Small scale topography influence on the formation of three convective systems observed during COPS over the Vosges Mountains

Numerical modelling of the airflow and precipitating convective systems are performed to better understand the role of topography for the triggering of convection over a moderate mountain region during the Convective and Orographically induced Precipitation Study (COPS) campaign. A non-hydrostatic cloud scale model with two nested domains is used which permits to zoom from the mesoscale environment of south-western Germany/eastern France, into the Vosges Mountains and finally into the small-terrain of the field experiment, increasing the grid resolution to well represent the orography of the region. Using radar observations, a classification of the location of the convection initiation was established during the COPS project, which considers that the convective systems form either on the mountain ridges or on the lee side of the massif. The three simulated cases of this study, corresponding to either position of convection initiation, compare well with available observations of local thermo-dynamical conditions, high resolution X-band radar reflectivity, Vienna Enhanced Resolution Analysis (VERA) of the surface horizontal wind and water vapour retrieval through GPS integrated water vapour 3D tomography. It was found that the convection generation is largely influenced by the Vosges topography. Even for a quite similar synoptic horizontal wind field, the relief acts differently for the studied cases. However, the convective systems are not formed solely by the mountains, but require inputs of moisture, proper stability, and some supportive mesoscale environment. Therefore, their representation in the model requires also a detailed knowledge of the local atmospheric conditions.