Francesco De Biasio

Wind Fields From C- and X-Band SAR Images at VV Polarization in Coastal Area (Gulf of Oristano, Italy)

This work deals with the spatial characteristics of the wind fields evaluated from synthetic aperture radar (SAR) images and simulated by the weather research and forecasting (WRF) atmospheric model in the Gulf of Oristano, a small coastal area about 10 km × 18 km wide in western coast of Sardinia (Western Mediterranean Sea). The SARderived wind fields have been obtained analyzing images of the COSMO-SkyMed, Radarsat-2, and Sentinel-1A satellites through a fully two-dimensional continuous wavelet transform (2-D-CWT) method. The analysis of the wind directions has shown that the model variability is limited if compared to that inferred by 2-D-CWT method, which mostly respects the variability evidenced by in situ data. As the use of model directions to compute the SAR wind fields is a standard in many studies, the impact on the SAR wind speed retrieval of using the model instead of the SARderived directions has been assessed: differences of wind speed greater than ±10% occur for about the 20% of data. The spatial variability of the SAR and model wind speed fields results quite different at both local and domain scales. The knowledge of the spatial variations of the surface wind fields can be very important for the oceanographic applications and constitutes the added value brought by SAR in the description of the coastal wind. For this reason, the SAR-derived wind fields should be taken as reference in many kind of applications.

Exploiting the Potential of Satellite Microwave Remote Sensing to Hindcast the Storm Surge in the Gulf of Venice

The potential of active microwave satellite observations of sea surface height (radar altimetry) and of sea surface wind (radar scatterometry) has been exploited for storm surge modeling purposes. The altimetry observations were assimilated into a storm surge model (SSM) with a dual 4D-Var system, in order to obtain the best possible surge level field as initial condition to reforecast runs. The scatterometer wind data were instead used to improve the accuracy of the wind fields of a global atmospheric model used as forcing to the SSM through a procedure that has been proved to be able to reduce the differences between the model winds and the scatterometer observations. Hindcast experiments were performed to test the sensitivity of the SSM to the altimetry data assimilation and to the modified wind forcing. Remarkable improvements of the storm surge level hindcast have been obtained for what concerns the modified model wind forcing, while encouraging results have been obtained with the altimeter data assimilation.

Techniques to extract the structure of the marine atmospheric boundary layer from SAR images

Two techniques suitable for detecting non-periodic backscatter structures in the SAR images are presented: the Variable Interval Space Averaging (VISA) and the two-dimensional Continuous Wavelet Transform (CWT2) analyses. Both methods have been tested over SAR images taken under different geophysical situations. Despite these techniques require the definition of subjective parameters and the knowledge of the spatial scales of interest, the results indicate that they succeed in the identification of the non-periodic backscatter structures present in the SAR images, which may be referred to the imprint of the atmospheric boundary layer. This will allow the quantitative estimate of the size, number and orientation of the backscatter structures. On the contrary, when periodic structures as the wind rolls are present, only the CWT2 yields good results. An interesting development of these technique will consist of the possibility to distinguish atmospheric from oceanographic features.

Improvements of storm surge forecasting in the Gulf of Venice exploiting the potential of satellite data: the ESA DUE eSurge-Venice project

ABSTRACT The northern Adriatic Sea is affected by storm surges, which often cause the flooding in Venice and the surrounding areas. We present the results of the eSurge-Venice project, funded by the European Space Agency (ESA) in the framework of its Data User Element programme: the project was aimed to demonstrate the potential of satellite data in improving storm surge forecasting, with focus on the Gulf of Venice. The satellite data used were scatterometer wind and altimeter sea level height. Hindcast experiments were conducted to assess the sensitivity of a storm surge model to a model wind forcing modified with scatterometer data and to altimeter retrievals assimilated with a dual 4D-Var system. The modified model wind forcing alone was responsible for a reduction of the mean difference between modelled and observed maximum surge peaks from −15.1 to −8.2 cm, while combining together scatterometer and altimeter data the mean difference further reduced to −6.0 cm. In terms of percent, the improvements in the reduction on the mean differences between modelled and observed surge peaks reaches 46% using only the scatterometer data, and 60% using both scatterometer and altimeter data.

Towards the wind direction determination in RADARSAT-2 polarimetrie images

Abstract The interpretation of SAR images of the sea surface is difficult, due to the complexity of the geophysics and of the interaction mechanisms between electromagnetic and sea waves. The determination of the wind direction is crucial for the evaluation of the wind speed, but its retrieval is still an open issue. One of the few methods able to extract the sea surface wind from SAR data only has been developed and extensively applied to Envisat ASAR images in the past years, using the two-dimensional wavelet transform to detect the backscatter signature related to locally coherent wind cells. A preliminary analysis on the applicability of this method to RADARSAT-2 fully polarimetric images has been conducted to verify if polarimetry may improve the detection of backscatter imprints related to the wind direction.