BOUNDARY LAYER STRUCTURE OVER THE ALPS DURING MAP FROM AIRBORNE LIDAR DATA S. F. J. De Wekker 1 , I. Colbeck 2 , K. Eleftheriadis 4 , C. Kiemle 5 , M. Kossmann 6 , S. Nyeki 2,3 , and D. G. Steyn 1 1 University of British Columbia, Vancouver, Canada 2 University of Essex, Colchester, Essex, England 3 Paul Scherrer Institut, Villigen, Switzerland 4 INT-RP, "NCSR" Demokritos, Athens, Greece 5 DLR, Oberpfaffenhofen, Germany 6 University of Canterbury, Christchurch, New Zealand An aerosol lidar aboard the DLR Falcon research aircraft has been used to examine characteristics of the convective boundary layer (CBL) over parts of the Alps and Po basin during MAP. North-south transects were made over the Alps in the vicinity of Innsbruck and roughly northwest-southeast transects were made over the Po Basin. The lidar measurements were conducted on September 24 and 29, 1999, which proved suitable for investigation of the CBL. Lidar allows the aerosol backscatter ratio (B) to be measured, which can give useful information on the spatial boundary layer structure, in particular the top of the CBL (which is commonly detected by a sharp transition from high to low values of B, see e.g., Steyn et al., 1999). An example is given in Figure 1, which represents a 250 km cross-section from the Alps to the Adriatic Sea, crossing the Po Valley. The total flight time was about 2 hours, with a mid-transect time of 1405 LST (= UTC + 1). It can be seen that over the Alps, the boundary layer reaches higher altitudes and that the structure is rather inhomogeneous with large spatial variations in the CBL depth. Over the Po Basin, the boundary layer structure and CBL depth are more homogeneous. The increased values of the backscatter ratio near the boundary layer top are due to the presence of convective clouds. Note that the boundary layer structure over mountainous terrain can be complicated by the presence of boundary layers that are formed over a variety of scales (small/large valleys, slopes, ridges, etc). For example, a boundary layer that was created over high elevation terrain may be advected over a boundary layer that was created over a valley floor located at a lower elevation. The presence of thermally driven circulations also tends to make the boundary layer structure less homogenous than over flat terrain. Figure 1: Lidar transect of the aerosol backscatter ratio illustrating the boundary layer structure over the Alps (on 24 September 1999).