Exploration into the assimilation of sub-millimetre frequencies observed by the Arctic Weather Satellite

The sub-millimetre channels in the 325 GHz band on the Arctic Weather Satellite have been assimilated operationally at ECMWF since July 2025. Prior work has shown they have a positive impact on the forecast when used alongside the 183 GHz humidity-sounding channels, and that they have a greater sensitivity to cloud ice scattering than traditional microwave frequencies. There are other upcoming EUMETSAT missions exploring the sub-millimetre, such as the EPS-Sterna constellation that also covers the 325 GHz band, and the Ice Cloud Imager which extends up to 664 GHz. These higher frequencies are expected to have an even greater sensitivity to cloud ice, motivating an exploration into the assimilation impact of the 325 GHz band. In this report, we investigate whether the sub-millimetre channels remain beneficial when assimilated without the 183 GHz channels, and whether their stronger sensitivity to cloud ice is reflected in the forecast of ice clouds and snow. We perform single-observation experiments to identify potential mechanisms and evaluate the full impact of the 325 GHz channels through four-month Observing System Experiments.

We determine that the information from the sub-millimetre channels is best exploited when used alongside 183 GHz. On the ArcticWeather Satellite, the 183 GHz channels are better at constraining the humidity profile, complementing the additional information provided by the 325 GHz band. We expect that within a data assimilation system, other observations of the 183 GHz band would sufficiently constrain the humidity field, whether or not they are co-registered, facilitating the exploitation of these sub-millimetre frequencies. When these sounding bands are combined, we find an improved characterisation of the cloud ice in the analysis, particularly in regions with strong convection. This leads to improvements in the 12H forecast scores for total column frozen water in these regions, in addition to the improvements to the humidity and wind at longer lead times. This study is the first investigation into the assimilation of sub-millimetre frequencies in all-sky conditions, and highlights the positive impact expected from upcoming satellites observing these frequencies.