Intercomparison of Hyperspectral Infrared Sounders with Simulated Radiances from GNSS-RO Inputs

Radio occultation (RO) profiles, obtained using Global Navigation Satellite System (GNSS) signals refracted by the atmosphere, are an important component in numerical weather prediction (NWP), long-term climate studies, and validation of other sounding products. Similarly, hyperspectral infrared (IR) sounders also provide accurate and stable observations of the atmosphere for NWP and calibration of other instruments. Hyperspectral IR sounders on orbit include the Infrared Atmospheric Sounding Interferometer (IASI) on board MetOp satellites, such as MetOp-C that launched in November 2018, and the Cross-track Infrared Sounder (CrIS) instruments onboard the Suomi National Polar Partnership (NPP) and NOAA-20 satellites. In June of 2019, the CrIS on-board Suomi NPP was reconfigured to use redundant electronics to rectify an anomaly that resulted in the loss of the midwave infrared (MWIR) band in March of that year and recover the full capabilities of the instrument. With the introduction of new instrument components as part of the Side-2 electronics, the Suomi NPP CrIS radiance data must be recalibrated and validated despite many years of operational data production. In this intercomparison study, GNSS RO data provides an accurate, stable, and high vertical resolution reference against which IR soundings from the recently launched MetOp-C IASI and the recently commissioned S-NPP CrIS can be validated. Differences between radiances observed by the IR sounders and those simulated by the Community Radiative Transfer Model (CRTM) using GNSS RO derived profiles as an input are analyzed and the biases characterized. GNSS RO data used comes from the GRAS instruments onboard the MetOp satellites, COSMIC, and KOMPSAT 5.