Factors influencing cloud area at the capping inversion for shallow cumulus clouds

Large-eddy simulations of marine cumulus clouds are performed. Aspects of the simulations are varied in order to identify those parameters that play a role in determining when the cumulus clouds spread out under the capping inversion into stratocumulus. These include the strength of the surface fluxes, which also affects the strength of the cumulus mass transport, the strength of the capping inversion, the moisture content of the free atmosphere, the strength of cloud-top radiative cooling and whether there is significant precipitation or sedimentation of cloud drops. Tests are also performed to evaluate sensitivity to the model resolution. As in previous studies, having sufficient resolution at the capping inversion is found to be important for robust simulation of any stratiform cloud that forms there. In addition, the cloud cover is found to be sensitive to many of the physical processes investigated. In the end, though, a clear and robust dependence emerges on the cloud-top entrainment instability parameter, kappa. For large kappa, a typical (small) shallow cumulus cloud cover is seen. For kappa below the threshold for buoyancy reversal, the cumulus spreads into a solid sheet of stratocumulus beneath the inversion. Consistent with previous studies, there is no abrupt transition and the cloud cover decreases smoothly as kappa increases. The conclusion is reached that varying other parameters in the simulations affects the cloud cover through their impact on the size of the jumps across the inversion and thence kappa. The implications for turbulent mixing parametrizations in general circulation models are discussed. (C) Crown Copyright 2009. Reproduced with the permission of HMSO. Published by John Wiley & Sons, Ltd.