|Title||Cloud and precipitation overlap in simplified scattering radiative transfer.|
|Series/Collection||EUMETSAT/ECMWF Fellowship Programme Research Reports|
|Authors||Geer, AJ, Forbes, R, Bauer, P|
|Event Series/Collection||EUMETSAT/ECMWF Fellowship Programme|
|Place of publication||Shinfield Park, Reading|
Scattering radiative transfer calculations are used in the all-sky assimilation of microwave imager observations. For accurate simulations it is important to treat the cloud and precipitation overlap carefully. The current approximation for the cloud and precipitation fraction is not fully consistent with the representation of moist physics in the Integrated Forecasting System (IFS). The impact of going from the current representation to one that is more consistent with the moist physics is examined. Compared to reference radiative transfer simulations using the Independent Column Approach, rms errors are typically reduced. However, microwave imager first guess (FG) departures are not significantly affected. This is explained by the fact that FG departures are dominated by systematic model biases and by forecast errors (e.g. the inability to simulate cloud and rain in exactly the right place and time). Despite the lack of impact on the FG departures, it is still hoped to implement this change operationally, and to include it as an optional feature in the RTTOV-10 release.