Evaluation of Commercial and Public GNSS RO observations during the Atmospheric River Reconnaissance Campaigns

Abstract:

Among the new commercial Smallsat datasets, the Spire Global Navigation Satellite System (GNSS) Radio Occultation (RO) observations add significantly to current operational and research RO missions. Increasing the density of RO with Smallsats as well as increasing the quality and SNR of RO with the next generation COSMIC-2 constellation is expected to improve analyses of the atmospheric state, which is crucial for numerical weather forecasts. RO provides high vertical resolution observations of water vapor which could be useful in determining the depth of a moist layer and its vertical structure, for example in atmospheric rivers (ARs). These bands of enhanced low-level horizontal water vapor transport direct moisture from the tropics to the west coasts of continents, often resulting in heavy precipitation. The AR Recon 2019 campaign released dropsondes in six intensive observation periods that took place over the northeastern Pacific. Spire provided RO profiles to study these cases as part of the NASA Smallsat pilot evaluation. The Spire RO profiles are compared to operational RO data distributed by the National Center for Environmental Prediction in their “GPSROBUFR” file, as well as dropsondes and the ERA5 reanalysis.

The ability of the RO datasets to resolve moisture in the low-level jet, subsiding air poleward of the front, and ascending moisture in the warm sector and core of the AR is described. Quantitative comparisons are made with dropsonde data near occultations in time and space. RO is also evaluated with data assimilation (DA) experiments that use the DA system quality control as an objective measure of data quality. The Spire data has comparable performance and number of rejected observations to other constellations at upper tropospheric levels. However, the apparent similarity in performance in the lowest levels may be an artifact of the unrealistically smooth nature of the Spire RO profiles. This should be investigated further before relying on the data for improved lower tropospheric analyses.

The current negotiated NASA and NOAA agreements with Spire require each agency to purchase the data separately, and prohibits the data from being distributed openly beyond NASA and NOAA for research purposes. There is no current agreement negotiated with the National Science Foundation for access to the data.