The impact of surface and boundary-layer conditions on convection-related parameters over mountainous terrain

The field campaign ‘Convective and Orographically-induced Precipitation Study' (COPS) was performed in south-western Germany and eastern France in summer 2007. Within the COPS-context this study focuses on the process chain of soil moisture, surface fluxes, conditions of the convective boundary layer (CBL), and convection-related parameters. The results are different for valley and mountain sites. Only in the Rhine valley the ratios of sensible and latent heat to the net radiation, H0/Q0, and E0/Q0, respectively, reveal a dependence on soil moisture. H0/Q0 is lower and E0/Q0 is higher at higher soil moisture. A correlation of the diurnal increase of the equivalent potential temperature, Θe, with the energy supplied by H0 and E0 is only found for the stations in the Rhine valley and in the valleys of the Black Forest mountains. However, the correlation coefficients are low, indicating that the CBL conditions are influenced to a great extent by advective processes, too. Furthermore, only a weak dependence of the CBL depth on the sensible heat flux is found for valley sites and is non-existent at the mountain crest. The convective indices in the whole COPS domain are found to depend on Θe in the CBL. Convective inhibition (CIN) is positively correlated with the strength of the CBL-capping inversion and negatively with the CBL height: the higher the CBL, the lower the upper threshold of CIN. The frequency of low CIN is higher in the Black Forest mountains than in the Rhine valley, which facilitates convection initiation over the mountain sites.