Narrowing the blind zone of the GPM dual-frequency precipitation radar to improve shallow precipitation detection over oceans

Abstract The Dual-frequency Precipitation Radar onboard the Global Precipitation Measurement core satellite cannot observe precipitation near the surface because of surface clutter contamination (blind zones). The blind zone is an obstacle to detect shallow precipitation. This study detects shallow precipitation by lowering the clutter free bottom (CFB) over the ocean as much as possible and using data that the original operational algorithm judged as being in blind zones. We used statistical methods to estimate the constant CFB for each scanning angle and each 1° × 1° grid of latitude and longitude. Here, the proportion of Ku-band radar (KuPR) radar reflectivity that exceeds the precipitation detection threshold (15.46 dBZ) is calculated for each scanning angle and each range bin. Because the precipitation frequency for all data is 15% or less, range bins with 90% or more of radar reflectivity exceeding the threshold value are estimated as blind zones in this study. As a result of lowering the CFB and improving the precipitation determination method to detect shallow isolated precipitation, the estimated precipitation frequency increased by more than 10% over all high-latitude oceans. The local increase rate reached over 100% in subtropical oceans near the west coasts of South America and Africa. Unlike the original operational algorithm or previous studies, this improved precipitation detection near the west coast of South America enabled the diurnal variation in the precipitation frequency to be detected for the first time.