Sensitivity of retrieved atmospheric profiles from infrared radiances to physical and statistical parameters of the data assimilation system

The direct assimilation of satellite radiances is now operational in a few forecast centres, providing global temperature (T) and moisture (Q) informa tion. The critical parameters which influence the quality of the resulting analysis are mainly the selection of channels, the respective errors of the background field and radiance observations, and the quality of the radiati ve transfer model. These various aspects are studied from sensitivity exper iments based on 1-D variational assimilations using the ensemble of 19 infr ared channels (HIRS) of the NOAA-14 satellite. It is shown that significant improvements in the retrievals would be obtain ed if the radiance observation error (measurement plus radiative transfer), currently estimated to be about equal to that of the background (in radian ce units), were decreased. This could in principle be achieved by improving the forward radiative transfer model (RTM). Two RTMs suitable for radiance assimilation are compared in terms of analyzed increments, Jacobians, brig htness temperature and equivalent background error Important differences ar e noted for all of these interrelated measures. The existence of air-mass d ependent biases of fast radiative transfer models of the order of 1.5 K is confirmed in several channels from additional comparison with a line-by-lin e model. The importance of correctly specifying surface emissivity and the effective angle for downward calculations is demonstrated. The paper also evaluates, in some detail, the impact of uncertainties on th e background error covariance matric. The uncertainty on the skin temperatu re (T-s) error affects mostly the retrieval of that parameter; it has a mod est impact on the T and Q profiles in the low troposphere. The uncertainty on the e-e elements has more impact than that on the T-T elements. Off-diag onal elements of the background error covariance matrix ave very important as they impose smoothness and level-to-level consistency, especially for Q retrievals. Finally, T-s-T correlations, often ignored, could result in sig nificant improvements in the retrieval of temperature at low levels. Resear ch issues are discussed in the conclusion.