Applicability of the iterative backward retrieval method for the GPM dual-frequency precipitation radar

The dual-frequency precipitation radar (DPR) on the core satellite of the Global Precipitation Measurement (GPM) mission will measure the radar reflectivity factor in the Ku-band and Ka-band. A rain-rate retrieval algorithm that does not require surface reference was developed (called the MA04 method). However, MA04 cannot give the true solution in some cases of heavy rainfall. MA04 is a simplified version of the iterative backward retrieval method (IBRM) and the IBRM is equivalent to the forward retrieval method with a constraint. The purpose of this study is to clarify the essential conditions under which the IBRM and MA04 can give the true solution.