Evaluation of All-weather Sea Surface Wind Speed Product from GCOM-W/AMSR2 Microwave Radiometer Under Tropical Storm Conditions

The Global Change Observation Mission – Water (GCOM-W) satellite was launched by the Japan Aerospace Exploration Agency (JAXA) in May 2012. It carries the Advanced Microwave Scanning Radiometer 2 (AMSR2). The all-weather sea surface wind product (ASW) was produced as a research product of AMSR2 using the 7 and 10 GHz channels to avoid contaminations due to heavy rain and provides data in high wind speed ranges, such as tropical cyclones. Spatial resolution of the product is 50 km.

The AMSR2 ASW product was evaluated by using data obtained around hurricanes by airborne Stepped-Frequency Microwave Radiometers (SFMR), which were calibrated by in situ measurements by wind using dropsondes. The data were processed and provided by NOAA/NESDIS/STAR. The SFMR wind data were smoothed along the flight track over 50 km to match with the spatial resolution of AMSR2 ASW product. The AMSR2 ASW product and smoothed SFMR data were collocated allowing spatial and temporal separations of 15 min and 30 km, respectively. Only data points of wind speed higher than 15 m/s were utilized in the analysis. The bias and root-mean-square (rms) difference of the comparison between the AMSR2 ASW product and smoothed SFMR measurements were -1.94 and 3.26 m/s, respectively. The value of rms difference satisfies the requested accuracy of 7 m/s. No systematic trends depending on wind speed or rain rate were discernible.

To assess spatial distribution of high winds around tropical cyclones, the 50-kt radius derived from the ASW wind fields also compared with the Japan Meteorological Agency (JMA) typhoon best-track data. The two data sets showed reasonable agreement. It is shown that the AMSR2 ASW product has a reliable accuracy under extreme wind conditions in hurricanes. The data will be a powerful tool for studies of air-sea interactions under tropical cyclones.