Estimating Winds from Synthetic Aperture Radar under Typhoon Conditions

Abstract

LONG-TERM GOALSThe Impacts of Typhoons on the Ocean in the Pacific (ITOP) program is a multi-national field campaign that aims to study the ocean response to typhoons in the western Pacific Ocean. The main goals are to understand the formation and dissipation of cold wakes and how ocean eddies affect typhoons. CMRE's activities within ITOP are related to SAR wind retrieval and therefore will be helping to better understand air-sea fluxes under extreme wind speeds and improve the knowledge and forecast of surface wave field under typhoons as well as the Typhoon forcasting. OBJECTIVESCMRE (former NATO Undersea Research Center) is part of the ITOP SAR Team working on extracting wind and wave information from synthetic aperture radar (SAR) data. Their major objectives are the development and implementation of a SAR Typhoon Processing System (SARTyPS) for near real time processing of SAR data with respect to estimating wind and sea state under tropical cyclone conditions. It is anticipated to incorporate the SAR-retrieved information into numerical models in particular with respect to tropical cyclone forecasting.The main objectives for CMRE are the development of algorithms that are capable of extracting wind information from SAR images in near real time and fully automated. The SAR activity within ITOP is a close cooperation with the Center for Southeastern Tropical Advanced Remote Sensing (CSTARS) of the University of Miami, General Dynamics Advanced Information Systems (GDAIS) and the Applied Physics Laboratory (APL) of the University of Washington. APPROACHIn a first step prior to wind retrieval all SAR data have to be accurately calibrated for the normalized radar cross section (NRCS). In addition all acquisitions performed in the scanning mode also have to be corrected for scalloping as well as beams and nadir ambiguities. SAR data, which were acquired at cross polarization (pol) have to be corrected for the contribution of the noiseequivalent sigma naught (noise floor) of the system to the NRCS. Following these data calibration steps the wind retrieval can be undertaken. In case of SAR data collected at C-and X-band in co-pol, meaning vertical (V) or horizontal (H) polarization in transmit and receive, the detailed methodology