Estimates of observation-error characteristics in clear and cloudy regions for microwave imager radiances from numerical weather prediction

This article provides estimates of effective observation errors and their inter-channel and spatial correlations for microwave imager radiances currently used in the European Centre for Medium-Range Weather Forecasts (ECMWF) system. The estimates include the error contributions from the observation operator used in the assimilation system. We investigate how the estimates differ in clear and cloudy/rainy regions. The estimates are obtained using the Desroziers diagnostic. The results suggest considerable inter-channel and spatial error correlations for current microwave imager radiances, with observation errors that are significantly higher than the measured instrument noise. Inter-channel error correlations are even stronger for cloudy/ rainy situations, where channels with the same frequency but different polarizations show error correlations larger than 0.9. The findings suggest that a large proportion of the observation error originates from errors of representativeness and errors in the observation operator. The latter includes the errors from the forecast model, which can be significant in the case of humidity or cloud and rain. Assimilation experiments with single SSM/I fields of view highlight how the filtering properties of a four-dimensional variational assimilation system are changed when inter-channel error correlations are taken into account in the assimilation. Depending on the first-guess (FG) departures in the channels used, increments can be larger as well as smaller in comparison with the use of diagonal observation errors. Copyright (C) 2011 Royal Meteorological Society