The assimilation of SSM/I and TMI rainfall rates in the ECMWF 4D‐Var system

A recent version of the European Centre for Medium-Range Weather Forecasts four-dimensional variational (4D-Var) assimilation system (40 km horizontal resolution with a 12-hour window) is used to examine the comparative impact of including satellite-derived rainfall rates from SSM/I and TMI radiometers within the tropics. The methodology is similar to the one proposed by Marecal and Mahfouf (2002) where Total Column Water Vapour (TCWV) retrievals in rainy areas from a simplified 1D-Var assimilation are introduced in the 4D-Var system. An improved methodology for the estimation of rain rate retrieval errors proposed by Bauer et al. (2002) is used. Three one-month experiments are undertaken: a control run (no rain rate assimilation), a TMI run (assimilation of TMI-derived rain rates) and a SSM/I run (assimilation of SSM/I-derived rain rates). The corrections of TCWV in rainy areas introduced in the 4D-Var are very similar between SSM/I and TMI because they are dominated by the ‘no rain’ information. The impact of TMI and SSM/I assimilations is positive on forecast scores, both in the extratropics and in the tropics. Results from the SSM/I run show a larger positive impact which tends to demonstrate the benefit of the increased number of data from the SSM/I with respect to TMI. Copyright © 2005 Royal Meteorological Society.

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