Arctic Weather Satellite Assessment and Assimilation at ECMWF

Abstract. The Arctic Weather Satellite (AWS) is a ground-breaking small satellite from ESA. Its goal is to measure microwave sounding radiances of sufficient quality for improving weather forecasts from a rapidly developed, low-cost mission. AWS is a pathfinder for the proposed EUMETSAT Polar System (EPS) Sterna constellation, which would represent a paradigm shift for operational satellite meteorology. The payload of AWS is a newly developed passive microwave (MW) sounder, with traditional temperature and humidity sounding channels near 54 and 183 GHz, plus novel humidity-sounding channels near 325 GHz. In this paper, first the radiometric performance of AWS is evaluated in reference to the ECMWF data assimilation system and heritage sounders, and then assimilation trials are presented to gauge the impact of AWS on forecast performance. The assimilation of AWS follows the all-sky method as applied to other MW radiometers in the ECMWF system, with the notable addition of the first-ever sub-millimetre wavelengths from the 325 GHz channel suite. Channel biases and noise estimates are generally in line with those of heritage instruments; AWS performance is similar to that of equivalent channels of AMSU-A and MWHS-2 in the 50 and 183 GHz bands, respectively, but effective noise for temperature sounding is higher than that of ATMS after spatial averaging. Nine months of experimentation show that adding AWS to the assimilation improves short-range forecasts of humidity, winds, and temperature. Geopotential height and winds are improved in the Southern Hemisphere through day 4. Despite its small size, AWS is a high-performing radiometer with data quality sufficient for operational assimilation in NWP. It has been assimilated operationally at ECMWF since July 2025.