Aspects of the convective boundary layer structure over complex terrain

Measurements are presented of the development of the convective boundary layer in the transition zone from the Upper Rhine valley to the Northern 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 stations. The analysed boundary layer structure shows a strong influence of the underlying terrain. Until noon, a nearly terrain following capping 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 boundary layer growth at the mountain ridge by forcing the capping inversion towards the elevation of the terrain. A mountain induced secondary circulation system was observed on the western facing slopes of the Black Forest. This secondary circulation system affected the heat budget and therefore the growth of the convective boundary layer over the mountain slopes. The advection of cold air by up-slope winds lowered the heating rate near the ground and was able to generate an inversion above the up-slope wind layer. In the late afternoon, the terrain following structure of the capping inversion diminished and the capping inversion tended to form a horizontal plane. Prognostic formulae for boundary layer growth are discussed for different sites of the terrain. While in the valley good agreement is found between calculated and observed boundary layer depths, the calculations for the mountain ridge overestimate the obserations by up to a factor of 5 if a boundary layer growth equation derived for homogeneous terrain is used.