Overview of the VICIRS Study: A VIcarious Calibration Tool for MWI and ICI Using RadioSoundings

The EUMETSAT VICIRS ('VIcarious Calibration tool for MWI and ICI using RadioSoundings') study developed a vicarious calibration tool for the two conicalscanning radiometers, the MicroWave Imager (MWI) and the Ice Cloud Imager (ICI), planned to fly from 2026 onwards aboard the Metop-SG (Meteorological Operational satelliteSecond Generation) B-satellites as part of the EUMETSAT Polar System (EPS) program. The combined use of MWI and ICI radiometers will provide an unprecedented set of microwave passive measurements, from 18.7 GHz up to 664 GHz, on the same platform. Vicarious calibration using radiosoundings is particularly important for MWI and ICI channels that are unprecedented and therefore cannot be compared with reference observational data from space. The VICIRS tool has been developed to collect clear-sky match-ups between MWI/ICI and radiosoundings from the high-quality, low-density Global Climate Observing System (GCOS) Reference Upper-Air Network (GRUAN) archive, as well as from a more comprehensive, homogenized, higher-density archive, the Radiosounding HARMonization (RHARM) dataset, which also provides estimations of measurement uncertainty at all pressure levels. Numerical Weather Prediction (NWP) profiles are also used to fill gaps in surface parameters and in data above the radiosoundings top levels. The tool provides a statistical analysis of the difference between observed and simulated brightness temperatures, considering the total uncertainty emerging from various sources (e.g., instrumental uncertainties, forward model uncertainties, spatial and temporal mismatches) for each MWI/ICI channel. The VICIRS tool has been tested with the MWI/ICI L1B test data product collocated with RHARM RS, and a dataset of match-ups between observations from NASA's Global Precipitation Measurement (GPM) Microwave Imager (GMI) and GRUAN/RHARM RS.