WCD-04. Evaluation of Physical Processes within the GFSv16 PBL

Abstract
Parameterization of the atmospheric planetary boundary layer (PBL) has the challenge of accurately representing the effects of small-scale turbulent mixing as well as the impact of the larger-scale, convectively driven mixing that can take place in an unstable atmosphere. In the current operational version of the Global Forecast System (GFS), the local transport by turbulent eddies is represented by an eddy diffusion scheme based on the prognostic turbulent kinetic energy. The nonlocal transport is a function of the prognostic updraft and downdraft mass fluxes within the PBL. In this presentation, we evaluate the performance of the GFSv16 PBL scheme in unstable atmospheric conditions, using comparisons with field campaigns and idealized studies. Such conditions frequently lead to the formation of shallow cumulus clouds, which play an important role in tropical weather, yet remain a particular forecast challenge. Results from single-column model simulations are presented to demonstrate the sensitivity of the scheme to parameter variations. The magnitudes of the local and nonlocal contributions to the mixing are evaluated, and compared with those in other operational PBL schemes, in an effort to better understand, and potentially improve, the representation of the mixing processes.