Clouds
6. Ed Pavelin
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Radiances from advanced infrared sounders such as AIRS and IASI are now an important source of information for global NWP. In order to make the best use of the data from these instruments, it has been necessary to devise techniques for assimilating infrared radiances in cloudy conditions. The Met Office has developed a technique allowing the direct assimilation of infrared radiances with some sensitivity to cloud. This technique avoids problems caused by limitations in current radiative transfer models used in radiance assimilation.
The cloud top pressure and effective cloud fraction are first estimated using a minimum residual method (Eyre and Menzel, 1989), assuming the cloud to be a grey body at a single pressure level. A non-linear 1D-Var analysis is then carried out (Rodgers, 2000), whereby the cloud top pressure and cloud fraction are analyzed simultaneously with the atmospheric temperature and humidity profile. The retrieved cloud top pressure is then used to select a sub-set of channels for assimilation that have only small sensitivity to the atmosphere below the cloud top. These selected radiances are then directly assimilated in 4D-Var, and the retrieved cloud parameters are used to ensure that the cloud effects are adequately represented.
Figures:
Figure1: Retrieved effective cloud fraction from 6 hours of IASI data
Figure2: Simulated temperature Jacobians for cloudy conditions, before (left) and after (right) selection of channels for assimilation.
This technique is has been used operationally for AIRS data since July 2008, and for IASI data since March 2010, and has been shown to provide significant improvements in forecast accuracy.
References:
Eyre, J.R. and Menzel, W.P., 1989. Retrieval of cloud parameters from satellite sounder data: a simulation study. J. Appl. Meteorol., 28, 267-27
Pavelin, E.G., English, S.J. and Eyre, J.R., 2008. The assimilation of cloud-affected infrared satellite radiances for numerical weather prediction. Q. J. R. Meteorol.Soc., 134, 737-749
Rodgers, C.D., 2000. Inverse methods for atmospheric sounding: theory and practice. World Scientific, Singapore.