Development of spaceborne dual frequency precipitation radar for the global precipitation measurement mission

It is essential to measure global precipitation not only for the research of the climate change but also for the water resources management. In order to satisfy the requirements, the Global Precipitation Measurement (GPM) mission was proposed jointly by US and Japan. The basic concept of the GPM is to provide three hourly global precipitation maps using eight constellation satellites equipped with microwave radiometers and a core satellite equipped with the Dual-frequency Precipitation Radar (DPR) and a microwave radiometer. The DPR that uses radio waves of 14 and 35 GHz is now being developed in Japan. The DPR will observe three-dimensional precipitation structure and will provide essential data for microwave rain retrieval. GPM is partly a follow-on mission of the Tropical Rainfall Measuring Mission (TRMM), but the GPM will extend the observation to cold regions where solid precipitation frequently exists. Rain retrieval algorithms that use the DPR data are also being developed. Using two wavelength data, two parameters in the raindrop size distribution could be retrieved, which would result in precise rain retrieval. The retrieval of solid precipitation rate is another challenge. Several algorithms including a combination with the microwave radiometer would be applied to the DPR. It is important for the DPR algorithm validation to compare between precipitation rate through the calculation of DPR algorithm and that of the directly observed precipitation rate over the validation site. For this purpose, the most important and difficult issue is to construct the database of the physical parameters for the precipitation retrieval algorithms of DPR from the ground-based data using well-calibrated instruments.