On the Divergence and Vorticity of Sar Derived Wind Fields

The presented study was carried out in the framework of the AO project COTAR (ID 3183). A new approach is described to extend the point by point approach taken in most previous studies about SAR wind retrieval. The idea is to analyse the spatial dynamics of the SAR derived two dimensional (2-D) wind fields by estimating quantities like the 2-D divergence or vorticity, which are well known key parameters in the description of atmosphere dynamics. These quantities have a clear physical interpretation as they are key elements in the numerical modeling of the atmosphere. The presented analysis tool is based on a new SAR wind retrieval technique, which ensures a certain smoothness of the resulting 2-D wind field. Different examples of atmospheric features imaged by the ENVISAT ASAR and ERS-2 SAR are discussed including stable conditions, atmospheric fronts and hurricanes. For some of the case studies a combination of ERS-2 and ENVISAT scenes taken with 30 minutes time gap are used. These pairs allow a better separation of atmospheric and oceanic features and also provide some additional information of the wind field evolution in time. They can e.g. be used to test the Taylor hypothesis, which is often used in studies on mesoscale atmospheric dynamics. The presentation summarizes the consequences of the investigation for SAR wind field retrieval approaches. In particular a proposal is made to use a priori knowledge on parameters like convergence as a regularisation condition in SAR wind measurement schemes.