Quantitative analysis of Bora event in the Adriatic Sea by means of SAR-based techniques and the ETA model

Bora is a north-eastern wind which blows in the Adriatic sea and is characterized by multiple surface wind jets linked to the orography of the Dinaric Alps ([1], [2], [3]). It is a cold, intense, low level wind which is known to strongly modify the sea surface status ([4]). As the Bora usually causes detectable wind-derived features in SAR imagery, well-suited image processing algorithms have been implemented in order to extract wind direction directly from SAR images. In particular, SAR sea surface wind directions have been obtained by exploiting both the Local Gradient (LG) method ([5]) and the relatively novel technique based on the use of 2D continuous wavelet transform (CWT2) ([6], [7], [8]) on ENVISAT C-band ASAR data. The exceptional Bora events of the beginning of February 2012 over the Gulf of Trieste have been studied by means of two Wide Swath (WS) mode ASAR images, acquired on the 2nd and 5th of February and available from ESA archive. The retrieved wind directions extracted by the above mentioned methods have been used to estimate the wind speeds with a classical inversion procedure applied to the forward semi-empirical backscatter model CMOD-5 ([9]). Indeed, CMOD-5 improves performances of the previous CMOD-4 for high wind conditions, as those expected during Bora events. Moreover, a recent version of the atmospheric ETA model ([10]), which is a three-dimensional, primitive equation, grid-point model, has been also adopted to estimate the low level wind fields corresponding to the Bora events under investigation.