Evaluation and assimilation of <i>Arctic Weather Satellite</i> data in the AROME‐Arctic forecast system

Abstract The Arctic Weather Satellite ( AWS ) has been launched on August 16, 2024 as a prototype and proof of concept for the future European Organisation for the Exploitation of Meteorological Satellites (EUMETSAT) Polar System (EPS)–Sterna constellation of small satellites. The AWS platform carries an innovative microwave sounder instrument, providing information on temperature, humidity, and surface states. Many studies recently emphasized the critical value of assimilating microwave data in polar regions, demonstrating how improvements also extend to midlatitudes. However, due to the combined uncertainties in the model state and radiative transfer models, the use of microwave observations is still restrained to clear‐sky conditions over open ocean and snow‐free land surfaces in our regional Application de la Recherche à l'Opérationnel à Méso‐échelle (AROME)‐Arctic forecast system. In this article, we present the new capabilities of AWS over the polar regions for July and December 2025. The AWS instrument demonstrated similar performance to other operational microwave instruments currently assimilated in the AROME‐Arctic system. No visible asymmetric patterns in the residual bias of AWS first‐guess departures (FGd) were found, thanks to multiple updates of the local on‐ground processing chain. Observation errors and thinning distances have been tuned using FGd and other existing instruments as reference. The assimilation of AWS demonstrated an overall neutral to positive impact on the 3‐h forecast fields through an improvement (up to 0.6%) of the fit to other satellite instruments (e.g., microwave and infrared data). In terms of forecast scores, the assimilation of AWS in clear‐sky conditions has shown neutral to positive impacts on the forecast skills of the AROME‐Arctic system, with some significant improvements of the humidity at 700 hPa and close to the surface (about 1% for surface and 3%–6% for radiosonde observations). The AROME‐Arctic system is presently being set up towards AWS all‐sky assimilation.