Analysis of the SAR-derived wind signature over extra-tropical storm conditions

Abstract

The monitoring and forecasting of tropical and extra-tropical storm tracks and intensities are strategic for the protection of coastal infrastructures and residents. The Synthetic Aperture Radar (SAR) can potentially be very useful for such purposes. In fact, its high spatial resolution and its capability to observe the sea surface in all weather conditions and at all times (regardless of the day or night-time) makes it one of the best candidate instrument for these goals. Several empirical forward models or geophysical model functions (GMFs), which relate the normalized radar backscatter cross section to the sea surface wind vector, have been developed and successfully used for a wide variety of scatterometer and SAR systems at different frequencies and polarizations. The GMFs have high accuracy under no-rain and low-to-moderate conditions, although for C-band radar systems good-quality winds are also derived under rainy conditions. For high wind speed measurements, the accuracy rapidly decreases due to saturation of the co-polarized backscattered intensity and the reduction of friction between the sea surface and the wind. An improved high-wind GMF for C-band and SAR spatial scales (most GMF developments are based on scatterometer data) as well as new high-wind GMFs for other frequencies (e.g., X-band) are required for the successful wind retrievals under tropical and extratropical cyclone conditions. In this study, the radar backscatter sensitivity to high winds (as given by the different GMFs) is revisited for both C-band and X-band systems, as well as for co-polarized and cross-polarized beams, using the NOAA P-3 flight winter campaign data from January-February 2017. During this campaign, the NOAA P-3 plane, equipped with several sea-surface wind sensing systems, i.e., the Step Frequency Microwave Radiometer (SFMR), the Imaging Wind and Rain Profiler (IWRAP), and dropsondes, underflew both Sentinel-1 (S-1) and Cosmo-SkyMed (CSK) satellite passes under storm conditions in the North Atlantic region. These extra-tropical storms are characterized by vast areas of nearly uniform very high-wind conditions, up to 30-35 m/s. The sensitivity analysis will therefore provide a comprehensive view of the main sensitivities of the radar backscatter (e.g., co-polarized, cross-polarized, polarization difference, polarization ratio) to relatively high winds, for a variety of incidence angles. Moreover, two wind retrieval approaches commonly used by the SAR community, i.e., the azimuth cut-off method and the combined radar backscatter and doppler centroid scheme, will be used to derive the sea surface wind field for the mentioned SAR scenes and validated against the mentioned NOAA wind data sources as well as collocated scatterometer and Soil Moisture Active Passive (SMAP) derived winds