Daytime Development of the Boundary Layer over a Plain and in a Valley under Fair Weather Conditions: A Comparison by Means of Idealized Numerical Simulations

Abstract

he daytime thermal structures of the valley boundary layer (VBL) and of the convective boundary layer (CBL) above a plain, as revealed by idealized large-eddy simulations, are compared. Simulations in the two environments consider similar thermal forcing, thus allowing an analysis of the atmospheric heating processes in the VBL and CBL in light of the volume-effect theory, traditionally invoked to explain the larger diurnal temperature ranges observed in valleys. It is found that, after an equal input of thermal energy, the atmospheric volumes affected by thermal perturbations in the CBL and in the VBL are comparable. Although the boundary layer top is higher in the VBL than in the CBL, the average VBL depth is approximately equal to the CBL depth, since the ground elevation is nonuniform in the valley. Accordingly, the volume-averaged potential temperature increments in the CBL and VBL are comparable. Nevertheless, surface air temperature variations are larger in the VBL, while differences in the thermal structures of the CBL and the VBL are found to be larger at elevated levels. These effects are related to the heat and mass transfer processes associated with upslope flows and midvalley subsidence. As far as the simulated CBL and VBL cases are representative of two asymptotic regions (respectively, far up valley and far over the plain) of a plain?valley system with a horizontal floor, their comparison provides insight in the mechanisms responsible for the generation of the pressure contrasts driving a daytime plain-to-valley wind at lower levels and possibly a valley-to-plain upper flow.