Wind field from SAR over the Venice lagoon, Italy

The Venice Lagoon (45.15°-45.6° N and 12.1°-12.8° E) , located in the northern part of the Adriatic Sea (Fig. 1), northeastern part of Italy, extends ~50 km by 10 km along a North- East axis. The lagoon is subjected to the main wind systems of the northern Adriatic Sea, i.e. the south-easterly Sirocco and the north-easterly Bora, the former blowing roughly parallel to the Adriatic Sea major axis producing the well known storm surges over the city, the latter blowing roughly parallel to the lagoon major axis producing local storm surges inside the lagoon. SAR is the only instrument at present available to retrieve the wind field over such small areas at a spatial resolution below 1 km. This is possible once robust methodologies to extract the wind direction from SAR images, without external informations, are available. In this context, the use of wind directions from atmospheric models should be avoided because of their well know difficulty to account for the small spatial scale variations of the wind in coastal and semienclosed areas as the the Venice lagoon. A technique based on deep residual network (ResNet), a variant of Convolution Neural Network (Zanchetta and Zecchetto, 2021) is, at present, the best approach to the determination of the wind direction necessary to compute the wind speed through the available Geophysical Model Functions. The results of the wind fields retrieval by our ResNet technique on 48 Sentinel-1 SAR images are shown in terms of statistics from comparisons with in-situ data and description of the wind spatial structure typical of the above mentioned winds (Fig. 2). Comparisons with the ESA OCN wind products are also provided, to stress the importance of our results which do not depend on external direction and provide a better spatial coverage of the Venice Lagoon and of the coastal areas.