A general circulation model cannot resolve the detailed vertical structure of the stratocumulus-topped PBL. Therefore, in parameterizing the stratocumulus-topped PBL it is useful to split the effect of cloud-top radiative cooling to two bulk portions: one that cools the entire mixed-layer air and the other that balances the entrainment effect (the latter can then be directly related to the entrainment rate). To address this issue, we adopt the large-eddy-simulation technique. We first performed a smoke (or dust) cloud simulation in which latent heating was ignored so that longwave radiative cooling at the cloud top was the only buoyancy source for turbulent transport, and found that a certain fraction of cloud-top radiative cooling is used to cool the whole mixed-layer air. However, in a stratocumulus simulation in which latent heating exists, very little longwave radiative cooling is found to cool the mixed-layer air.
In this talk, I would like to address the following questions: Why does the role of cloud-top radiative cooling differ between the smoke-cloud and the stratocumulus cases? Is it typical of the marine stratocumulus-topped PBL that the positive buoyancy flux in the cloud layer is maintained mainly by the evaporation/condensation effect? All cloud-top radiative cooling exists entirely within cloud elements in our simulations, so how does that cooling promote entrainment in a dynamical sense?