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

Abstract. 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 the Mesoscale Alpine Programme (MAP). Only data over the sea surface are used to avoid the contamination of the surface emissivity. Moreover, the rainy data are filtered out because the assimilation algorithm of 3DVAR does not take into account the scattering processes. SSM/I data are assimilated in two different ways: as Brightness Temperature directly, or as Precipitable Water and surface wind speed retrieved from the Brightness Temperature. The effect of the thinning of the observations has been studied and a set of sensitivity test cases has been carried out; the one by one removal of the SSM/I frequencies from the initial dataset allows to evaluate their impact on the Initial Conditions. A few experiments are performed using these new Initial Conditions to initialize the MM5 (PSU/NCAR) model. The results show that the assimilation of the retrieved quantities, i.e. Precipitable Water and surface wind speed, does not produces large improvement in the Initial Conditions. Vice versa, the assimilation of the Brightness temperatures produces a large variability of the Initial Conditions. The forecast experiments show that the model is very sensitive to the 22GHz and 37GHz frequencies.