Large-eddy simulation of evaporatively driven entrainment into cloud-topped mixed layer

The cloud-top entrainment instability (CTEI) is a hypothesized rapid cloud evaporation due to a positive feedback between cloud-top entrainment and enhanced turbulence associated with buoyancy reversal (BR). Numerous studies have been devoted to investigating the role of CTEI in cloud breakup, with ambiguous results.

In this study, CTEI is extensively investigated using large-eddy simulations (LESs). Experiments with BR were designed so as not to have any source of turbulent kinetic energy production except entrainment due to evaporative cooling. A total of 147 (a) LESs were performed. Three marine stratocumulus cases (FIRE, ASTEX, DYCOMS-II) were also simulated to compare with the BR experiments.

The results of the BR experiments do show the hypothesized positive feedback. Rapid cloud dissipation, i.e., within several hours, is simulated in some cases, but some clouds subject to CTEI do not fully evaporate in a ten-hour simulation. A hypothesized dependence of the results on the liquid water mixing ratio is confirmed; however, with a typical stratocumulus cloud water amount, the feedback is weak. A transition from stable to unstable conditions was also simulated for one stratocumulus case.

When CTEI takes place, the spontaneous entrainment leads to cloud dissipation in the absence of cloud-building processes such as surface evaporation. However, cloud dissipation can be prevented by sufficiently strong cloud-building processes.