An analysis of the Constant Volume Balloon (CVB) flights, performed by the Laboratoire d’Aérologie (LA), the Centre National d’Etudes Spatiales (CNES) and the Swiss Meteorological Institute (SMI) during the Mesoscale Alpine Program (MAP), is presented in the frame of the Foehn events analyzed above the high Rhine valley (FORM experiment). The cases of southerly flow over the Rhine valley are well documented with 56 CVB flights: 33 trajectories above the Alps crest between 3000 and 6000 meters height and 23 inside the valley during 10 IOPs (2, 4, 5, 8, 9, 10, 12, 13, 15 and 16). The SMI and CNES CVBs are instrumented with a Vaisala radiosonde measuring pressure, temperature, humidity and wind ( differential GPS system). The CNES CVBs are instrumented also with a second pressure sensor giving the CVB internal pressure monitoring the CVB volume. The CVB responses to vertical motion are modelized, specially in the cases of lee waves.

The results presented in this paper are relative to (i) the mesoscale flow organization deduced from the CVB flights (flow rotation, heating and drying behind the mountain ridge), ( ii) the trapped lee waves characteristics (location, amplitude, period, intensity) , (iii) the flow organizations in the Rhine valley, and (iv) some comparisons with high resolution numerical simulations made with MESO-NH model using eulerian and lagrangian approaches.