A climatological stratocumulus-to-cumulus transition model derived from the surface energy budget

The transition from a stratocumulus topped to a trade-wind cumulus topped marine boundary layer is associated with a large contrast in area-averaged cloud radiative influence on the top of the atmosphere energy budget. Potential changes in the characteristics of the transition in a perturbed climate (e.g. the average location of the transition) can thus significantly either strengthen or dampen the initial perturbation. Models, spanning a wide range of complexity and sophistication, have been developed to increase the understanding of this cloud transition. In this study we proceed from the energy budget of the ocean surface layer to derive a simple model explaining this climatologically important cloud cover transition. More precisely we investigate the transition occurring off the coast of California. The ECMWF INTERIM reanalysis product indicates the climatological net surface energy flux (i.e. the sum of radiative and turbulent fluxes) to be small in the region associated with the transition. The two major surface fluxes of energy of the region are the absorbed solar radiation and the turbulent flux of latent heat. The former is strongly dependent on the cloud cover and the latter depends on the sea surface temperature through the temperature dependence of the saturation mixing ratio (i.e. the Clausius-Clapeyron relation). By using climatological averages of parameters related to the radiation and the air-sea fluxes a cloud cover model is proposed that manages to explain the transition from stratocumulus to trade-wind cumulus surprisingly well.