Atmospheric humidity and temperature sounding from the CubeSat TROPICS mission: Early performance evaluation with MiRS

Abstract

Atmospheric humidity and temperature are the two most important geophysical variables for numerical weather forecasting and relevant Earth studies, contributing to the reduction of over 60% forecast errors. Conventional large satellite microwave radiometers provide the primary sounding data of moisture and temperature, with the V-band (near 50 GHz) and G-band (near 183 GHz) as the main spectra. In recent years, CubeSats are rapidly developing with compact sizes, novel designs, and low cost. The NASA TROPICS is an ongoing CubeSat constellation mission for studying tropical meteorology and storm systems, with the first Pathfinder satellite launched on 30 June 2021, which will provide a median revisit rate better than 60 min. TROPICS has innovative channels at F-band (near 118 GHz) and 205 GHz, providing a new perspective for atmospheric sounding. In this study, we have extended the NOAA MiRS system and applied it to TROPICS Pathfinder early-phase, provisionally calibrated data, focusing on moisture and temperature. We have examined the retrieval quality and compared with the ECMWF analysis and MiRS retrievals from NOAA-20 ATMS. An experiment of subsetting ATMS to match up with TROPICS channels is also conducted. Here we focus on atmospheric humidity and temperature retrievals, since they are of primary importance for microwave sounders and is tied to the core of the TROPICS mission along with precipitation retrieval. Compared to traditional channels, TROPICS F-band and 205 GHz provide new information content with distinct sensitivity to moisture and hydrometeors, showing for example over 20 K larger dynamic range of brightness temperature at 205 GHz compared to 190 GHz. The retrieved total precipitable water compared to ECMWF has a correlation coefficient of 0.955, 0.985, and 0.977 for TROPICS, ATMS, and ATMS subset, respectively. For water vapor profile, the standard deviation of retrieval to ECMWF is 0.93, 0.76, and 0.80 g/kg for the three experiments, respectively, and regarding temperature, it is 2.5, 1.5, and 1.6 K, respectively. The retrieval shows dependence on surface type and clouds, where land and cloudy conditions degrade retrieval compared to ocean and clear condition. The early performance is promising, and the successful MiRS extension paves the way to explore improved data when the calibration validation is complete and from the forthcoming constellation.