Estimates of spatial and inter-channel observation error characteristics for current sounder radiances for NWP

TitleEstimates of spatial and inter-channel observation error characteristics for current sounder radiances for NWP
Publication TypeMiscellaneous
Year of Publication2009
AuthorsBormann, N, Collard, A, Bauer, P
Secondary TitleTechnical Memorandum

This paper uses three methods to estimate and examine observation errors and their correlations for clearsky sounder radiances used in the ECMWF assimilation system. The study considers sounder-radiances from the main instruments currently in use, ie., AMSU-A, HIRS, MHS, AIRS, and IASI. The analysis is based on covariances derived from pairs of First Guess and analysis departures. The methods used are the so-called Hollingsworth/L¨onnberg method, a method based on subtracting a scaled version of mapped assumed background errors from FG-departure covariances, and the Desroziers diagnostic. The findings suggest that mid-tropospheric to stratospheric temperature sounding channels for AIRS and IASI and all AMSU-A sounding channels show little or no inter-channel or spatial observation error correlations, and estimates for the observation error are close to the instrument noise. Channels with stronger sensitivity to the surface show larger observation errors compared to the instrument noise, and some of this error is correlated spatially and between channels. Short-wave infrared temperature sounding channels also appear more prone to spatial observation error correlations. The three methods show good consistency for these estimates. Estimating observation errors for humidity sounding channels appears more difficult. A considerable proportion of the observation error for humidity sounding channels appears correlated spatially for short separation distances, as well as between channels. Observation error estimates for humidity channels are generally considerably larger than those provided by the instrument noise. Our statistics suggest that assumed background errors for tropospheric temperature are inflated (by about 30-60%), whereas there is little indication for background error inflation for stratospheric temperatures.