Large-Eddy Boundary Layer Entrainment
Journal of the Atmospheric Sciences
A series of large-eddy simulations have been performed to explore boundary layer entrainment under conditions of a strongly capped inversion layer with the boundary layer dynamics driven dominantly by buoyant forcing. Different conditions explored include cloud-top cooling versus surface heating, smoke clouds versus water clouds, variations in cooling height and optical depth of longwave radiation, degree of cloud-top evaporative instability, and modest wind shear. Boundary layer entrainment
... ayer entrainment involves transport and mixing over a full range of length scales, as warm fluid from the region of the capping inversion is first transported into the boundary layer and then mixed throughout. While entrainment is often viewed as the small-scale process of capturing warm fluid 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 find instead that the entrainment rate is often limited by the boundary layer-scale eddy transport and is therefore surprisingly insensitive to the smaller scales of mixing near the inversion. The fraction of buoyant energy production available to drive large eddies that is lost to entrainment rather than dissipation was found to be nearly constant over a wide range of simulation conditions, with no apparent fundamental difference between top-versus bottom-driven or cloudy versus clear boundary layers. In addition, it is found that for quasi-steady boundary layers with dynamics driven by cloud-top cooling there is an effective upper limit on the entrainment rate for which the boundary layer dynamics just remains coupled, which is often approached when the cloud top is evaporatively unstable.