High-Resolution Large-Eddy Simulations of Flow in a Steep Alpine Valley. Part I: Methodology, Verification, and Sensitivity Experiments

Abstract

This paper investigates the steps necessary to achieve accurate simulations of flow over steep, mountainous terrain. Large-eddy simulations of flow in the Riviera Valley in the southern Swiss Alps are performed at horizontal resolutions as fine as 150 m using the Advanced Regional Prediction System. Comparisons are made with surface station and radiosonde measurements from the Mesoscale Alpine Programme (MAP)-Riviera project field campaign of 1999. Excellent agreement between simulations and observations is obtained, but only when high-resolution surface datasets are used and the nested grid configurations are carefully chosen. Simply increasing spatial resolution without incorporating improved surface data gives unsatisfactory results. The sensitivity of the results to initial soil moisture, land use data, grid resolution, topographic shading, and turbulence models is explored. Even with strong thermal forcing, the onset and magnitude of the upvalley winds are highly sensitive to surface processes in areas that are well outside the high-resolution domain. In particular, the soil moisture initialization on the 1-km grid is found to be crucial to the success of the finer-resolution predictions. High-resolution soil moisture and land use data on the 350-m-resolution grid also improve results. The use of topographic shading improves radiation curves during sunrise and sunset, but the effects on the overall flow are limited because of the strong lateral boundary forcing from the 1-km grid where terrain slopes are not well resolved. The influence of the turbulence closure is also limited because of strong lateral forcing and hence limited residence time of air inside the valley and because of the stable stratification, which limits turbulent stress to the lowest few hundred meters near the surface.

On the boundary‐layer structure over highly complex terrain: Key findings from MAP

“…Overall the numerical modelling studies showed that simply increasing spatial resolution without incorporating Chow et al (2006). This figure is available in colour online at www.interscience.wiley.com/qj 945 improved surface data gives unsatisfactory results.…”

“…As the single most critical parameter in obtaining good correspondence between simulated and observed flow characteristics, the soil moisture distribution was identified (De Wekker et al, 2005;Chow et al, 2006). A successful approach to obtain enough spatial detail in the soil moisture distribution consists of using a detailed distributed hydrological model.…”

“…Modelled net radiation shows improvement around sunrise and sunset if the models take shadowing effects into account (Colette et al, 2003;De Wekker et al, 2005;Antonacci and Tubino, 2005;Chow et al, 2006). However, effects on the overall flow are limited because of the strong lateral boundary forcing from the larger grids in the nesting procedure where terrain slopes are not well resolved.…”

Analysis of second‐order moments in surface layer turbulence in an Alpine valley

“…characterising the real valley environment. In particular, Chow et al (2006) clearly demonstrated the need for high-resolution datasets with a proper nesting procedure, in order to achieve agreement between simulations (obtained through the Advanced Regional Prediction System; ARPS) and observations. Furthermore, Weigel et al (2006) confirmed the reduced growth of the well-mixed layer determined by both cold-air advection in the alongvalley direction and subsidence of warm air from aloft.…”

“…Furthermore, Weigel et al (2006) confirmed the reduced growth of the well-mixed layer determined by both cold-air advection in the alongvalley direction and subsidence of warm air from aloft. Routinely operated mesoscale models have also been tested for their reliability in reproducing the structure of the convective boundary layer in the context of a valley (Doran et al, 2002;De Wekker et al, 2005;Chow et al, 2006;Weigel et al, 2007b), allowing the identification of the processes which require far greater care when compared to measurements.…”