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

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 cooling of the cloud layer during part of the day. Second, shortwave heating decreases exponentially from a maximum at cloud top, which tends to stabilize and evaporate the cloud layer. In a deep cloud layer radiative heating is restricted to the upper part of the cloud, which warms at a faster rate than the lower part of the cloud; hence, decoupling can occur within the cloud layer. Vertical mixing in the cloud is limited, and multiple cloud layers may form. Third, the maximum shortwave heating is displaced below the maximum longwave cooling, creating a divergent flux that generates convection in the upper part of the cloud layer that, in turn, promotes entrainment. In a deep cloud layer, shortwave radiative heating can affect the decoupling of a cloud and subcloud layer only if longwave cooling is reduced sufficiently to allow longwave radiative heating of cloud base to warm the lower part of the cloud at a faster rate than the subcloud layer is heated by the sea surface. In a shallow cloud layer, shortwave radiation may penetrate to cloud base to provide an additional heat source to decouple the cloud from the subcloud layer.These results highlight the difficulty of predicting the formation, evolution, and dissipation of marine stratocumulus clouds.