During IOP 9c on Friday, July 20, 2007, a mesoscale convective system related to a vorticity maximum at the east side of a jet over eastern France, propagated northeastward and a gust front of the system reached the COPS region in the morning. During the rapid passage of the gust front from west to east through the Rhine valley, the convective weather activity was significantly reduced. When reaching the western slope of the Black Forest the air has been lifted up to 1000 m in order to propagate over the hilly terrain of the low mountain range. More to the East the sky was still free of any clouds and undisturbed radiation causes high air temperature of up to 30 °C near the earth's surface leading to a thermally driven secondary circulation directed to the West. At noon convergence took place between the cold air of the gust front propagating to the East and modified by the hilly Black Forest and the hot and humid air ascending in the East. A north south orientated convergence line was formed which led to a re-intensification of convection over the mountains, causing severe convection over the eastern Black Forest and the Swabian Alps.
The transformation of the stable air mass passing the Rhine Valley into an air mass of high instability when passing the Black Forest is studied by analysis of airborne measurements and data of ground based in situ and coupled remote sensing instruments. Upper tropospheric forcing and ground based triggering of lifting processes are discussed with special emphasis of the high spatial and temporal variability of atmospheric stability and humidity within the atmospheric boundary layer.
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