Fine-Scale Winds in Super Intense Hurricanes Observed from Spaceborne Synthetic Aperture Radar (SAR)

We present high resolution remote sensing analysis of historical strongest hurricanes from synthetic aperture radar (SAR), with focuses on the fine scale wind speed retrieval, quality flag and rain correction etc. Wind speed retrieval under high wind conditions has become successful, benefitting from the high quality of RADARSAT-2 SAR dual-polarization mode observations, and the recent development of geophysical model functions for cross-pol and co-pol data. Many studies have confirmed the promising capability of SAR to provide hurricane winds over the ocean surface, pointing to a potential application for SAR winds in operational hurricane monitoring and forecasting. However, the quality of hurricane retrieved winds under strong precipitation areas is always a concern. In this study, we have analyzed recent super intense hurricanes, including Patricia (2015), Irma (2017), Maria (2017) during their most intense stages. We firstly demonstrate the effect of precipitation on SAR backscattered signals, based on RADARSAT-2 SAR images and collocated rain measurements from hurricane reconnaissance missions of the National Hurricane Center (USA). Secondly, we introduce a novel methodology to evaluate rain effects in the retrieved SAR winds from SAR measurements themselves, based on the different imaging mechanisms of the radar backscattering from the ocean surface of cross-pol and co-pol SAR observations. The retrieved rain area was validated by GOES satellite radiometer measurements and SFMR hurricane hunter aircraft data. A quality index was introduced for each wind cell from a given SAR image to assess the quality of the retrieved wind speed. Furthermore, we present a methodology to correct the winds that are of poor quality (due to rain contamination) within the hurricane eyewall center area, based on a theoretical physical profile along each radial direction. The rain corrected hurricane wind fields are validated by the hurricane surface wind analysis product (HWIND). The quality flagged SAR winds can be used as ground truth for hurricane surface winds, and the rain corrected wind field can be helpful for operational hurricane forecasting and related marine forecasts. The proposed methodologies are easy to be adapted in operational applications. The capability of SAR to monitor intense hurricanes over the ocean surface is of vital importance, for improved predication of hurricane tracks and intensities, especially under present climate change projections which point to increased number of intense hurricanes in the future.