Turbulent Inversion: A Missing Piece in the Puzzle of Stratocumulus Entrainment?

Abstract:

We investigate high-resolution airborne data collected in the course of the Physics of Stratocumulus Top (POST) project. Analysis of 8 different cases of marine stratocumuli, based on ~400 aircraft penetrations across the Entrainment Interfacial Layer (EIL), reveals a fine-layered structure of the cloud top and capping inversion. Despite maximum static stability, the inversion and the whole EIL remain turbulent with maximum turbulence intensity just below the cloud top. Anisotropy of velocity fluctuations in the turbulent inversion and the cloud top mixing sublayer indicates significant effect of static stability. The thickness of the EIL corresponds to the thickness of the shear layer in the cloud top region, and adapts to the velocity- and buoyancy-jump between the cloud layer and the free troposphere in such a way, that the gradient Richardson Number across the EIL and its sublayers remains critical. This, in turn, suggests that shear at the cloud top, not radiative cooling or large convective eddies in the boundary layer, is the source of the observed turbulence. This last supposition is confirmed by LES simulations based on data from the POST and DYCOMS-II campaigns. They document the importance of shear-generated turbulence in stratocumulus top dynamics and suggest that entrainment into the stratocumulus topped boundary layer depends on fine-scale properties of this turbulence.