Cloud-top entrainment instability

From Glossary of Meteorology

cloud-top entrainment instability

A condition whereby entrainment of dry air into a cloud top causes even more entrainment, leading to the dissipation of the cloud.

When the entrained air mixes with cloudy air, evaporation of the cloud drops into the mixture causes the mixture to cool. As this cool mixture sinks, it generates a turbulent circulation that can cause more entrainment, thereby continuing the process until the cloud dissipates. While there is still debate about the requirements for such a process to occur, one of the first suggestions was that this instability will occur when Δθe < Δθe critical, where Δθe is the difference of equivalent potential temperature from just above to just below cloud top, and Δθe critical is a critical value that is near zero. While the exact value of Δθe critical has yet to be determined, it depends on the buoyancy of the air (i.e., on virtual potential temperature) and on the temperature change possible due to latent heat changes when cloud droplets evaporate.