Synergic use of Sar Imagery and High- Resolution Atmospheric Model to Estimate Wind Vector over the Mediterranean Sea

An experiment whose aim is the retrieval of surface wind fields from SAR imagery coupled to a high resolution mesoscale numerical atmospheric model in semi-enclosed sea basin, is presented. A sea region belonging to North-Western Mediterranean Sea, which spans in W-E direction from Corsica (8.8 E) to Italian coast (10.5 E) and in N-S direction from Lygurian Sea (44.0 N) to North Tyrrhenian Sea (42.2 N), was selected as test area. Two consecutive ERS-2 SAR frames from the pass of March 30, 2000, along with a set of NOAA/AVHRR and MODIS images acquired on the same day were used for the analysis. SAR wind speeds and directions at 10 m above the sea surface were retrieved from predictions of the semi-empirical backscatter models CMOD4 and CMOD-IFREMER, which describe the dependency of the normalized radar cross section (NRCS) on wind vector and ERS-2 SAR image geometry. Surface wind vectors predicted by the meteorological ETA model were exploited as guess input to SAR wind inversion procedure to describe atmospheric conditions in the area, according a Bayesian approach recently proposed in literature. ETA is a three-dimensional, primitive equation, grid-point operational model running at the National Centers for Environmental Prediction of the U.S. National Weather Service. The model was adapted to run on selected regions of the Mediterranean basin with a nested very high, up to about 4.0 Km, resolution. The latter feature makes ETA model particularly suitble for its use in combination with SAR images. Besides, to simulate and predict several specific atmospheric weather phenomena, ETA outputs also include the vertical distributions of physical parameters such as air pressure, temperature, moisture up to about 25 Km. Apart some discrepancies in sparse and small areas, an overall agreement between SAR inversion results and ETA predictions was found. More importantly, it was found that the inversion methodology was not able to resolve wind speed modulations due to the manifestation of an atmospheric gravity wave, which occurred in the analyzed area as a result of the terrain disturbance to the air flow imposed by the peninsula located North of Corsica. Temporal evolution of the wave propagation phenomenon was allowed by inspection of NOAA/AVHRR and MODIS images through the detection of a cloud band associated to the atmospheric wave. A wave propagation model describing waves in the atmosphere owing to the disturbing action on the primary air flux by terrain features was thus used to account for the observed surface wind speed modulation on SAR image. Synergy with ETA model outputs was further exploited as atmospheric parameters up-wind the atmospheric wave were considered as input to the wave propagation model.