The Alpine mountain-plain circulation: Airborne Doppler lidar measurements and numerical simulations using MM5 and LM

On summer days radiative heating of the Alps produces a rise of air above the mountains, and a resulting inflow from the Alpine foreland. This leads to a horizontal transport of polluted air from the forelands to the Alps, and a vertical transport from the boundary layer into the free troposphere at the mountains. We investigated this mountain-plain circulation in southern Germany (“Alpine Pumping”) using an airborne scanning Doppler lidar, dropsondes, and numerical models (MM5 and the German Local Model (LM)). The measurements were part of the VERTIKATOR campaign (Vertical Transport and Orography) in summer 2002. Comparison of dropsonde and lidar data has proofed that the lidar is capable of measuring the wind direction and wind speed of this weak flow towards the Alps (1–3 ms-1). The depth of the flow was up to 1500 m, and it extended ~80 km into the Alpine foreland. Lidar data are volume measurements (horizontal resolution ~5 km, vertical resolution 100 m), and therefore ideal for the investigation of the flow structure, and for a quantitative evaluation of numerical models. Even the vertical velocities measured by the lidar agree with the mass budget calculations by sign and magnitude. We quantified the transports, and studied the structure of the circulation. Furthermore, we evaluated numerical simulations with the MM5 model (grid dist.=2 km) and the LM (grid dist.=7 km), and showed that these models are able to reproduce the structure of the mountain-plain circulation. However, both of them overestimated the mass flux by 10–50%.