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

Abstract

The mixed-layer stratocumulus model first developed by Lilly is extended to include liquid-water-dependent solar optical properties and infrared 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 mid-day, 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.