Sea surface wind measurement enhanced by stepped frequency microwave radiometer: A comprehensive approach to validation and sensitivity analysis

The National Institute of Atmospheric Sciences (NIMS) employs its KingAir 350H research aircraft for various missions, including the "Severe Weather" (SW) mission. This mission aims to enhance understanding of severe weather events like heavy rain, snowfall, and typhoons through advanced observations, ultimately improving numerical model accuracy. A key instrument aboard the aircraft is the Stepped Frequency Microwave Radiometer (SFMR). Mounted underneath, this C-band radiometer measures ocean brightness temperature to calculate sea surface wind speed and rain rate. Our study compared wind speeds derived from the SFMR with those obtained from dropsondes deployed along the aircraft's flight path. We also investigated the sensitivity of SFMR wind speed estimates to sea surface temperature and salinity, both serving as initial input data. Between 2021 and 2023, 118 SW-mission flights around the Korean peninsula. SFMR data revealed an average wind speed of 8.7 m/s and a maximum of 22.1 m/s. Sea surface temperature and salinity data from buoys and research stations were interpolated to 0.1˚ intervals for sensitivity analysis. Calibration coefficients derived from a dedicated calibration flight on October 26, 2022, were applied to re-calculate wind speeds, which were then compared with pre-calibration estimates and wind speeds obtained from dropsondes at 30m, 150m, and 500m above the sea surface. Notably, the strongest correlation was observed between SFMR-derived wind speeds and those at 500m. This study demonstrates the potential of NIMS aircraft SFMR data for accurate sea surface wind speed measurements, particularly when employing calibration and considering the influence of lower-level wind patterns. Further research can refine these techniques and enhance our understanding of air-sea interaction.