Profiler RASS observations and numerical model simulation of cold air pool removal during foehn

One of the scientific objectives of the Mesoscale Alpine Programme (MAP, Drobinski et. al. 2007) addressed the four dimensional variability of the foehn flow in the Rhine valley. The interaction between the foehn and cold-air pools frequently filling the Rhine Valley is of crucial importance for the flow evolution in the valley.

Within the framework of the experimental field study an instrumentation of unprecedented density was set up for 2.5 months including surface and radio stations, a variety of remote-sensing instruments and several research aircraft.

One of the two wind profilers was our boundary layer profiler-RASS (WTR). It was located near Rankweil in the middle of the Rhine Valley. Thanks to the RASS capability (Radio Acoustic Sounding System) and the FM-CW operation mode (frequency modulated-continuous wave) it was possible to get profiles of wind and temperature from nearly ground level up to a height of around 1000m, with high resolution in time (30min) and in height (60m). This is not possible with other profiling systems like radio sounding stations systems even with the present dense ascents intervals of 3h.

One of the ten intensive observing periods has been selected to illustrate the life cycle of a long lasting foehn event, during which the descending of the warm Foehn air and the erosion of the cold air pool was very pronounced.

Numerical simulations have been performed with the MM5 model for this selected event. Model calculation and WTR measurements of wind and temperature are in good agreement. The timing of the foehn onset was accurately simulated and the erosion of the cold pool and the break through of the Foehn down to the valley floor is well matched.

( Drobinski, P. et al: Foehn in the Rhine Valley during MAP: A review of is multiscale dynamics in complex valley geometry. Q.J .R. Meteorol. Soc. 133 897-916; 2007)