Multi-Scale LG-Mod Analysis for a More Reliable SAR Sea Surface Wind Directions Retrieval

Abstract

An improved version of the Local-Gradient-Modified (LG-Mod) algorithm for Sea Surface Wind (SSW) directions retrieval by means of Synthetic Aperture Radar (SAR) images is presented. A “local” multi-scale analysis of wind-aligned SAR patterns is introduced to improve the LG-Mod sensitivity to SAR backscattering modulations, occurring locally with various spatial wavelengths. The Marginal Error parameter is redefined, and the adoption of the Directional Accuracy Maximization Criterion (DAMC) allows for the novel Multi-Scale (MS) LG-Mod to automatically select the local processing scale that may be regarded as optimal for pattern enhancement, once a discrete set of scales has been already fixed. Hence, this optimal scale successfully gives evidence to guarantee the best achievable local direction estimation. The assessment of the MS LG-Mod is carried on both simulated SAR images and a Sentinel-1 (S-1) dataset, consisting of 350 Interferometric Wide Swath Ground Range Multi-Look Detected High-Resolution images, which cover the region of the Gulf of Maine. In the latter case, the removal of artifacts and non-wind features from SAR amplitudes is mandatory before directional estimations. In situ wind observations gathered by the National Oceanic and Atmospheric Administration National Data Buoy Center (NOAA NDBC) are exploited for validation. The findings obtained from S-1 data confirm the ones from simulated patterns. The MS LG-Mod analysis performs better than each single-scale one in terms of both percentages of reliable directions and directional Root Mean Square Error (RMSE) values achieved.

A Review for Retrieving Wind Fields by Spaceborne Synthetic Aperture Radar

“…It cannot measure multiple azimuth angles, which makes it impossible to directly retrieve WS and direction information [9]. Based on the typical features of wind streaks, many methods have been proposed, such as Fourier transform [45][46][47], LG [38,48], and wavelet analysis [49,50]. LG has been widely exploited because the result of the WD has higher resolution and precision.…”

Uncertainty Analysis in SAR Sea Surface Wind Speed Retrieval through C-Band Geophysical Model Functions Inversion

“…Those S-1 images characterised by a clear visibility (i.e., by human eye) of wind induced linear patterns, such as those from WSs [32], BLRs [33,44] or any other wind aligned patterns, hereafter "wind rows", are chosen aiming at extracting the local SSW direction exploiting such SAR signatures. In particular, the ultimate multi-scale version of the LG-Mod algorithm [40] is applied to serve this purpose.…”

“…Among the several methods for SSW direction extraction from SAR data [34][35][36][37][38][39], the local gradient-modified (LG-Mod) method [11] and its multi-scale (MS) version [40], developed by the same authors of this work, are suitable to locally provide not only the mean wind direction but also its related uncertainty within each examined WVC. The capability of the LG-Mod algorithm to furnish a spatial distribution of both SSW direction and the related uncertainty can be fruitfully exploited, whereas NWP models give all the wind direction estimates with a constant maximum value of the uncertainty (e.g., 20 • for ECMWF [12]).…”

“…The capability of the LG-Mod algorithm to furnish a spatial distribution of both SSW direction and the related uncertainty can be fruitfully exploited, whereas NWP models give all the wind direction estimates with a constant maximum value of the uncertainty (e.g., 20 • for ECMWF [12]). As shown in [11,40], each wind direction map can be accompanied by an associated uncertainty map. As a consequence, following the above-cited approach, the spatial distribution of both the SSW speed, along with the corresponding uncertainty, may be generated, in turn, in order to fulfil the further lack of wind accuracy information, especially near the coast.…”

“…This may affect the quality flag provided by Copernicus. On the contrary, the definition of the SSW direction uncertainty provided by the MS LG-Mod depends only on the directional content evaluated from the NRCS in the WVC after the masking of unusable points [40], and then independently from any external NWP model reliability in such areas. As described in the paper, the SSW speed uncertainty does not depend on any external wind data source as well.…”