The assimilation of cloud-affected infrared satellite radiances for numerical weather prediction

A practical technique for the assimilation of cloud-affected infrared radiances is presented. The technique is best suited to advanced infrared sounders such as AIRS and IASI. Radiances are first pre-processed by a one-dimensional variational analysis (1D-Var) scheme, where cloud parameters (cloud-top pressure and effective cloud fraction) are retrieved simultaneously with atmospheric profile variables. The retrieved cloud parameters are then passed to a variational data assimilation system, where they are used to constrain the radiative transfer calculation in the assimilation of a reduced set of channels. The channel selection is chosen to reduce the sensitivity to errors in the forward modelling of radiation originating below the cloud top. The performance of this technique is explored by means of a 1D-Var study using simulated measurements. It is demonstrated that the technique has the potential to allow the assimilation of a significant proportion of cloud-affected infrared sounding measurements, possibly bringing valuable benefits to an operational NWP system. (C) Crown Copyright 2008. Reproduced with the permission of the Controller of HMSO and the Queen's Printer for Scotland. Published by John Wiley & Sons, Ltd.