Limitations of scatterometry high wind speed retrieval

Abstract

Over the last year, the University of Massachusetts (UMass) Microwave Remote Sensing Laboratory (MIRSL) has worked with the National Oceanic and Atmospheric Administration (NOAA) Office of Research Applications (ORA) and Hurricane Research Division (HRD) to better characterize the Ku and C band ocean backscatter response to high wind speeds (greater than 20 m/s). MIRSL and NOAA conducted aircraft experiments aimed at sampling the ocean backscatter during the 1999 hurricane season. Participating in Hurricane Research/Reconnaissance flights through Hurricanes Brett, Dennis and Floyd, MIRSL collected off nadir ocean backscatter measurements with its Ku and C band scatterometers, KUSCAT and CSCAT. Coincident wind speed measurements were provided by GPS dropsondes and the C-band Simultaneous Frequency Microwave Radiometer (SFMR-2). Coincident precipitation measurements were provided by the SFMR-2. During these flights, surface wind speeds from 1 m/s to 65 m/s were sampled with the scatterometers in precipitation free areas. This paper will focus on wind events exceeding 15 m/s. For the first time, a hard saturation in Ku and C-band backscatter is shown. The Ku-band and C-band backscatter measurements are mapped to two kilometer alongtrack pixels, each pixel containing seventy-two five degree azimuth bins. The coincident SFMR rain rate estimates are used to remove all data collected in the presence of precipitation. A three term Fourier cosine series is fit to each alongtrack backscatter pixel. These coefficients are plotted versus the coincident SFMR and GPS dropsonde wind speed estimates. At approximately 40 to 45 m/s the mean backscatter reaches a maximum value. Above this wind speed, the backscatter no longer increases and begin to decrease.