Influence of convective clouds on the turbulence structure of the boundary layer measured by Doppler Lidar

Cloud venting is a very effective process for exchanging mixed layer air masses with the free atmosphere, but only active cumulus clouds contribute to this process. During August 2004 the 2 µm Doppler-Lidar of the Forschungszentrum Karlsruhe was operated with a fixed beam pointing vertically upward to measure thermals generating the development of the convective boundary layer. The growing of the boundary layer as well as clouds which passed occasionally over the Lidar are detected using the aerosol backscatter information from the lidar. Cumulus clouds showed different influence on the turbulent state of the middle and upper boundary layer depending on the state within their life cycle. Active and forced cumulus clouds were connected to strong upwinds underneath and in their base, on the other hand passive clouds at the end of their life cycle are disconnected from the underlying boundary layer and have no influence on vertical wind speed and turbulence. Results from the 2004 measurements are verified with data from the CSIP 2005 campaign. σ_w profiles as well as power spectra derived from the 10 Hz Lidar data will be shown and compared to literature.