Assessing TROPICS Pathfinder's Precipitation Retrieval Performance Through the NOAA Microwave Integrated Retrieval System (MiRS)

The NASA TROPICS mission encompasses a fleet of five CubeSats equipped with microwave radiometers, dedicated to investigating tropical meteorology and storm systems. Among these, the inaugural launch took place in 2021, known as the TROPICS Pathfinder, which adopted a sun-synchronous orbit. The remaining four were launched in 2023, following a low-inclination orbit of 30 degrees. In a departure from conventional microwave sounders, the TROPICS Microwave Sounder (TMS) employs F-band frequencies near 118 GHz and G-band channels reaching up to 205 GHz. To facilitate the retrieval of atmospheric and surface geophysical variables, we have expanded the capabilities of the Microwave Integrated Retrieval System (MiRS), a state-of-the-art 1D variational (1DVAR) algorithm developed at the NOAA Center for Satellite Applications and Research (STAR) for microwave radiometry (Yang et al. 2023). The performance of TROPICS in retrieving moisture and temperature stands at a level comparable to the ATMS. Specifically, the standard deviation for moisture retrieval is 0.93 g/kg for TMS and 0.76 g/kg for ATMS when compared against ECMWF analysis. For atmospheric temperature, the standard deviation stands at 2.5 K for TMS and 1.5 K for ATMS.

In this investigation, we center on assessing the derived precipitation data. The results highlight the efficacy of TROPICS in capturing the dynamics, spatial distribution, and seasonal patterns of precipitation. For instance, the 2021 Hurricane Ida landfall and its precipitation patterns are distinctly depicted by TROPICS. TROPICS consistently exhibits a well-distributed precipitation across CONUS when compared to Stage IV data. In addition to case studies, we perform a quantitative approach and compare TROPICS with other precipitation datasets, including N20 ATMS, GPM DPR, GMI, and Stage IV. This is conducted through collocating observations from TROPICS and these alternate products. Statistics and metrics such as the correlation coefficients and root mean square errors (RMSE) are analyzed. Specifically, when contrasted with GMI as a reference, TROPICS' rainfall yields a correlation coefficient of 0.5 and an RMSE of 2.0 mm/h, while N20 ATMS shows figures of 0.61 correlation and 1.5 mm/h RMSE. For Graupel water path (GWP), TROPICS records a correlation of 0.52 and an RMSE of 0.53 kg/m², in contrast to N20 ATMS, which demonstrates a correlation of 0.56 and an RMSE of 0.47 kg/m². Regarding Cloud liquid water (CLW), the metrics stand at 0.5 correlation and 0.08 mm for TROPICS, with N20 ATMS having 0.71 correlation and 0.07 mm, respectively. When compared against GPM DPR rainfall, TROPICS exhibits a correlation of 0.31 and an RMSE of 2.97 mm/h, whereas ATMS scores 0.41 correlation and 3.14 mm/h RMSE. Noteworthy is a marginal degradation in precipitation retrieval over land than over the ocean. This preliminary analysis underscores the value of the TROPICS mission for precipitation measurement and demonstrates the successful integration of TROPICS processing capability within the MiRS retrieval algorithm framework.

Reference: Yang J. X., Y. K. Lee, C. Grassotti, K. Garrett, Q. Liu, W. Blackwell, R. Vincent Leslie, T. Greenwald, R. Bennartz, S. Braun, Atmospheric Humidity and Temperature Sounding from CubeSat TROPICS Mission: Early Performance Evaluation with MiRS, Remote Sensing of Environment, 2023.