|Title||A parameterization of cloud overlap as a function of wind shear and its impact in ECMWF forecast|
|Year of Publication||2015|
|Authors||Di Giuseppe, F, Tompkins, AM|
|Secondary Title||Technical Memorandum|
Six months of CloudSat and CALIPSO observations have been divided into over eight million cloud scenes and co-located with ECMWF wind analyses to identify an empirical relationship between cloud overlap and wind shear for use in atmospheric models. For vertically continuous cloudy layers, cloud decorrelates from maximum towards random overlap as the layer separation distance increases, and we demonstrate a systematic impact of wind shear on the resulting decorrelation length scale. As expected, cloud decorrelates over smaller distances as wind shear increases. A simple, empirical linear fit parameterization is suggested that is straightforward to add to existing radiation schemes, although it is shown that the parameters are quite sensitive to the processing details of the cloud mask data and also to the fitting method used. The wind shear-overlap dependency is implemented in the radiation scheme of the ECMWF integrated forecast system. It has a similar magnitude impact on the radiative budget as that of switching from a fixed decorrelation length scale to the latitude-dependent length scale presently used in the operational model, altering the zonal mean, top-of-atmosphere, equator-to-mid latitude gradient of short-wave radiation by approximately 2 W m−2 .