9.5      Evaluating and tuning the orographic gravity wave drag scheme in the RAP model.

Toy, Michael D., Joseph B. Olson, Tanya G. Smirnova, Jaymes S. Kenyon, NOAA/Earth System Research Laboratory (NOAA/ESRL), and Cooperative Institute for Research in Environmental Sciences, Boulder, Colorado, John M. Brown, and Georg A. Grell, NOAA/ESRL

The operational 13-km Rapid Refresh (RAP) and 3-km convection-allowing High-Resolution Rapid Refresh (HRRR) are hourly-updating forecast models run at NCEP.  The experimental version of RAP and HRRR are currently based on WRF Model Version 3.9.  In this version, developed at NOAA-GSD, we have developed a suite of subgrid-scale parameterizations that represent drag forces imparted to the atmosphere by unresolved topography.  The large-scale (horizontal grid spacing 5km) gravity wave drag (GWD) scheme, which has long been part of WRF, is used in the RAP model.  In order to evaluate the scheme, we used the method of Kruse and Smith to diagnose vertical momentum fluxes associated with gravity waves from high-resolution (0.75km) model runs, and compared these to the resolved and parameterized fluxes of the RAP model.  Our results show that GWD at 13km horizontal resolution is within the “gray zone” scale at which GWD is partially resolved.  The method provides guidance for directly “tuning” the parameterization.