Improved surface treatment for all-sky microwave observations

Cycle 48r1 of the Integrated Forecasting System (IFS) contains numerous improvements to the surface representation for all-sky microwave observations. For the first time, mid-frequency microwave window channels (in the region of 37 GHz, 89 GHz and 150 GHz) are assimilated over land surfaces, bringing sensitivity to water cloud, rain and snow in the atmosphere. This makes use of a new technique to extrapolate surface emissivity retrievals (made at lower frequencies) up to the frequencies of the assimilated channels. Further, and recognising that at least 20% of all observations can be mixed scenes (combinations of water, land and sea-ice), the treatment of mixed surfaces has been improved. In particular, the emissivity retrieval and extrapolation is now restricted to the land and sea-ice part of the scene, while the water part is represented with a successful physically-based model. Where possible, this also uses a land-sea mask tailored to the field of view of the observational data, rather than relying on the model land-sea mask. There is also more precise screening of sea-ice and surface snow, so that observations can now be assimilated at higher latitudes, in colder seasons, and closer to coastlines. Overall, this package increases data usage by between 4% and 33% in most actively assimilated channels, with the biggest effects in the northern hemisphere (NH), in winter, and in polar regions. This improves forecasts in the NH higher latitudes out to around day 3. This work is a precursor to further developments aiming to realise a full ‘all-sky all-surface’ assimilation of microwave radiances in the IFS.