Unified treatment of dry convective and stratocumulus-topped boundary layers in the ECMWF model

The unified parametrization of the dry boundary layer and stratocumulus in the ECMWF model is described using the eddy diffusivity mass-flux framework. Model simulations are compared to observations at a few selected research field experiments, to ISCCP cloud cover, to LIDAR from space cloud observations, to SYNOP observations over Europe and to the climatology. It is shown that the new stratocumulus formulation improved the quality of the stratocumulus simulations rather dramatically. The key ingredients are: (i) mixing in moist conserved variables, (ii) diffusion related to cloud-top cooling, (iii) an additional mass-flux term, (iv) an entrainment parametrization and (v) a switching between shallow cumulus and stratocumulus based on lower tropospheric stability. To illustrate the relevance of various aspects, the sensitivity to different parameters is also presented. Copyright (C) 2011 Royal Meteorological Society