WCD-13. Using a process-based analysis to optimize the orographic drag parameterization in the NOAA Unified Forecast System

Abstract
Parameterizing gravity wave drag (GWD) and low-level flow blocking due to unresolved topography in numerical weather and climate prediction models is known to be vital for providing accurate forecasts. The challenges to accurately representing these subgrid processes include the representation of the subgrid topography itself, the calculation of the expected momentum flux from upward propagating gravity waves and the levels at which it is dissipated via wave breaking, and the existence of a broad horizontal “gray zone” over which orographic GWD acts, which spans from ~1 to ~100s of kilometers. Efforts to constrain the effects of subgrid topography are ongoing. A recent study provides orographic GWD and blocking data provided by high-resolution atmospheric models for the purpose of analyzing and tuning drag parameterizations. In this presentation we demonstrate the process of optimizing the orographic drag components of the Unified Gravity Wave Physics suite of parameterizations in the NOAA Unified Forecast System (UFS) using physically based stress values based on the study. The impact on forecast skill scores with the global UFS at various horizontal resolutions will be demonstrated.