Wind Direction Extraction from SAR in Coastal Areas

Stefano Zecchetto

doi:10.3390/rs10020261

Abstract

This paper aims to illustrate and test a method, based on the Two-Dimensional Continuous Wavelet Transform (2D-CWT), developed to extract the wind directions from the Synthetic Aperture Radar (SAR) images. The knowledge of the wind direction is essential to retrieve the wind speed by using the radar-backscatter versus wind speed algorithms. The method has been applied to 61 SAR images from different satellites (Envisat, COSMO-SkyMed, Radarsat-2 and Sentinel-1A,B), and the results have been compared with the analysis wind fields from the European Centre for Medium-range Weather Forecasts (ECMWF) model, with in situ reports and with scatterometer data when available. The 2D-CWT method provides satisfactory results, both in areas a few kilometres from the coast and offshore. It is reliable as it produces good direction estimates, no matter what the characteristics of the SAR are. Statistics reports a success in the SAR wind direction estimates in 95% of cases (in 83% of cases the SAR-ECMWF wind direction difference is < ± 20 ∘ , in 92 % < ± 30 ∘ ) with a mean directional bias B θ < 7 ∘ . The SAR derived wind directions cannot be said to be validated, as the data available at present cannot be really representative of the wind field in the coastal area. However, the figures given by SAR winds are highly valuable even not properly validated, providing an independent and unique view of the spatial variability of the wind over the sea, which is possible by using the 2D-CWT method to derive the wind directions.

Highlights

The possibility to get the wind field from the Synthetic Aperture Radar (SAR) images is extremely important in coastal areas, which are not covered at present by satellite wind data
The results are provided in terms of comparison between the SAR derived and the European Centre for Medium-range Weather Forecasts (ECMWF) wind directions averaged over the area imaged by SAR
The meteorological situation present at the SAR pass time, typical of this area, is shown in bottom left panel of Figure 7, which reports the wind field from ECMWF temporally interpolated at the SAR pass time: a strong southeasterly wind is blowing in the southern part of the area, while a northeasterly in its northern one

Summary

Introduction

The possibility to get the wind field from the Synthetic Aperture Radar (SAR) images is extremely important in coastal areas (let’s say the areas within ≈ 20 km from the coast), which are not covered at present by satellite wind data. SAR images of the sea provide evidence of a large set of geophysical phenomena: besides oceanographic features related to wind waves and swell, ocean internal waves and sea currents, atmospheric phenomena related to the wind rolls [3,4,5,6,7] and atmospheric gravity waves [8], not all of them locally related each other. These phenomena have different spatial scales and spatial layouts, besides being located in different areas, and often are all present in a SAR image

Objectives

Methods

Results

Discussion

Conclusion