Mechanisms initiating deep convection over complex terrain during COPS

Abstract

Precipitating convection in a mountain region of moderate topography is investigated, with particular emphasis\non its initiation in response to boundary-layer and mid- and upper-tropospheric forcing mechanisms.\nThe data used in the study are from COPS (Convective and Orographically-induced Precipitation Study) that\ntook place in southwestern Germany and eastern France in the summer of 2007. It is found that the initiation\nof precipitating convection can be roughly classified as being due to either: (i) surface heating and low-level\nflow convergence; (ii) surface heating and moisture supply overcoming convective inhibition during latent\nand/or potential instability; or (iii) mid-tropospheric dynamical processes due to mesoscale convergence lines\nand forced mean vertical motion. These phenomena have to be adequately represented in models in order\nto improve quantitative precipitation forecast. Selected COPS cases are analysed and classified into these\ninitiation categories. Although only a subset of COPS data (mainly radiosondes, surface weather stations,\nradar and satellite data) are used here, it is shown that convective systems are captured in considerable\ndetail by sensor synergy. Convergence lines were observed by Doppler radar in the location where deep\nconvection is triggered several hours later. The results suggest that in many situations, observations of the\nlocation and timing of convergence lines will facilitate the nowcasting of convection. Further on, forecasting\nof the initiation of convection is significantly complicated if advection of potentially convective air masses\nover changing terrain features plays a major role. The passage of a frontal structure over the Vosges –\nRhine valley – Black Forest orography was accompanied by an intermediate suppression of convection\nover the wide Rhine valley. Further downstream, an intensification of convection was observed over the\nBlack Forest due to differential surface heating, a convergence line, and the flow generated by a gust front.