Effect of Marine Stratocumulus Clouds on the Ocean-Surface Heat Budget

The mixed-layer stratocumulus model first developed by Lilly is extended to include liquid-water-dependent solar optical properties and IR radiative fluxes. The ocean surface heat budget under these clouds is discussed as a function of ocean temperature, wind speed and large-scale divergence. Comparison of diurnally varying solar forcing with daily averaged forcing indicates the importance of the nonlinear effect of the clouds becoming thin during midday, when the Sun is strongest. Absorption of solar energy by the cloud is responsible for this: it tends to cut off turbulent entrainment, and the cloud top becomes lower; it heats the layer, and the cloud base rises. The ocean surface heat budget is generally negative (oceanic heating) under these clouds, and tends to become positive as the ocean temperature is raised. The climatic implications of this negative feedback and a similar feedback at the cloud-top level are discussed.