Results from the first UKMO IASI fast radiative transfer model intercomparison
SHERLOCK, Vanessa
An intercomparison of existing fast radiative transfer models (RTIASI, PFAAST) for the Infrared Atmospheric Sounding Interferometer IASI has been undertaken to assess their suitability for use in NWP. As it is anticipated that IASI Level 1C radiances will be assimilated directly in the variational framework, both the forward model error characteristics and the accuracy of modelled Jacobians are considered. Differences in the implementation of the fast models and between the line-by-line models used to generate the fast models are summarised. The impact of these differences are quantified and the implications for both future fast model developments and the specification of the forward model error covariance matrix are discussed. Fast model level 1C radiances are then compared with those simulated by each fast model's line-by-line generator for six representative atmospheres and temperature and water vapour Jacobians are compared for two extreme atmospheres (warm/humid and cold/dry) on three spectral intervals (15 microns CO2 band, the 6.7 microns H2O band and the 11 microns atmospheric window region). In general fast forward model error characteristics are satisfactory. On spectral intervals of interest for atmospheric sounding most channels have forward model errors which are significantly less than instrumental noise. Temperature
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