Discrimination of Light Precipitation from COWVR + TEMPEST, GMI, and ATMS Passive Microwave Radiometer Observations

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Abstract In the near future, the number of low-cost, SmallSat/CubeSat-sized passive microwave (MW) radiometric observations available from low-Earth-orbiting (LEO) satellites is expected to increase substantially, especially high-frequency (HF) sounders (near or exceeding 90 GHz). The number of sensors that include the constant-incidence angle, dual-polarization low-frequency (LF) channels (near or below 37 GHz) will remain steady or decline. Maintaining LF sensing capabilities in the global satellite constellation for precipitation is one of the key recommendations of the International Precipitation Working Group (IPWG). Physically, the use of HF-only sensors for precipitation skews the precipitation estimates toward indirect, ice-based (scattering) retrievals rather than the precipitation closer to the surface. Over certain Earth surfaces, a portion of light precipitation (2 mm h−1 or less) may remain undetected, which has ramifications for global satellite-based precipitation products. Observations from the SmallSat-sized Compact Ocean Wind Vector Radiometer (COWVR) and the Temporal Experiment for Storms and Tropical Systems (TEMPEST) duo are compared with the Global Precipitation Measurement (GPM) Microwave Imager (GMI) and the Advanced Technology Microwave Sounder (ATMS) for discriminating light precipitation over a variety of Earth surface conditions. Precipitation is discriminated using a surface emissivity–based transformation applied to selected channel combinations from each of these sensors using the GPM dual-frequency precipitation radar (DPR) products as precipitation references. The results presented in this study highlight the importance of the LF channels for detection of light precipitation over ocean and lightly vegetated surfaces and 50-GHz temperature sounding channels over heavily vegetated surfaces. Significance Statement In the future, the number of low-cost, SmallSat or CubeSat-sized passive microwave sensors in low-Earth orbit is expected to increase substantially, especially high-frequency sounders operating near or above 90 GHz. Over certain Earth surfaces, the exclusion of low-frequency channels below 37 GHz implies that a portion of light precipitation (less than 2 mm h−1) may remain undetected, which has ramifications for global satellite-based precipitation products. This study emphasizes the need to maintain low-frequency sensing capabilities in the global satellite constellation to represent the range of precipitation intensity. Selected combinations of low- and high-frequency channels from sensors onboard a SmallSat highlight the benefit of low-frequency channels to improve the detection of light precipitation over ocean and vegetated surfaces.

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