An economical PDF-based turbulence closure model for cloud-resolving models and global climate models

 
Poster PDF

Authors

Steven K. Krueger — University of Utah
Peter A Bogenschutz — National Center for Atmospheric Research

Category

Modeling

Description

Many coarse-grid cloud-resolving models now use horizontal grid sizes that resolve deep convection but not boundary layer clouds. How can these subgrid-scale (SGS) clouds be better represented in an economical way?

Our solution is to integrate several existing components: a prognostic SGS turbulence kinetic energy (TKE) equation, the trivariate assumed PDF method, a diagnostic second- and third-moment SGS turbulence closure, and a turbulence length scale related to the SGS TKE and eddy length scales. Our turbulence closure requires only one prognostic equation. This makes it economical, portable, and well-behaved. Our closure also uses a novel turbulence length scale that allows results to be nearly independent of horizontal resolution.

We implemented our approach, called SHOC (Simplified Higher-Order Closure), in a cloud-resolving model (CRM) and tested it against large-eddy simulation (LES) results. We also implemented it in a global model based on the Multiscale Modeling Framework (MMF) and evaluated the results using global observations. The CRM that we used is SAM (System for Atmospheric Modeling) developed by Marat Khairoutdinov. SAM-SHOC incorporates our new turbulence closure model.

Several LES benchmark cases were used to test SAM-SHOC. Our evaluations show that SAM-SHOC can realistically represent many boundary-layer cloud regimes in CRMs with horizontal grid sizes of 0.5 kilometers or larger, with practically no dependence on horizontal grid size. We also performed a SHOC test within the SP-CAM (Super-Parameterized Community Atmospheric Model). In SP-CAM, the representation of shallow cumulus was improved, but subtropical stratocumulus were still severely under-represented, likely due to inadequate vertical resolution.

We are currently performing simulations of the Midlatitude Continental Convective Clouds Experiment (MC3E) field campaign with SAM-SHOC for a range of horizontal grid sizes.