Operational Use of AIRS Observations at the Met Office

Observations from the Atmospheric Infrared Sounder (AIRS) have been operationally assimilated into the Met Office’s Numerical Weather Prediction (NWP) model since 26 May 2004. AIRS observations are screened by a 1D-Var pre-processing stage before being passed to the Met Office’s 4D-Var data assimilation system. The 1D-Var pre-processing stage is used for quality control, bias correction, cloud detection, channel selection and monitoring. The final quality control step is to test for good convergence in the 1D-Var retrieval, using the 6-hour forecast field from the previous assimilation cycle as the background. Currently, only cloud-free fields-of-view over the sea are selected for data assimilation. A subset of 45 and 60 channels are assimilated for day-time and night-time fields-of-view respectively. Three pre-operational trials were carried out. Two one-month trials were run to determine the impact of the new system for summer and winter periods, and both indicated positive impact. The changes required to assimilate AIRS observations were added to the Met Office’s operational system as part of a package of satellite data changes. The entire package was tested by incrementally adding each component to a trial running in parallel to operations. The results of these pre-operational trials indicate a positive impact of around 0.4–0.5 on the global NWP index from the use of AIRS observations; a satisfactory result given the conservative nature of the initial system. The global NWP index is explained in annex 1 of Hilton et al. (2005). It is planned to continue to develop the system to better exploit the potential of AIRS observations.