An evaluation of the volume-effect theory by means of large-eddy simulations.

The daytime thermal structures of the valley boundary layer (VBL) and of the convective boundary layer above a plain (CBL), 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 increments observed in valleys. It is demonstrated that, after an equal input of thermal energy, the atmospheric volumes affected by thermal perturbations in the CBL and in the VBL are comparable. Therefore, the volume-averaged potential temperature increments are also comparable. However, the local temperature increments near the surface are considerably larger in the VBL. This is thought to be a consequence of the larger vertical extent of the VBL, favouring the entrainment of potentially warmer air from the free atmosphere. The largest imbalances between the thermal structures of the CBL and the VBL are shown to occur at elevated levels rather than at the surface. This seems to be related to the combined effect of the heat and mass transfers operated by upslope flows (resulting in local cooling) and mid-valley subsidence (local warming). On the overall, the comparison of the CBL and VBL simulations provides new insight in the processes that generate the pressure contrasts responsible for valley breezes. While no proper volume effect is found, a major role seems to be played by the different heat transfer processes occurring in the two environments, and in particular by the deeper mixing observed above valleys.