Evaluating and Tuning Orographic Gravity Wave Drag Parameterizations in Atmospheric NWP Models

The Global Systems Division (GSD) at the NOAA Earth System Research Laboratory is implementing a suite of orographic drag parameterizations for use in the operational FV3GFS global NWP model. These represent drag forces imparted to the atmosphere by unresolved (subgrid scale) topography. A major component of the suite is a large-scale (horizontal grid spacing >5km) gravity wave drag (GWD) scheme, which parameterizes the vertical transport of momentum by gravity waves throughout the atmospheric column. It is a challenge to evaluate such schemes due to the difficulty of determining the “true” subgrid-scale momentum transport. In this work, we used the method of Kruse and Smith to diagnose vertical momentum fluxes associated with gravity waves from high-resolution (750m) model runs, and compared these to the resolved and parameterized fluxes given by the GWD scheme at coarser resolutions. The drag suite is scale adaptive, and is suitable for use in the gray-scale range of horizontal grid spacing (~5-50km) for orographic GWD. Our results indicate the range of grid sizes for which GWD is partially resolved, and the method provides guidance for tuning the parameterization.