Improved Revisit Times of Microwave Observations of Precipitation: Recent Scientific Results from the Temporal Experiment for Storms and Tropical Systems (TEMPEST) Missions

Small satellite constellations provide the potential to improve spatiotemporal resolution of microwave observations of precipitation from low-Earth orbit.  Shorter revisit times are essential to improve understanding of the development and evolution of extreme precipitation systems, in turn improving numerical weather prediction and accuracy of parameterization of extreme weather events in global climate models.  To this end, Temporal Experiment for Storms and Tropical Systems (TEMPEST) was proposed in 2013 as a constellation of 6U CubeSats in LEO to provide frequent observations of rapidly developing storms.  TEMPEST-D, the resulting NASA Earth Venture Technology Mission, demonstrated the first global observations from a multi-frequency microwave radiometer on a CubeSat for nearly three years from 2018 to 2021. TEMPEST-D exceeded expectations for scientific data quality, instrument calibration, radiometer stability, and mission duration. TEMPEST-D brightness temperatures were validated using double-difference intercomparison with scientific and operational microwave sensors, including GPM/GMI and four Microwave Humidity Sounders (MHS), operating at similar frequencies to TEMPEST-D channels at 87, 164, 174, 178 and 181 GHz. TEMPEST-D performance was shown to be comparable to or better than much larger operational sensors, in calibration accuracy, precision, stability and instrument noise, during its nearly 3-year mission.

A nearly identical TEMPEST flight spare was produced by JPL alongside TEMPEST-D for risk reduction.  The TEMPEST flight spare was made available to the U.S. Space Force to demonstrate low-cost space technologies for improving global weather forecasting. TEMPEST was then integrated with the Compact Ocean Wind Vector Radiometer (COWVR) produced by NASA/JPL for the U.S. Air Force. COWVR and TEMPEST were launched together as the Space Test Program – Houston 8 (STP-H8) on December 21, 2021, and deployed on the ISS Japanese Experiment for at least 3 years of operations. COWVR and TEMPEST have performed complementary observations of Earth’s oceans and atmosphere from the ISS nearly continuously since January 8, 2022. Atmospheric retrievals of water vapor profiles, clouds, and precipitation from COWVR/TEMPEST-H8 are performed collaboratively by JPL and Colorado State University.

Atmospheric inversion techniques have been developed to retrieve water vapor altitude profiles, as well as single-layer cloud liquid water and cloud ice water, from TEMPEST brightness temperatures, using ECMWF Reanalysis v5 (ERA5) data as an initial guess. These retrievals are enhanced through the inclusion of geostationary infrared data from GOES-16 ABI channels, increasing the number of levels and reducing the error of water vapor retrieval, particularly in the upper troposphere. 

The accuracy and precision of TEMPEST-D brightness temperatures have previously been validated using clear-sky oceanic observations.  Recent studies have extended the validation of both TEMPEST-D and TEMPEST-H8 to include observations of tropical cyclones, hurricanes, and typhoons using GPM-GMI passive microwave brightness temperatures and GPM-DPR active microwave vertical cumulative reflectivity.  These passive/active microwave intercomparisons employ techniques developed for quantitative evaluation of the cross correlation between TEMPEST-D and RainCube observations of tropical cyclones, hurricanes, and typhoons.  Such passive/active microwave observations also provide the basis for the development of surface rain rate estimates and retrieval of the vertical structure of precipitation from combined TEMPEST and DPR observations.