9.3      Scale-aware tests of the MYNN-EDMF PBL and shallow cumulus scheme with a novel framework.

Angevine, Wayne M., and Joseph Olson, CIRES, University of Colorado, and NOAA GSL

Making parameterizations for boundary layer turbulence and cumulus convection behave correctly as model grid spacing decreases from ~10km to a few hundred meters is a topic of current research.  At these scales, turbulence and cumulus are partially resolved, so the parameterized proportion should decrease. We present tests of the scale-aware aspects of the MYNN-EDMF PBL and shallow cumulus scheme in WRF with a novel framework.  The multi-column model (MCM) is a partially-convection-permitting setup that allows for changing grid spacing while testing the scheme’s behavior in a well-constrained, idealized situation.  MCM consists of a grid with many columns (32x32) using doubly-periodic boundary conditions (like LES).  The grid is initialized and forced like a single-column model, but with tiny perturbations to the initial soil moisture to break symmetry.  The MYNN-EDMF scheme is available in WRF, is used in the operational RAP and HRRR models in the U.S., and is under active development. The MCM results point out the important distinction between grid spacing and resolution:  The effective resolution of a mesoscale model is 4-8 * deltax, whereas the coarsened LES used to formulate previous scale-aware functions have 2 * deltax resolution.  We find that the best results on a 600-m MCM grid are obtained with the parameterization fully active.  The parameterized convection is somewhat weak at 3 km, which may also contribute to better results at full strength on the 600-m grid.