Comparison of rain fractions over tropical and sub‐tropical ocean obtained from precipitation retrieval algorithms for microwave sounders

We compare the fractional occurrence of precipitation (rain fraction) over ocean derived using the Global Satellite Mapping of Precipitation algorithm for the Advanced Microwave Sounding Unit (GSMaP_AMSU) and the Microwave Surface and Precipitation Products System Day 2 rainfall algorithm (NOAA_AMSU) for the Kwajalein radar site and over tropical and subtropical ocean. The rain fractions of GSMaP_AMSU and NOAA_AMSU are lower than that of Kwajalein radar estimates because of failure to detect areas of light rain. Over tropical and subtropical ocean, the rain fraction of GSMaP_AMSU is closer to that obtained using a microwave imager (MWI) and little different from that of Tropical Rainfall Measuring Mission Precipitation Radar (PR) data, whilethe rain fraction of NOAA_AMSU is much smaller than that obtained using MWI or PR data. In the case of the edge of the South Pacific Convergence Zone where the PR observes scattered shallow rain, while NOAA_AMSU fails to detect the scattered rain, GSMaP_AMSU detects the scattered rain through consideration of the scattering index, which is the difference in brightness temperature (Tb) between 89 and 150 GHz. Although the scattering index is designed on the basis that Tb decreases in response to scattering by precipitation at these frequencies and increases rapidly with frequency, there are emission and scattering regimes. Furthermore, the scattering index also responds to emission in light rain with a low concentration of cloud liquid water. As a result, the light rain pixel can be detected using the scattering index to take advantage of the emission signature from raindrops.