Implementation of the Newly Developed Orographic Drag Scheme in the Weather Research and Forecast Model

Abstract A reasonable representation of the complex terrain impacts including orographic anisotropy in numerical models is crucial for improving weather and climate simulations especially in the mountainous region. In this study, we developed the orographic drag scheme that includes orographic anisotropy for all flow directions based on Lambert projection grid and implemented it into the Weather Research and Forecast (WRF) model. We then conducted two monthly simulations for Tibetan Plateau (TP) using the updated WRF model with the new drag scheme and that with orographic anisotropy for eight representative directions (8X) to validate the updated model against station observation and reanalysis data. The new scheme is shown to reduce the root‐mean‐square‐error (RMSE) in the upper‐level zonal wind speed (above 50 hPa) by 10%–20% in the north of the TP and by 5%–10% on the northern TP region. This is achieved by better representation of the orographic parameters that perturbed the launching gravity wave momentum flux from the surface and altered its breaking level aloft, thereby impacting the zonal wind vertically. The weakened drag over the west TP raised the breaking level and enhanced zonal wind below 100‐hPa while weakened zonal wind above; strengthened drag over the north of TP lowered the breaking level and weakened zonal wind below 50 hpa while enhance it above. The results indicate the importance of explicit orographic anisotropy representation for modeling of regional circulation in mountainous regions.