QPF research in low-mountain regions using the COPS data set: Precipitation statistics, predictive skills of models, and the first process study of IOP4b

This presentation gives an overview of the major results achieved during the Convective and Orographically-induced Precipitation Study (COPS). COPS is a WWRP Research and Development Project (RDP) dedicated to improving QPF in low-mountain regions and for studying the predictability of heavy precipitation events. The presentation consists of three parts: First of all, new high-quality statistics of atmospheric variables such as precipitation during the COPS observations period in summer 2007 in southwestern Germany/eastern France are presented. These data provide the backbone for process studies and model verification. Secondly, the predictive skill of an ensemble of state-of-the-art mesoscale models is studied using advanced skill scores. The results point to specific deficiencies of model physics and model initialization. Particularly, still strong systematic errors in the simulation of thermally-induced flows and the 3D distribution of humidity exist. Finally, a heavy convective precipitation event is investigated during IOP4b on June 20, 2007. Between a ridge stretching from southern Scandinavia over the eastern Alps to the Adriatic Sea and a low over Ireland, a strong southwesterly mid-tropospheric flow transported a moist and unstable air mass to the COPS area ahead of a diffuse frontal zone located from southern Denmark towards central Spain. In the early afternoon, the interaction of the mesoscale flow and thermally-induced flow systems caused hot spots for local initiation of convection and precipitation. Only low amounts of precipitation were produced by these events. However, in the late afternoon, positive vorticity advection arrived in the COPS domain. This was caused by a frontal wave along the eastern exit of the upper level trough associated with the low pressure system. The increased instability resulted in a strong amplification of convection and rapid development of deep convective cells. These cells organized in a large mesoscale convective system with heavy precipitation including hail. Thus, IOP4b is another excellent example illustrating the importance of the constructive interaction of small-scale/large-scale processes in orographic terrain determining the development and duration of heavy precipitation events.