The influence of mesoscale circulation systems on triggering convective cells over complex terrain

The project VERTIKATOR (Vertical Exchange and Orography) focused on the initiation and transport processes as well as the model representation of convective systems above low mountain ranges in central Europe. Secondary circulation systems developing during daytime seem to be responsible for triggering convection and subsequent precipitation. The 7 km version of the Lokal-Modell (LM) using the Tiedtke scheme for convection parameterization does not correctly resolve convection initiation forced by slope and valley winds as well as the time, amount, and location of precipitation. All results indicate that the operational version of the LM produces precipitation averaged over large areas without any internal structure over longer time periods than in reality. For simulating single cells with more realistic precipitation concerning time and amount of rainfall, a higher horizontal and vertical resolution is necessary. This is because of better resolved orography and convection initiation by boundary layer circulations. Furthermore, inhomogeneities of the temperature and humidity fields, which are essential for the generation of convection and subsequent rainfall events, are expected to be much too smoothed when using the 7 km grid. The operationally used closure for convection parameterization depends on the large-scale humidity convergence and on turbulent fluxes of latent heat at the surface. Both processes are decisive for the onset and intensity of convection. It has been shown that humidity convergence over the mountain ridges is described by the LM with a 2.8 km grid resolution in a much more realistic way. The better representation of the flow patterns in mountainous areas improves forecasting in these regions. (c) 2005 Elsevier B.V. All rights reserved.