported in the literature. Then, a numerical model of an idealised 2D mountain–valley system with mountain slope angle of 20° is developed to simulate thermally driven winds. A couple of top surface boundary conditions (BC) and various combinations of temperature initial conditions (IC) are tested. A transient solver for buoyant, turbulent flow of incompressible fluids is used. Contrary to classical approaches where buoyancy is set as a variable of the problem, here temperature linearly dependent with altitude is imposed as BC on the slope and successfully leads to thermally driven wind generation. The minimum fluid domain height needed to properly simulate the thermally driven winds and the effects of the different setups on the results are discussed. Slip wall BC on the top surface of the fluid domain and uniform temperature IC are found to be the most adequate choices. Finally, valleys with different widths are simulated to see how the mountain–valley geometry affects the flow behaviour, both for anabatic (daytime, up-slope) and katabatic (nighttime, down-slope) winds. The simulations correctly reproduce the acceleration and deceleration of the flow along the slope. Increasing the valley width does not significantly affect the magnitude of the thermally driven wind but does produce a displacement of the generated convective cell.

Journal of Hydrometeorology, 2010

The inhomogeneous snow distribution found in alpine terrain is the result of wind and precipitation interacting with the snow surface. During major snowfall events, preferential deposition of snow and transport of previously deposited snow often takes place simultaneously. Both processes, however, are driven by the local wind field, which is influenced by the local topography. In this study, the meteorological model Advanced Regional Prediction System (ARPS) was used to compute mean flow fields of 50-m, 25-m-, 10-m-, and 5-m grid spacing to investigate snow deposition patterns resulting from two snowfall events on a mountain ridge in the Swiss Alps. Only the initial adaptation of the flow field to the topography is calculated with artificial boundary conditions. The flow fields then drive the snow deposition and transport module of Alpine3D, a model of mountain surface processes. The authors compare the simulations with partly new measurements of snow deposition on the Gaudergrat ridge. On the basis of these four grid resolutions, it was possible to investigate the effects of numerical resolution in the calculation of wind fields and in the calculation of the associated snow deposition. The most realistic wind field and deposition patterns were obtained with the highest resolution of 5 m. These high-resolution simulations confirm the earlier hypothesis that preferential deposition is active at the ridge scale and true redistribution-mainly via saltation-forms smaller-scale deposition patterns, such as dunes and cornices.

Meteorologische Zeitschrift - METEOROL Z, 2007

For generating highly resolved wind fields in the Alpine region the presented work focuses on the evaluation of a hybrid dynamic- diagnostic downscaling procedure. The diagnostic model CALMET is driven by the dynamic model MM5, which is nested into ECMWF's re-analysis ERA-40. Near surface winds are downscaled via multiple nesting from ~120 km to 200 m grid spacing without ingestion of any further observational data for the test period between 7 September to 15 November 1999 within a mountainous study area (140 km x 70 km) located in the eastern Alps (Hohe Tauern). Two MM5 grid spacings for driving the diagnostic model were evaluated, 5 km and 10 km, respectively. Detailed error statistics based on observations from surface stations show drastic improvements of modelled air-flows compared to the grossly deviating driving data (ERA-40). The overall bias relative to the station-averaged observed mean wind speed of 5.8 m/s is systematically reduced from -4.1 m/s to -0.7 m/s. In gen...