Crossing the “Unknown Land” in the Parameterization of Subgrid Turbulent Mixing

Jian-Wen Bao

PSL Forecast and Modeling Development Team

Tuesday, Aug 02, 2016, 2:00 pm
DSRC Room 2A305

Abstract

It has been recognized that the conventional parameterization of subgrid turbulent mixing suitable for either mesoscale modeling or large-eddy simulation (LES) cannot be expected to operate properly on a grid spacing that is comparable to the typical size of the largest energy-containing eddies. The range of such a grid spacing is referred to as “terra incognita” (i.e., “unknown land”) in the parameterization of subgrid turbulent mixing by Wyngaard (2004). To help cross this “unknown land,” we have developed an energetically coherent and grid-spacing adaptive parameterization scheme based on the full 3-dimensional prognostic equation of turbulent kinetic energy (TKE) in the Advanced Research Weather Research and Forecasting (WRF) model (ARW).



This presentation provides an overview of the theoretical background for the development of the new scheme and highlights its desirable properties using results from a series of idealized dry convective boundary layer (CBL) simulations, in which the new scheme is compared with the conventional 1-D TKE-based schemes for parameterizing vertical sub-grid turbulent mixing (i.e., planetary boundary layer schemes) against an LES dataset. To demonstrate the grid-spacing adaptive effect of the scheme, simulations are carried out for the same case with progressively varying grid sizes of 500 m, 1 km, 3 km and 9 km. We will first compare the characteristics of the simulated CBL using the new and conventional schemes. We will then illustrate the importance of including the non-local components in the subgrid vertical stress and buoyancy flux. Finally, comparing the results from the simulations against the LES dataset, we will show the feasibility of using the new scheme in the ARW model in lieu of the conventional planetary boundary layer parameterization schemes.