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Data assimilation by 3DVAR of data from the Special Sensor Microwave/Imager (SSM/I)has been performed to study the IOP2b case (19-21 September 1999) of theMesoscale Alpine Programme (MAP). Only data over the sea surface are used to avoidthe contamination of the surface emissivity. Moreover, the rainy data are filtered outbecause the assimilation algorithm of 3DVAR does not take into account the scatteringprocesses. SSM/I data are assimilated in two different ways:as Brightness Temperature directly, or as Precipitable Water and surfacewind speed retrieved from the Brightness Temperature. The effect of thethinning of the observations has been studied and a set of sensitivity test caseshas been carried out; the one by one removal of the SSM/I frequenciesfrom the initial dataset allows to evaluate their impact on the Initial Conditions.A few experiments are performed using these new Initial Conditionsto initialize the MM5 (PSU/NCAR) model.The results show that the assimilation of the retrieved quantities, i.e. PrecipitableWater and surface wind speed, does not produces large improvement in theInitial Conditions.Vice versa, the assimilation of the Brightness temperatures producesa large variability of the Initial Conditions. The forecast experiments show thatthe model is very sensitive to the 22GHz and 37GHz frequencies

3DVAR assimilation of SSM/I data over the sea for the IOP2b MAP case