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Assimilation of SSMIS humidity‐sounding channels in all‐sky conditions over land using a dynamic emissivity retrieval
Author(s) -
Baordo F.,
Geer A. J.
Publication year - 2016
Publication title -
quarterly journal of the royal meteorological society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.744
H-Index - 143
eISSN - 1477-870X
pISSN - 0035-9009
DOI - 10.1002/qj.2873
Subject(s) - emissivity , depth sounding , environmental science , remote sensing , sky , radiative transfer , data assimilation , atmospheric radiative transfer codes , meteorology , diffuse sky radiation , scattering , physics , geology , optics , oceanography
The extension of all‐sky assimilation of Special Sensor Microwave Imager/Sounder (SSMIS) humidity‐sounding channels to land surfaces is investigated in this article. The scattering index, which is able to discriminate between cloudy and precipitating regions over land, is used as a predictor to develop a ‘symmetric’ model for observation error. This formulation is able to increase the observation error in those scenes that are more difficult to model because of radiative transfer and ‘mislocation’ errors. The implementation of an instantaneous emissivity retrieval from SSMIS observations is also presented. In clear‐sky scenes, emissivity retrievals appear better at capturing daily differences in surface conditions compared with emissivity atlas values. In the presence of clouds, retrievals show 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 this kind of problem is around 10%. In these cases, an estimate from an emissivity atlas can be substituted instead. Together, the new observation‐error model and the instantaneous emissivity retrievals were adopted for the assimilation of SSMIS 183 GHz channels over land in all‐sky conditions. Assimilation experiments showed that the assimilation system is not degraded and the improvements in analysis and forecast scores are about the same as those 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 the European Centre for Medium‐Range Weather Forecasts (ECMWF) in 2015, covering both SSMIS and four Microwave Humidity Sounder (MHS) instruments.