Processes driving deep convection over complex terrain: a multi-scale analysis of observations from COPS IOP 9c

The ‘Convective and Orographically-induced Precipitation Study’ (COPS) analyses the processes driving deep convection over complex terrain. Convection initiation (CI) is mainly not only expressed by a single process, but by a variety of them, which interact on different scales in time and space and finally can lead to deep convection. A study of such a case over inhomogeneous terrain is presented in this article. Data from the COPS network of stations are taken to identify the time and location of CI. Inmany cases this is not the same location as the first convective clouds, showers or even thunderstorms. It is shown that the interaction of the CI processes onmultiple scales locally leads to either deep convection and severe storms or calm weather. The boundary conditions between the different outcomes are narrow. During IOP 9c (20 July 2007), a mesoscale convective system (MCS) embedded in a surface low over eastern France propagated north-eastward and a gust front of the MCS reached the COPS area. During the passage of the gust front through the Rhine valley, convective activity was significantly reduced. The gust front reached the slope of the Black Forest, and the warm and humid air above the mountain range prior to the gust front was lifted up to 2000mwithin a short time period. Within the air prior to the gust front aloft from the surface, CI started along a north–south oriented line above the crest. Due to insolation ahead of the gust front, a convergence line developed. The interaction of local-scale orographic winds, the regional-scale gust front, the mesoscale convergence line, and the synoptic-scale cold front led to a squall line. Finally, individual severe convective cells formed along the outer section of the synoptic-scale cold front.