The performance of RAMS in representing the convective boundary layer structure in a very steep valley

Abstract

Abstract. Data from a comprehensive field study in the Riviera Valley of Southern Switzerland are used to investigate convective boundary layer structure in a steep valley and to evaluate wind and temperature fields, convective boundary layer height, and surface sensible heat fluxes as predicted by the mesoscale model RAMS. Current parameterizations of surface and boundary layer processes in RAMS, as well as in other mesoscale models, are based on scaling laws strictly valid only for flat topography and uniform land cover. Model evaluation is required to investigate whether this limits the applicability of RAMS in steep, inhomogeneous terrain. One clear-sky day with light synoptic winds is selected from the field study. Observed temperature structure across and along the valley is nearly homogeneous while wind structure is complex with a wind speed maximum on one side of the valley. Upvalley flows are not purely thermally driven and mechanical effects near the valley entrance also affect the wind structure. RAMS captured many of the observed boundary layer characteristics within the steep valley. The wind field, temperature structure, and convective boundary layer height in the valley are qualitatively simulated by RAMS, but the horizontal temperature structure across and along the valley is less homogeneous in the model than in the observations. The model reproduced the observed net radiation, except around sunset and sunrise when RAMS does not take into account the shadows cast by the surrounding topography. The observed sensible heat fluxes fall within the range of simulated values at grid points surrounding the measurement sites. Some of the scatter between observed and simulated turbulent sensible heat fluxes are due to sub-grid scale effects related to local topography.

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

“…In the Riviera Valley, RAMS (Pielke et al, 1992) was successfully used at 330 m resolution (De Wekker et al, 2005) and the Advanced Regional Prediction System LES code (Xue et al, 2000) was used in a horizontal resolution as fine as 150 m Weigel et al, 2006) with excellent results (i.e. very good correspondence to observations).…”

“…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.…”

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

“…The latter are required for the development of models suitable for simulating specific environmental processes in various applications, such as weather prediction, air quality management, meteorological assistance to agriculture and assessment of energy efficiency of buildings. Indeed, various recent contributions, mainly based on high-resolution numerical simulations adopting advanced models (Gohm et al 2004;Rampanelli et al 2004;Zängl et al 2004a,b;De Wekker et al 2005;Weigel et al 2007b), have confirmed that atmospheric processes occurring in mountain valleys not only require suitable grid choices, but also specific settings of physical parameterisation, as well as the suitable representation of factors (e.g. land use, topographic factors, shading, etc.)…”

“…These issues are relevant, not only for outlining a theoretical framework for understanding turbulent processes in valleys, but also for providing a sound basis for appropriate turbulence parametrisations (Fast and Darby, 2004;De Wekker et al, 2005 -in particular their conclusions). The latter are required for the development of models suitable for simulating specific environmental processes in various applications, such as weather prediction, air quality management, meteorological assistance to agriculture and assessment of energy efficiency of buildings.…”

“…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.…”