Foehn analysis in the Rhine Valley with a FM-CW boundary layer Radar-RASS during the MAP experiment

One of the scientific objectives addressed the four-dimensional variability of the Foehn flow in the Rhine valley. To investigate Foehn phenomena a very dense observing network was installed during the MAP special observing period in the Rhine valley target area. Our Radar-RASS was continuously operating in the middle of the Rhine valley south of Lake Constance.

The Radar-RASS has a 5-beam geometry for two bistatic radiofrequency (rf) and for one acoustic (a) antennas. The rf and a antennas are emitting continuous waves that are frequency modulated with a saw tooth (FM-CW-Doppler-RADAR), to provide high average transmitted power. Thanks to this unique technical design redundant wind profiles are derived for both the clear air and the RASS signal with high resolution in time and height. The temperature profiles are estimated simultaneously from the RASS signal.

During 77 days of measurements there were 17 days with South Foehn at the Radar-RASS site. We have selected three different Foehn situations (IOP2, IOP15, and IOP16) for our presentation. Foehn event IOP2 was a strong deep Foehn event. Its origin was essentially dynamic. In the second event (IOP15) the cold air pool in the valley was removed very slowly but steadily. In the third event (IOP16) the warm Foehn air did not reach the valley floor, very sharp gradients of temperature had been detected only some 100 m above ground level.

We will present the three-dimensional structure and dynamics of these Foehn events by means of time series and time-height plots of wind and temperature. Numerical simulations for two Foehn events (IOP2, IOP15) will be used to interpret our measurements in respect to local forcing, mesoscale forcing and synoptic forcing.