Radiative Transfer and Turbulence in the Cloud-topped Marine Atmospheric Boundary Layer

Abstract

Abstract The effects of longwave and shortwave radiative heating on the coupling between stratocumulus clouds and the boundary layer is investigated using a one-dimensional second-moment turbulence-closure model. The decoupling of a stratiform cloud from the subcloud layer is often a precursor to cloud break up and the transition to scattered cumulus clouds or clear sky. Coupling between cloud and subcloud layers is found to be very sensitive to cloud depth and subcloud layer sensible and latent heat fluxes. In particular, a strong moisture flux can maintain weak coupling between the cloud and subcloud layers so that the lower part of the cloud layer may continue to develop despite the formation of a stable temperature gradient between the top of the subcloud layer and cloud base.The effect of shortwave heating on decoupling is threefold. First, shortwave heating directly offsets the net longwave cooling at cloud top by as much as 30% (in February at latitude 29°N), reducing or eliminating the overall coo...