Plume Budgets in Clear and Cloudy Convective Boundary Layers

Abstract

From results of large-eddy simulations of the clear convective boundary layer and of a stratus-topped boundary layer, we evaluate the budgets of mass, momentum, heat, moisture, and turbulent kinetic energy within “plumes” that are formed by “updrafts” or “downdrafts.” Each grid cell is classified as part of updrafts or downdrafts according to the sign of the resolved vertical velocity. By means of the divergence theorem and Leibniz' rule, the mixing flux across the interface between plumes can be computed from volume integrals. The general form of the budget equation is deduced and compared to previous two-stream models. Both the mean convective circulation and small-scale turbulent motions contribute to the mixing flux across the interface. The small-scale fluxes are largest near the inversion layer at the top of the boundary layers and are important also in the surface layers. Vertical and horizontal velocities in plumes are strongly influenced by the vertical mean pressure gradient and horizontal pressure forces at the plumes lateral surfaces. For the cloudy case, the plume budgets differ from those in the clear boundary layer because of latent heat release and radiation cooling near the cloud top. We find stronger downdrafts because of buoyancy and pressure forces. In both the clear and the cloudy cases, most of the kinetic energy of turbulence within the upper part of the downdrafts comes from the updrafts through lateral mixing, not from buoyancy forcing.