High-Resolution Large-Eddy Simulations of Flow in a Steep Alpine Valley. Part II: Flow Structure and Heat Budgets

Abstract

This paper analyzes the three-dimensional flow structure and the heat budget in a typical medium-sized and steep Alpine valley, the Riviera Valley in southern Switzerland. Aircraft measurements from the MAP-Riviera field campaign reveal a very pronounced valley-wind system, including a strong curvature-induced secondary circulation in the southern valley entrance region. Accompanying radio soundings show that the growth of a well-mixed layer is suppressed, even under convective conditions. Our analyses are based on the MAP-Riviera measurement data and the output of high-resolution large-eddy simulations using the Advanced Regional Prediction Sys-

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

“…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. 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.…”

“…They speculate that the surface heat fluxes, and thus w * , are essential for the heating of the valley atmosphere, either directly by heat flux divergence, or indirectly via thermally driven cross-valley circulations. Indeed, Rampanelli et al (2004) and Weigel et al (2006) showed that overshooting slope winds induce subsidence of potentially warmer air over the valley core. Both studies identified this process as the key driver of valley heating.…”

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).…”

“…This figure is available in colour online at www.interscience.wiley.com/qj 945 improved surface data gives unsatisfactory results. The encouraging conclusion is that simulations using LEStype models can be made so accurately that the model runs may be used to investigate flow characteristics such as driving mechanisms for valley flow or the heat budget in a valley (Weigel, 2005;Weigel et al, 2006).…”

Analysis of the diurnal development of a lake-valley circulation in the Alps based on airborne and surface measurements

“…These will be performed in the near future and will provide the basis for the comparison with the results reported in the present paper. As a matter of fact, the meteorological fields produced by RK inherently provide a degree of spatial detail that is comparable with the resolution of large-eddy simulations, which are able to resolve explicitly the ABL structures typically associated with thermally driven winds (see examples in Catalano and Cenedese, 2010;Catalano and Moeng, 2010;Chow et al, 2006;Lehner and Whiteman, 2012;Weigel et al, 2006). Numerical sensitivity analyses will be also carried out to investigate the effects of the valley orientation and curvature, of land use inhomogeneities and of local topographic features.…”