How well do we need to simulate cloud radiative effects to simulate stratocumulus?

Abstract:

Using Large-Eddy Simulations with a simple representation of the cloud radiative effect, we confirm that the cloud radiative cooling at the top of the cloud is crucial to the simulation of the equilibrium, stratocumulus-capped boundary layer. In the perspective of representing this radiative cooling in a lower-resolution model, we study how spatial averaging can alter this equilibrium. Vertical averaging over large layers, or introducing some cooling above the cloud top destabilizes the stratocumulus equilibrium. On the other hand, the stratocumulus equilibrium is not very sensitive to horizontal averaging of the cloud radiative effect. Furthermore, we investigate whether the cloud radiative effect can be computed from the large-scale variables. We show that the variance of the liquid water path has to be taken into account. These experiments provide a set of necessary conditions to represent the cloud radiative effect in a larger-scale model such as a GCM or a CRM well enough to simulate the interaction between turbulence and cloud radiative effect essential to stratcumulus.