Crosswind measurements with scintillometers at 500 m above valley floor during foehn

Several of the intensive observation periods during the Mesoscale Alpine Programme's (MAP) Special Observing Period were concerned with foehn in the Alpine valleys. The Rhine valley between Chur (Switzerland) and the Lake of Constance was the target area for such measurements. The Paul Scherrer Institute operated two crosswind scintillometers from mid-September through mid-November. The 6.5-km light paths crossed the valley at about 500 m above valley floor. Measurements were taken once per 7 s during the SOP.

The measurements revealed a variety of phenomena of foehn flow. In one case, horizontal crosswind speeds of more than 35 m s^-1 were sustained for several hours, while the vertical component showed sustained downward flow of around 6 m s^-1 at one location and upward flow of 2 m s^-1 at the other, indicating the existence of a stationary wave. Foehn onset was generally more gradual than foehn breakdown, which was often associated with a cold front entering the area. Scintillation strength, an indicator for turbulence measured as the standard deviation of the light intensity fluctuation, does not always show systematic features related to foehn events. In two cases however, a strong increase of scintillation strength preceded the onset of foehn which happened around 0800 Central European Time (CET). In the late night a cold air pool in the valley typically reaches a thickness of more than 500 m, i.e. the scintillometer paths are within the cold pool. After local sunrise the foehn flow gradually erodes this cold air from the top downwards. The strong scintillation signal indicates shear-induced turbulence in the region where the foehn impinges onto the cold air.