Evaluation of the EPS-Sterna 325 GHz channels in the Ensemble of Data Assimilations

Evaluation of the EPS-Sterna 325 GHz channels in the Ensemble of Data Assimilations
Date Published
Eumetsat Contract Report
Katie Lean
Niels Bormann
Abstract The proposed EPS-Sterna constellation of small satellites will carry a microwave (MW) sounding instrument comprising frequencies around 50-60 GHz and 183 GHz with a long heritage of use in Numerical Weather Prediction (NWP) but will also include a novel set of four humidity-sounding channels at 325 GHz. These higher, sub-millimetre MW frequencies have not been flown before on a space-borne platform. To allow evaluation of their additional benefit for NWP, a strategy is developed here to simulate and assimilate the 325 GHz channels in an all-sky framework.
A key aspect of this strategy is that observation error modelling has been adapted for the new 325 GHz channels. The model follows the usual all-sky observation error model of assigning larger errors in cloudy regions, but uses a new cloud indicator which exploits the cloud signal extracted from the lowest peaking 325 GHz channel. Compared to using the scattering index employed for the 183 GHz humidity-sounding channels, the model with the new cloud indicator leads to a more Gaussian distribution when considering background departures normalised by the assigned observation error.
EDA experiments show that the 325 GHz channels produce a similar positive impact as the 183 GHz channels when each channel set is added separately to the temperature sounding channels at 50 GHz. When added on top of the 50 GHz and 183 GHz sounding channels, the impact of the 325 GHz channels is mostly neutral, with some benefits for relative humidity in the mid-troposphere. Some inflation of the observation errors assigned to the 325 GHz channels is found necessary to achieve this impact. It is hypothesised that this is due to the presence of neglected observation error correlations, and better handling of these may be required to realise more benefit from the combined channel set in the future.
URL https://www.ecmwf.int/en/elibrary/81561-evaluation-eps-sterna-325-ghz-channels-ensemble-data-assimilations
DOI 10.21957/f53d05c057