Large-Eddy Simulation of Evaporatively Driven Entrainment in Cloud-Topped Mixed Layers

Abstract

Cloud-top entrainment instability (CTEI) is a hypothesized positive feedback between cloud-top entrainment and enhanced turbulence associated with buoyancy reversal. A sufficiently strong positive feedback is hypothesized to lead to the destruction of the cloud. Numerous studies have investigated the possible role of CTEI in cloud breakup, with ambiguous results. In this study, CTEI has been extensively investigated using many large-eddy simulations. An idealized experimental design has been used so as not to have any source of turbulence kinetic energy production except for entrainment due to evaporative cooling. A new method has been used to estimate the entrainment rate and to identify the inversion base and top. The results of the experiments do show the hypothesized positive feedback when the Randall?Deardorff CTEI criterion is met. When CTEI takes place in the numerical experiments, entrainment develops spontaneously through buoyancy reversal and, as a result, leads to cloud dissipation. Cloud dissipation within several hours is simulated in the cases with strong instability. A hypothesized dependence of the strength of the evaporatively driven turbulence on the cloud-top liquid water mixing ratio is confirmed. As expected, with a typical stratocumulus liquid water mixing ratio, the evaporatively driven turbulence is weak. Additional simulations with longwave radiation, surface latent heat flux, or both suggest that sufficiently strong radiative cooling can prevent cloud destruction by CTEI. For this reason, CTEI usually does not result in cloud dissipation in realistic cases.