Satellite measurements affected by precipitation have not been assimilated in the Met Office Numerical Weather prediction models before now. At infrared and visible wavelengths clouds are opaque and no information from below the cloud top is available, therefore cloudy and precipitating fields of view from infrared and visible instruments are screened out before assimilation. Microwave remote sensing techniques have the ability to make retrievals in precipitating conditions, these data will provide important information for weather prediction when assimilated. The first step towards assimilating precipitation affected microwave radiances is the modelling of scattering at microwave frequencies by precipitation sized hydrometeors allowing calculation of brightness temperatures or radiances based on the state of the atmosphere given by the model.

RTTOVSCATT is a version of the radiative transfer model RTTOV7 with the capability to handle the scattering of radiation by hydrometeors. Work at ECMWF (Moreau et al, 2002) concluded that the preferable method of treating scattering of radiation in terms of numerical accuracy and computing time is the delta-Eddington approximation. The Eddington approach to scattering approximates the radiance vector and the phase function to the first order so that only one angle is required for the scattering calculations. This method has been incorporated into the RTTOV framework allowing the computation of radiances and brightness temperatures in scattering atmospheres. Further documentation can be found by clicking on the NWP-SAF Reports link.

The Satellite Radiance Assimilation Group (SRAG) is currently researching precipitation-affected satellite microwave radiances. Recently developed scattering modules for precipitation are being incorporated into the radiative transfer model RTTOV to compare model brightness temeratures (calculated from NWP model fields of cloud and precipitation) with observed SSM/I and AMSU brightness temperatures, and in this way, it is hoped that we can gain insight into how the Met Office model may respond to assimilation of this kind of data.

Another aspect of the work is a comparison between the precipitation-affected AMSU-B brightness temperatures generated by RTTOVSCATT and those produced by ARTS (Atmospheric Radiative Transfer System), a RT model developed at Bremen University. It is hoped that such a comparison will lead to a greater understanding of precipitation and ice cloud modelling at millimetre wavelengths.