The simulation of a deep convective cloud in complex orography: the 15 July 2007 case from COPS

The COPS (Convective and Orographically-induced Precipitation Study) was an international campaign, the field phase of which took place in the Black Forest, Germany, in July-August 2007. During COPS a very deep, isolated cloud formed over complex terrain, during a period of high pressure. This event (the only one of its kind observed during the COPS campaign) represents a particularly challenging test case and constitutes an ideal scenario for model verification: the ability to simulate such a cloud is crucial for precipitation forecasting in complex terrain.

The Weather Research and Forecasting (WRF) model will be used to simulate this event. The ability of the WRF boundary- and surface-layer schemes to properly represent the atmosphere above complex orography is not well understood, however: sensitivities are known to exist, which is unsurprising given the large degree of freedom in choosing physical parametrisations. For this study, the sensitivity tests are restricted to those which directly affect the boundary layer specification.

From the group of simulations, and via comparison with observational data such as radiosondes, the potential problems in the available boundary layer schemes (such as boundary layers being too well-mixed)will be demonstrated. The importance of a good representation of soil moisture will also be shown. Out of all the tests performed for this case, only one run produced results which bore resemblance to the observed cloud (indeed, the remaining runs did not produce any cloud at all). The modelled structure of the boundary layer, and specification of surface moisture, are seen to be of crucial importance.

This study has important implications for the forecasting of deep convective clouds in complex terrain - and for convection in general - and illustrates the important physical processes which need to be adequately represented for a realistic model simulation.