Daytime heat transfer processes related to slope flows and turbulent convection in an idealized mountain valley

The mechanisms governing the daytime development of thermally driven circulations in idealized two-dimensional valleys are investigated by means of large eddy simulations. In particular, the impact of slope winds and turbulent convection on the heat transfer from the vicinity of the ground surface to the core of the valley atmosphere is examined. Subsidence in the valley core, arising to compensate the upward mass flux related to upslope flows, generates a top-down warming mechanism, as opposed to the bottom-up warming resulting from turbulent convection at the valley floor. Advective top-down warming and convective bottom-up warming are shown to antagonize each other, the former prevailing in the morning and the latter in the afternoon. An evaluation of the depth of the atmospheric layer affected by the slope wind system is also provided. The free atmosphere descent mechanism first described by Whiteman and McKee (1982) does not seem to affect the entire air column up to indefinite heights: rather, it appears to be limited to the depth below the level of neutral buoyancy of air parcels in the thermal plumes devoloping on mountain tops. Due to the occurrence of slope circulations, the boundary layer appears to have peculiar features in mountainous regions: the valley boundary layer (VBL) may include a CBL, but is not limited to it. Elevated layers, where the atmosphere is warmed and becomes weakly turbulent due to advection of heat and TKE, are also included in it. A non-turbulent stable region where air subsides towards the valley floor to compensate upward motion along the slopes is also embedded in the VBL, at least in the early stages of its development. As the daily heating cycle progresses, the CBL and elevated stable layers may merge into a unique mixed layer. These features are consistent with previous findings from airborne measurements.