Proceedings with Extended Abstracts (single PDF file) - Radio ...

estimating two independent and redundant wind profiles. A description of the profiler is givenin Bauer-Pfundstein, 1999.3. Model DesignThe numerical simulation is performed with the non-hydrostatic model Meso-NH based onthe anelastic equation system. Two simulations domains are used with the two-way interactivegrid-nesting technique of Stein et al. (2000), the horizontal mesh size being 10 km and 2.5km. Domains and physical parametrizations are as described in Lafore et al. 1998, except forthe use of the most recent radiation scheme of ECMWF (Morcrette et al.2001).The simulation starts at 12 UTC November 5 th. It is initialized with two mesoscale analyses,performed at the resolution and on the horizontal grid of each of the two nested models, withthe CANARI/DIAGPACK assimilation system, developed at Meteo-France and based onoptimal interpolation (Calas et al. 2000). Lateral boundary conditions are provided by timespaceinterpolation between operational analyses of the ARPEGE system of Meteo-France,available every 6 hours.4. Experimental Data and Model ResultsThe descending of one strong Foehn in the area of Rankweil from upper layers down to thesurface can be seen best by means of time-height cross sections of wind and temperature.Figs. 2 and 3 are showing the virtual potential temperature and the S-N wind component,respectively measured by the WTR. Strong south wind was detected first at an altitude of 1.2km around midnight November 4 th . It lasted more than 19 h until the Foehn penetrates downto the valley floor. The descending was not uniform but with some intermittent periods.Nevertheless an overall descending rate of 33 m/h is estimated. More details of this Foehnevent will be found in Jaubert et al. 2003.Four periods of IOP15 are evident:1. onset of Foehn, heating and eroding of the stagnant cold pool,2. break through of Foehn to the surface,3. intermittence of Foehn,4. arrival of the perturbation and end of Foehn.Fig. 4 is showing simulated potential temperature along a S-N transect in the Rhine valley forfour selected time periods (16, 19, 23 UTC, Nov. 5th and 6 UTC, Nov. 6th). Left hand side ofFig. 4 is a similar viewgraph of the wind parallel to the cross-section and Fig.5 is themodelled air flow near the surface (90 m level). When comparing the depth of the cold airpool for different time periods measured by WTR to the depth simulated by Meso-NH, themodel simulation is very close to the measurements.6. ConclusionThanks to the excellent resolution in time and height of temperature and wind fields measuredby the WTR it is possible to get a very detailed picture of the evolution of this Foehn event.The model simulations are in good agreement with the observations. So it will be possible toquantify the physical processes involved in the removal of the cold pool and the instationarynature of the Foehn flow. This will be a significant key to better understanding and, hopefully,to forecast Foehn episodes.291