Intercomparison of Mesoscale Model Simulations of the Daytime Valley Wind System

Abstract

Three-dimensional simulations of the daytime thermally induced valley wind system for an idealized valley–plain configuration, obtained from nine nonhydrostatic mesoscale models, are compared with special emphasis on the evolution of the along-valley wind. The models use the same initial and lateral boundary conditions, and standard parameterizations for turbulence, radiation, and land surface processes. The evolution of the mean along-valley wind (averaged over the valley cross section) is similar for all models, except for a time shift between individual models of up to 2 h and slight differences in the speed of the evolution. The analysis suggests that these differences are primarily due to differences in the simulated surface energy balance such as the dependence of the sensible heat flux on surface wind speed. Additional sensitivity experiments indicate that the evolution of the mean along-valley flow is largely independent of the choice of the dynamical core and of the turbulence parameterization scheme. The latter does, however, have a significant influence on the vertical structure of the boundary layer and of the along-valley wind. Thus, this ideal case may be useful for testing and evaluation of mesoscale numerical models with respect to land surface–atmosphere interactions and turbulence parameterizations.

Influence of along-valley terrain heterogeneity on exchange processes over idealized valleys

“…The WRF model has been successfully applied for idealized simulations of thermally driven flows in the kilometre scale (Rampanelli et al, 2004;Schmidli et al, 2011;Wagner et al, 2014b) and for large-eddy simulation (LES) studies (Catalano and Moeng, 2010;Catalano and Cenedese, 2010;Wagner et al, 2014a, b) in the past.…”

“…The used valley topography is similar to the model terrain applied in Schmidli et al (2011). The modelling domain of the reference setup (REF, see Table 1) has an extent of 200 km in the along-valley and 40 km in the crossvalley direction.…”

“…In order to vary the model topography in along-valley direction (i.e. narrowing valley, inclined valley floor), the terrain computation of Schmidli et al (2011) is extended following Riday (2010). The along-valley (y-direction) mountain height h y is defined as…”

“…However, apart from the study of Li and Atkinson (1999), in which valleys with different floor inclinations and narrowing valley cross sections were used, in most modelling studies valleys with homogeneous along-valley topography were investigated (e.g. Rampanelli et al, 2004;Schmidli and Rotunno, 2010;Schmidli et al, 2011;Schmidli, 2013;Wagner et al, 2014a, b). When considering real valleys such as the Inn Valley, the Isar Valley or the Oetz Valley in the European Alps, it is evident that the valley geometry typically changes in the along-valley direction.…”

Some Requirements for Simulating Wildland Fire Behavior Using Insight from Coupled Weather—Wildland Fire Models

“…Also, whether winds penetrate down into narrow canyons and drainages can determine whether a fire progresses. In addition, the simulated daytime slope and valley wind system, an important factor in fire weather, varies from one mesoscale model to another [75]. They note large differences in local flow evolution, particularly near the surface, primarily in the time of onset due to differences in terms calculated in the surface energy budget.…”