ASPECTS OF THE CONVECTIVE BOUNDARY-LAYER STRUCTURE OVER COMPLEX TERRAIN

Measurements are presented of the development of the convective bounda ry layer in the transition zone from the Upper Rhine valley to the Nor thern Black Forest during one special observation period of the TRACT campaign conducted in September 1992. The data used in this study were obtained from airborne instruments as well as from ground-based stati ons. The analysed boundary layer structure shows a strong influence of the underlying terrain. Until noon, a nearly terrain following cappin g inversion developed. However, advective processes proved to play an important role in the boundary layer structure over the hilly terrain. So, the large-scale air flow caused suppression of the convective bou ndary layer growth at the mountain ridge by forcing the capping invers ion towards the elevation of the terrain. A mountain induced secondary circulation system was observed on the western facing slopes of the B lack Forest. This secondary circulation system affected the heat budge t and therefore the growth of the convective boundary layer over the m ountain slopes. The advection of cold air by up-slope winds lowered th e heating rate near the ground and was able to generate an inversion a bove the up-slope wind layer. In the late afternoon, the terrain follo wing structure of the capping inversion diminished and the capping inv ersion tended to form a horizontal plane. Prognostic formulae for boun dary layer growth are discussed for different sites of the terrain. Wh ile in the valley good agreement is found between calculated and obser ved boundary layer depths, the calculations for the mountain ridge ove restimate the observations by up to a factor of 5 if a boundary layer growth equation derived for homogeneous terrain is used. (C) 1998 Else vier Science Ltd. All rights reserved.