Large-Eddy Simulation of a Stratus-Topped Boundary Layer. Part I: Structure and Budgets

Abstract

The structure of a stratus-topped boundary layer is observed through large-eddy simulation which includes the interaction of longwave radiation and turbulence processes. This simulated boundary layer has a relatively warm and dry overlying inversion, a weak surface buoyancy flux, no solar heating, and an insignificant wind shear across the cloud top. The cloud top height and the layer-averaged buoyancy flux inside the cloud layer define a velocity scale appropriate for this of boundary layer. In the cloud layer, buoyancy generates the vertical component of the turbulent kinetic energy, while pressure effect transfer some of this energy into the horizontal components. In the subcloud layer, the only source of the vertical energy other than the surface buoyancy is import from above and the only source of the horizontal energy other than the mean shear is the vertical energy transferred through pressure effects. The profiles of the vertical velocity variance and kinetic energy flux in the stratus-topped boundary layer depend on the relative contributions of the surface beating and cloud-top cooling to turbulence. Therefore, the vertical velocity variance is decomposed into two components: one entirely due to surface heating and the other entirely due to cloud-top cooling; the dimensionless profile of the latter is presented.