Extended Abstract (564K)13.3
Unstationary and inhomogeneity aspects of the MAP IOP 12 south foehn event in the Rhine valley
Philippe Drobinski, Service d'Aéronomie, Paris, France; and C. Haeberli, E. Richard, B. Bénech, H. Berger, A. M. Dabas, P. H. Flamant, M. Furger, M. Lothon, and R. Steinacker
This paper presents an analysis of the local- to synoptic-scale dynamical processes driving the evolution of the foehn flow in the Rhine valley from 29 to 31 october 1999 during IOP 12 of the MAP field experiment. This analysis combines lidar, constant volume balloons, rawinsonde and surface measurements with Meso-NH numerical simulations. The foehn case of IOP 12 also known as the `Sahara dust-foehn' of the MAP-SOP displays several phases in its development in the Rhine valley and its tributaries: (1) the pre-foehn phase corresponds to a strong westerly synoptic flow. In the valley, the weak heating by the sun and the cloud cover prevented the development of thermally induced wind systems; (2) the increasing cross-Alpine pressure gradient supported the katabatic flow in the Rhine valley and its tributaries during the night from 29 october 1999 to 30 october 1999; (3) starting at 0300 UTC, a small zone of higher pressure established in the very northern parts of the Rhine valley. Due to steadily falling pressure, the pressure minimum in the Rhine valley moved to the area between Vaduz and Sargans. This caused a flow reverseal in the upper parts of the Rhine valley, and allows the cold air pool to pour into the region where katabatic drainage was present in phase (2); (4) the main reason for the intensification of the foehn flow in the Rhine and Seez valleys was the increasing cross-Alpine pressure difference which augmented from 6 hPa at 0600 UTC to 8.5 hPa at 1200 UTC on 30 october 1999. The foehn completely stopped by an approaching cold front on 31 october 1999 at 0200 UTC. A remarkable point during this foehn episode is the absence of rain on the windward side during this event.
Session 13, Gap Winds and Foehn I
Thursday, 20 June 2002, 8:00 AM-10:30 AM
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