Retrieval of Rain Rates for Tropical Cyclones From Sentinel-1 Synthetic Aperture Radar Images

The purpose of this study was to develop a method for retrieving the rain rate fromC-band (similar to 5.3 GHz) synthetic aperture radar (SAR) images during tropical cyclones (TCs). Seven dualpolarized (vertical-vertical [VV] and vertical-horizontal [VH]) Sentinel-1 (S-1) SAR images were acquired in the interferometricwide (IW) swath mode during the Satellite Hurricane Observation Campaign. These images were collocated with rain rates measured by the Stepped-Frequency Microwave Radiometers onboard National Oceanic and Atmospheric Administration aircraft. Wind speeds were retrieved from theVH-polarized SAR images using the geophysical model function (GMF) S1IW.NR. We determined the difference between the measured normalized radar cross section (NRCS) based on VV-polarized SAR and the predicted NRCS derived using the GMF CMOD5.N forced with wind speeds retrieved from VH-polarized SAR images. Rain cells were identified as regions in the images where the NRCS difference was greater than 0.5 dB or smaller than -0.5 dB. We found that the difference in the NRCS decreased and theVH-polarized wind speed increased with increasing rain rate. Based on these findings, we developed an empirical function for S-1 SAR rain retrieval in a TC, naming it CRAIN_S1. The validation of the CRAIN_S1 results with Tropical Rainfall Measuring Mission data resulted in a root mean square error of 0.58 mm/h and a correlation of 0.89. This study provides an alternate method for rain monitoring utilizing SAR data with a fine spatial resolution.

Automated summary

The purpose of this study was to develop a method for retrieving rain rate from C-band synthetic aperture radar (SAR) images during tropical cyclones (TCs). Seven dual-polarized Sentinel-1 SAR images were used, and rain rates measured by Microwave Radiometers were compared. Wind speeds were retrieved from SAR images using a geophysical model function. Rain cells were identified based on differences in normalized radar cross section (NRCS). An empirical function named CRAIN_S1 was developed for rain retrieval. Validation of the results showed a root mean square error of 0.58 mm/h and a correlation of 0.89.