Radiative transfer and turbulence in the cloud-topped marine atmospheric boundary layer
- Scripps Institution of Oceanography, La Jolla, CA (United States)
- Desert Research Institute, Reno, NV (United States)
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. Coupling between cloud and subcloud layers is found to be very sensitive to cloud depth and subcloud layer sensible and latent heat fluxes. 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 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%, 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. 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, promoted 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. 48 refs., 1 tab.
- OSTI ID:
- 5523005
- Journal Information:
- Journal of the Atmospheric Sciences; (United States), Vol. 49:16; ISSN 0022-4928
- Country of Publication:
- United States
- Language:
- English
Similar Records
Idealized Large-Eddy Simulations of Stratocumulus Advecting over Cold Water. Part I: Boundary Layer Decoupling
Improving Boundary-layer Turbulence and Cloud Processes in CAM with a Higher-order Turbulence Closure Scheme and ASR Measurement