Observation errors and their correlations for satellite radiances
Collard, Andrew ; Bauer, Peter
The assumed observation errors for tropospheric channels from AMSU-A (Advanced Microwave Sounding Unit) have recently been reduced considerably in the ECMWF system, contributing to a significant positive forecast impact in Cy37r2 of the Integrated Forecasting System (IFS). With this change more weight is given to AMSU-A observations in the assimilation system. The rather simple adjustment has been prompted by a study into estimating observation errors and their correlations for most satellite radiances used in the ECMWF system. It was found that observation errors for AMSU-A show only weak correlations spatially or between channels, and the observation error is instead dominated by uncorrelated instrument noise. This suggested that the data could be used more aggressively than previously thought, even if we assume uncorrelated observation errors as is currently done in the ECMWF system. For other instruments, such as IASI (Infrared Atmospheric Sounding Interferometer), the situation is more complex : while temperature-sounding channels mostly tend to behave in a similar way as those for AMSU-A, channels sensitive to water vapour or with strong surface contributions show considerable inter-channel or spatial error correlations. This article summarises the observation error estimation and highlights some of the implications.
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