Ocean Surface Wind Field Retrieval in Tropical Cyclones Using GNSS-R Delay Doppler Map Sequences

Spaceborne Global Navigation Satellite System Reflectometry (GNSS-R) from the Cyclone Global Navigation Satellite System (CYGNSS) constellation can provide better spatial and temporal sampling of ocean winds in the tropics than conventional polar orbiting scatterometers. Although Delay Doppler Maps (DDMs) generated from GNSS-R capture information about regions of the ocean surface spanning up to 90 km by 90 km, these observations are not generally invertible except at the specular point and along the ambiguity-free line. CYGNSS wind speed products are single-point retrievals or combinations of single-point retrievals based upon a limited portion of the DDM around the specular point over which the wind speed can be assumed to be uniform. The size of this region sets the spatial resolution. Our research will show results from the processing of a sequence of DDMs, taking advantage of the information shared between them and corresponding to overlapping surfaces on the ocean. A forward scattering model that relates wind field to the full DDM is used in an extended Kalman filter (EKF) to process the DDM sequence and generate wind speed retrievals over a uniformly gridded ocean surface covering a 90 km by 90 km swath sampled at every 10 km. The sequential algorithm was applied to CYGNSS data from the 2021 Atlantic hurricane season spanning hurricanes Grace, Ida, and Sam. When compared against HWRF winds across the entire swath, the wind retrievals are shown to have an improvement over the CYGNSS L3 v3.0 gridded winds, improving the RMSE from 5.4 m/s (CYGNSS) to 3.1 m/s (EKF), while maintaining a similar performance to the CYGNSS L2 v3.0 winds when compared at specular points only. The performance is specially improved at higher wind speeds (>25 m/s). Comparisons against other data sources like Dropsonde, flight level winds, and Stepped Frequency Microwave Radiometer (SFMR) will also be shown, with more storms being incorporated into the comparisons as well.