Relevance of the Mesoscale Entrainment Instability to the Marine Cloud-topped Atmospheric Boundary Layer

Abstract

Mesoscale variability in entrainment across the inversion capping the cloud-topped atmospheric boundary layer (CTBL) has been proposed as an explanation for mesoscale variability in cloud thickness. The relevance of this mechanism, called mesoscale entrainment instability, or MEI, to some typical atmospheric boundary layers is investigated. The results indicate that MEI is of relevance only if the potential temperature jump across the inversion is small, ?1?2 K, and the stable layer virtual potential temperature above is very strongly stratified, ?40 K km?1. Thus, MEI does not appear to be a viable explanation for mesoscale cellular convection in most CTBLs. Two parameters are also investigated whose effect on the growth rate can be substantial. They are the rate of horizontal turbulent diffusion and the effect of variations in solar heating due to cloud thickness fluctuations. Decreases in the horizontal turbulent transport rate do not greatly affect the growth rate but can substantially decrease the wavelength of the instability. The solar heating effect can as much as double the growth rate but probably not enough to make the instability significant.