Revisions to the Subgrid Orographic Parameterization of the NCEP Global Forecast System (GFS)

Abstract Lower-tropospheric enhanced gravity wave drag parameterization has been operational in the National Centers for Environmental Prediction (NCEP) Global Forecast System (GFS) since the late 1990s. The operational version of the scheme, as of January 2024 (GFS version 16), is based on the study by Kim and Arakawa, along with the addition of flow blocking. In contrast, an alternative subgrid orographic parameterization (SOP) algorithm rooted in the Kim and Arakawa scheme has been developed and operationalized in the Korean Integrated Model (KIM). The KIM’s SOP scheme (KSOP scheme) was implemented in the GFS model and evaluated by comparing it with the SOP scheme of GFS version 16 (GSOP scheme). In a comparative evaluation of the magnitudes of gravity wave and flow-blocking drag, it is evident that these two components play a significant role in the KSOP scheme, whereas the flow-blocking component is prevalent in the GSOP scheme. The 500-hPa anomaly correlation coefficient (ACC) forecast skill for medium-range forecasts is significantly improved in the Northern Hemisphere during both boreal summer and winter when the KSOP scheme is introduced, while slight degradation is observed in the Southern Hemisphere. The percentage improvements of the 500-hPa ACC scores in the Northern Hemisphere at 120 h for winter and summer are 140% and 35%, respectively. The precipitation forecasting skills in the CONUS region have also improved. Furthermore, accurate representation of the subgrid orography data is crucial for the success of the SOP scheme.