Similarities and differences of SAR derived wind fields using two different methods: The local gradient and the continuous wavelet transform methods

We present a discussion about the performances of two different methods to derive the wind field from SAR images. These are the method based on the Continuous Wavelet Transform (CWT2) (Zecchetto & De Biasio 2002, 2007) and on the estimation of the Local Gradient (LG) (Koch 2004). The CWT2 method extracts the backscatter signatures in the SAR image likely related to the structure of the marine atmospheric boundary layer; it derives the aliased wind direction from direction of the major axis of the cells-like detected structures and then resolves the ambiguity looking at the backscatter distribution inside them. The LG method divides the image into sub images, depending on the space grid over which the wind is requested; then it defines four image operators which are applied on the images to produce a map of valid points on which the local gradient directions are computed. The local wind direction is orthogonal to the more frequent gradient direction. The wind intensity is computed applying the CMOD4 or CMOD5 models for C-band scatterometer. The results of the two methods are compared with satellite scatterometer winds. They are relevant to ERS images taken over the Mediterranean Sea.