A Moist PBL Parameterization for Large-Scale Models and Its Application to Subtropical Cloud-Topped Marine Boundary Layers

Abstract

A new general purpose boundary layer parameterization that permits realistic treatment of stratocumulus-capped boundary layers (SCBLs) with coarse vertical resolution is described. It combines a 1.5-order turbulent closure model with an entrainment closure at the boundary layer top. Three different implementations of the entrainment closure, in which the boundary layer height is respectively prognosed, reconstructed from thermodynamic values at the grid points, or restricted to lie on a flux level of the host model grid, are tested in a single-column modeling framework at both fine and coarse vertical resolution. The first two approaches permit a stratocumulus top and base to lie between grid levels and evolve continuously with time, but are more complicated to implement in a three-dimensional model. The model performs very well in cases of dry convection, whatever the inversion implementation and the vertical resolution. With 15-mb or better vertical resolution, all approaches properly simulate mixing in SCBLs, including daytime cloud thinning and a transition to decoupling and conditional instability as SST increases. With coarser resolution, details of the implementation influence the simulated cloud thickness, which is systematically underestimated with the restricted inversion approach. A method for computing vertical advective fluxes at the boundary layer top that explicitly accounts for the inversion is presented; an essential component of the reconstructed inversion implementation, this vertical advection scheme also improves SCBL simulation at low resolution with a restricted inversion. For comprehensive simulation of boundary layer convection, this scheme should be coupled with a parameterization of shallow cumulus convection; this will be described in a forthcoming paper.