Advancing Precipitation Measurements from the NASA TROPICS Mission.

A major challenge for measuring precipitation from space is the need to properly capture the spatial and temporal variability of precipitation. This requires that all available observations from precipitation-capable sensors are exploited. Passive microwave sensors are fundamental in providing reliable observations since they relate to the precipitation particles themselves. Passive microwave sounding instruments have been developed for cubesats, such as for the Time-Resolved Observations of Precipitation structure and storm Intensity with a Constellation of Smallsats (TROPICS) mission. The TROPICS pathfinder was launched in June 2021 into a polar orbit, carrying the TROPICS Millimeter-wave Sounder (TMS) with frequencies ranging from 91.655 to 204.8 GHz. The Precipitation Retrieval and Profiling Scheme (PRPS) has been adapted for use with the TMS based upon a 4-channel ATMS-DPR a priori database. This surrogate database is built upon coincident observations from the NPP and NOAA-20 ATMS sensors and the precipitation retrievals from the GPM DPR. The main limitation of the current database is that it is limited to just four closely matched channels, one at 89/91 GHz and the three around the 183.31 GHz water vapour absorption lines. The limited number of TMS vs DPR observations early in the mission precluded their use to generate a viable database: however, a reasonable number of coincident observations are now available to provide an insight into the use of the full range of TMS channels in the retrieval scheme. A total of about 1200 coincident TROPICS/GPM overpasses are available since launch, providing about 2 million matched footprints. Comparisons between the full 12-channel TMS database and the 4-channel ATMS database show a good degree of improvement, although refinement of channel weighting is deemed necessary, not least due to the high inter-channel correlations present within the 118 and 183 GHz channel groups. However, validation of the resulting precipitation products against the IMERG precipitation product indicate that the retrievals from the TMS are comparable as those from similar cross-track sounding instruments (such as GPROF retrievals from the MHS and ATMS).