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This paper describes the development of a fractional cloud cover scheme which was implemented operationally in the ECMWF medium range forecast model in May 1985. the scheme is based on a diagnostic approach in which cloudiness is related empirically to the large-scale model variables, including convective activity. an example of the performance of the scheme is given, showing that a fair degree of skill is achieved in forecasting tropical and extratropical cloudiness. an attempt to verify the results using retrieved cloudiness from Nimbus 7 is described. Considerable difficulties were experienced due to the different height classifications of clouds in the model and in the satellite data. the importance of cloud radiative properties as well as cloud amount is considered briefly, based on a comparison of model earth radiation budget diagnostics with similar data from the NOAA polar-orbiting satellites. the sensitivity of the simulated outgoing long-wave radiance to changes in the prescribed cloud liquid water content is discussed. Two examples of the effects of cloudradiation interaction on boundary layer processes are described which demonstrate the importance of an integrated approach to the treatment of clouds, radiation and turbulent fluxes.
JM SLINGO (Wed,) studied this question.