Orography-Induced Gravity Wave Drag Parameterizations in the NCEP GFS

The lower tropospheric enhanced gravity wave drag (GWD) parameterization has been operational in the National Center for Prediction System (NCEP) Global Forecast System (GFS) since late 1990s. The operational version of the scheme, as of January 2024 (GFS version 16: v16), is based on the study of Kim and Arakawa, along with the addition of flow blocking. On the other hand, an alternative sub-grid orographic parameterization algorithm that is routed in the Kim and Arakawa scheme has been developed and operational in the Korean Integrated Model (KIM). This study implements the orographic parameterization scheme in KIM into the GFS model and its performance is evaluated in comparison to the GFS scheme. The 500 hPa anomaly correlation coefficients forecast skill for medium-range forecasts is significantly improved in the northern hemisphere in both boreal summer and winters when the KIM scheme is introduced. In contrast, slight degradation is found in the southern hemisphere. Precipitation forecasting skills in the CONUS region have also improved. It is also confirmed that the correct representation of sub-grid orography data is crucial to successfully representing sub-grid orography parameterization.