Alpine pumping as seen by a RADAR-RASS and simulation with the MM5 model

„Alpine Pumping“ (AP) denotes a regional circulation pattern which is generated during day time between a mountain range and its foreland in conditions of high solar radiation and weak synoptic pressure gradients. The air above the Alps is heated more quickly during the day than above the foreland. A heat low is generated and near-surface air gets convectively transported to higher levels. The air flows partly back towards the foreland at higher level and sinks above the foreland to compensate the air mass deficit induced by the inflow near the surface. AP is characterised by strong and localised updrafts, while the sinking motion above the foreland is slow and rather uniform over a larger area.

In July 2002 the field experiment „Alpine Pumping” was carried out in the framework of the German AFO-2000 research project VERTIKATOR (Vertical Exchange and Orography). Wind and temperature profiles were measured within Alpine valleys and above the foreland by various techniques. The special observation area of the field experiment was situated between Munich in the north, Innsbruck in the south and the rivers Lech and Inn near Rosenheim in the West and East. Aircraft, Sodars, lidars and our RADAR-RASS system have been operating during the field experiment to investigate the evolution of wind, temperature and aerosols within the lower atmosphere.

The RADAR-RASS was continuously monitoring wind and temperature profiles with a temporal resolution of 30min and with a height resolution of 60m Thanks to the excellent resolution in time and height of temperature, wind and turbulence measured by the RADAR-RASS a very detailed picture of the evolution of AP was obtained at a foreland site 50 km north of the Alps.

We will present MM5 simulations of the 4D structure of AP for a selected day in July when AP was strongest. For example,. time-height cross-sections of the meridional wind component and temperature calculated with the MM5 model at the location of the RADAR-RASS are compared with the observations and show good agreement.