A New Parameterization for Shallow Cumulus Convection and Its Application to Marine Subtropical Cloud-Topped Boundary Layers. Part II: Regional Simulations of Marine Boundary Layer Clouds

Abstract

The impact of physical parameterizations on simulations of cloud-topped marine boundary layers is investigated using the fifth-generation Pennsylvania State University?NCAR Mesoscale Model (MM5). Three-month MM5 simulations of the northeast and southeast Pacific during June?August 1987 are boundary forced with time- varying ECMWF analyses. Runs with four planetary boundary layer (PBL) parameterizations already implemented in MM5 are compared with runs using new parameterizations of boundary layer turbulence and shallow cumulus convection (ShCu) described in a companion paper. Numerous modifications to the MM5 that allow it to be used as a regional climate model are described. The simulated 3-month mean shortwave cloud radiative forcing (SWCF) and vertical structure of cloud-topped boundary layers in the northeast Pacific are sensitive to the PBL/shallow convection schemes. All four current MM5 PBL schemes [the Blackadar, Medium-Range Forecast (MRF), Burk?Thompson, and Gayno?Seaman schemes] produce overly shallow boundary layers with excessive SWCF throughout this region, especially in the transition from stratocumulus to trade cumulus where their SWCF errors range from 130 (Gayno?Seaman) to 200 W m?2 (MRF). These errors likely reflect inadequate vertical mixing by parameterized turbulence and shallow convection. The only shallow convection scheme available for MM5, the Grell scheme, was almost totally inactive in this region, so no shallow convection scheme was used for the above simulations. The Grenier? Bretherton (GB01) scheme, which entrains more aggressively above stratocumulus-capped convective layers, has much better regional SWCF and vertical structure. Without shallow cumulus convection, the scheme still produces excessive cloud in the transition regions; the main impact of the ShCu parameterization is to remove this bias. With all schemes, the near-surface air has a cool, dry bias, and surface turbulent fluxes are somewhat larger than observed. Sensitivity studies show that the SWCF is sensitive to a halving of the cloud droplet concentration, to plausible uncertainties in parameterized penetrative mixing at cumulus cloud tops and in stratocumulus entrainment, and (in the coastal zone) to horizontal resolution. Southeast Pacific simulations show that the GB01+ShCu scheme can also accurately represent another subtropical boundary layer cloud regime.