MESOSCALE BOUNDARY-LAYER EVOLUTION OVER COMPLEX TERRAIN .2. FACTORS CONTROLLING NOCTURNAL BOUNDARY-LAYER STRUCTURE

The development of the nocturnal boundary layer (NBL) over a sloping plateau upwind of a high mountain barrier is studied with a numerical model and field observations. Six numerical simulations and one observed case are used to describe the effects of wind speed, wind direction, and sunset mixed-layer depth on the NBL structure 6 h after sunset. When there is a component of wind into barrier, a two-layer structure develops. A 75-175-m-deep inversion layer that is topped by a 200-300-m-deep, less stable transition layer extends over the length of the plateau. Shear between the 3-4 m s-1 drainage winds in the inversion layer and the large-scale wind mix cold air vertically to build the transition layer. The inversion layer appears to be relatively insensitive to changes in the external parameters, but transition-layer depth is proportional to wind speed.