The mesoscale stability of entrainment into cloud-topped mixed layers

The Lilly-type models for stratocumulus-capped mixed layers are shown to allow for a mesoscale instability in which mesoscale fluctuations of buoyancy and humidity are reinforced in phase by entrainment. In a model of an AMTEX mixed layer, this mesoscale instability has a maximum growth rate of about 0.00002 per sec at wavelengths 30 times the depth of the mixed layer. The instability is able to account for the existence and broad scale of the stratiform cloud patterns known as mesoscale cellular convection.