All-sky assimilation of SSMIS humidity sounding channels over land within the ECMWF system

All-sky assimilation of SSMIS humidity sounding channels over land within the ECMWF system
Date Published
EUMETSAT/ECMWF Fellowship Programme Research Reports
Document Number
Fabrizio Baordo
Event Series/Collection
EUMETSAT/ECMWF Fellowship Programme
Abstract The extension of all-sky assimilation of SSMIS humidity sounding channels to land surfaces is investigated in this paper. A ‘symmetric' error model, which adaptively determines the size of observation errors, can be formulated using the scattering index as a predictor to identify cloudy and precipitating regions over land. This assigns larger observation errors in those situations more difficult to model because of radiative transfer and ‘mislocation' errors. The use of an instantaneous emissivity retrieval from SSMIS surfacesensitive channels is also explored. In clear-sky scenes, emissivity retrievals appear better at capturing daily differences in surface conditions, compared to emissivity atlas values. In the presence of clouds, retrievals have different behaviour. In the lower microwave frequencies (less than 50 GHz), emissivity estimates appear nearly as reliable as those in clear-skies, but at higher frequencies, as the magnitude of scattering increases, so does the error in the retrieval and the resultant emissivity estimate can be unphysically low or high. However, the retrieval still appears feasible at high frequencies in light cloud situations; the number of retrievals discarded due to these kind of problems is around 10%. In these cases, an estimate from an emissivity atlas can be substituted instead. Assimilation experiments are performed that demonstrate the feasibility of assimilating SSMIS 183 GHz channels over land in all-sky conditions: the assimilation system is not degraded and the improvements on analysis and forecast scores are about the same as those which are obtained by the equivalent clear-sky approach. The developments described in this study were an essential first step to create framework to allow the all-sky assimilation over land of other microwave humidity sounders: this started operationally at ECMWF in 2015, covering both SSMIS and four MHS (Microwave Humidity Sounder) instruments.