Accounting for correlated error in the assimilation of high-resolution sounder data

Until January 2013, data from the high-resolution sounder IASI were assimilated with a diagonal observation-error covariance matrix within the Met Office 4D-Var assimilation scheme, assuming no correlation between channels. The errors were inflated to account indirectly for known inter-channel error correlations. This is sub-optimal as it artificially down-weights observations from these instruments. The true nature of these correlations for IASI are estimated here using data from the Met Office 4D-Var assimilation scheme and a posteriori diagnostics based on analysis and background departures. The diagnosed matrices are symmetrised and reconditioned, to make them suitable for use in the operational assimilation scheme. These matrices have been tested in full assimilation experiments. The results of these experiments show that using the new matrices improves forecast accuracy due to more weight in the assimilation being given to the IASI observations, particularly those from water-vapour-sensitive channels.