LARGE-EDDY BOUNDARY-LAYER ENTRAINMENT

A series of large-eddy simulations have been performed to explore boun dary layer entrainment under conditions of a strongly capped inversion layer with the boundary layer dynamics driven dominantly by buoyant f orcing. Different conditions explored include cloud-top cooling versus surface heating, smoke clouds versus water clouds, variations in cool ing height and optical depth of longwave radiation, degree of cloud-to p evaporative instability, and modest wind shear. Boundary layer entra inment involves transport and mixing over a full range of length scale s, as warm fluid from the region of the capping inversion is first tra nsported into the boundary layer and then mixed throughout. While entr ainment is often viewed as the small-scale process of,capturing warm f luid from the inversion into The top of the boundary layer, this need not be the physics that ultimately determines the entrainment rate. In these simulations the authors End instead that the entrainment rate i s often limited by the boundary layer-scale eddy transport and is ther efore surprisingly insensitive to the smaller scales of mixing near th e inversion. The fraction of buoyant energy production available to dr ive large eddies that is lost to entrainment rather than dissipation w as found to be nearly constant over a wide range of simulation conditi ons, with no apparent fundamental difference between top- versus botto m-driven or cloudy versus clear boundary layers. In addition, it is fo und that for quasi-steady boundary layers with dynamics driven by clou d-top cooling there is an effective upper limit on the entrainment rat e for which the boundary layer dynamics just remains coupled, which is often approached when the cloud top is evaporatively unstable.