Combined use of SAR and scatterometer for the study of the atmospheric boundary layer

The paper illustrates a possible way of combining SAR images over the sea with scatterometer derived wind fields, to study the spatial characteristics of the marine atmospheric boundary layer (MABL). Both instruments are active microwave radars, relying upon the same physical mechanisms of electromagnetic wave-sea surface interaction. Sensing the sea surface with different spatial resolutions, they make possible to investigate the horizontal and vertical structure of the MABL from the mesoscale (< 100 km) down to the intermediate (< 10 km) and small (< 2 km) scales. An example of the multi-scale description of the MABL is provided analysing overlapping SAR image and scatterometer wind field in the Mediterranean Sea. Environmental conditions of this case study were characterised by moderate wind (8 to 10 m s −1) and unstable air-sea conditions. The radar backscatter modulations exhibited by the SAR image reveal the presence of largescale atmospheric variations, orographic wind distortions and wind rolls. The majority of the large-scale modulations of radar backscatter are obviously related to the wind speed;some other large-scale feature, which does not seem directly related to the wind, could be explained by the weak Ekman vertical velocity, derived from the scatterometer wind field. The analyses of the wind rolls have been performed through the bi-dimensional spectrum of the SAR image, which shows a two-scale orthogonal wave system with wavelengths of 7 km and 1.5 km. The paper points out the importance of other parameters, such as the sea and air temperatures, in the interpretation of the SAR images and in the full exploitation of the scatterometer winds

Wavelet analysis applied to SAR images to detect atmospheric structures

The aim of this work is to evaluate the possibilities offered by the wavelet analysis in the study of the spatial structure of the radar backscatter, which is linked to the spatial structure of the marine atmospheric boundary layer. Continuous wavelet analysis has been applied to a SAR image of the sea surface, previously analysed by the more classical Conditional Sampling technique. The main problem coped with has been to select, among the large number of wavelet maps produced by the analysis (256), those containing the more energetic backscatter structures. Since the phenomenon imaged by SAR was the convective turbulence, a multiscale and quasi-periodic or intermittent process, a selection of wavelet maps according to their mean energy was unsuccessful. On the contrary, a method based on the calculation of the standard deviation of the less probable highest wavelet amplitudes led to a right selection of the wavelet maps and to a convincing reconstruction of the map of backscatter structures, which resulted similar to that provided through the Conditional Sampling

Towards the wind direction determination in RADARSAT-2 polarimetrie images

AbstractThe 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

Wavelet Analysis for Wind Fields Estimation

Wind field analysis from synthetic aperture radar images allows the estimation of wind direction and speed based on image descriptors. In this paper, we propose a framework to automate wind direction retrieval based on wavelet decomposition associated with spectral processing. We extend existing undecimated wavelet transform approaches, by including à trous with B3 spline scaling function, in addition to other wavelet bases as Gabor and Mexican-hat. The purpose is to extract more reliable directional information, when wind speed values range from 5 to 10 ms−1. Using C-band empirical models, associated with the estimated directional information, we calculate local wind speed values and compare our results with QuikSCAT scatterometer data. The proposed approach has potential application in the evaluation of oil spills and wind farms

An automatic procedure to retrieve the windfields from SAR Images

This paper describes an automatic procedure to download and process SAR images in the coastal areas, in order to get the wind field. The images processed are the Envisat ASARWide Swath. The methodology to retrieve b the wind fields is based on the 2-D Continuous Wavelet Transform, developed in Zecchetto and De Biasio [2002] and applied in Zecchetto and De Biasio [2008]. About three images per day are processed since May 2008. Co-location of the SAR derived winds with those provided by QuikSCAT and Metop will permit to validate the methodology over a large variety of atmospheric regimes. Once validation will be accomplished, the procedure described in this paper will be used to derive the wind fields for coastal meteorology applications