The Wet Bias of Winter Precipitation Over Western Tibetan Plateau in RegCM4 Reduced by Including Anisotropic Topographic Factors in Sub‐Grid Turbulent Orographic Form Drag Scheme

The circulation and precipitation over complex terrains can be clearly influenced by the sub‐grid turbulent orographic form drag (STOFD). This study refines the Beljaars STOFD scheme by incorporating the anisotropic topographic factors. Both the original and modified STOFD schemes were implemented in the Regional Climate Model Version 4 (RegCM4). Results show that both STOFD schemes effectively reduce the overestimation of winter precipitation by the RegCM4 over the western Tibetan Plateau (TP). Adopting the original STOFD scheme increases the correlation coefficient (CC) and Taylor score (TS) of simulated winter precipitation over the western TP by 2.6% and 17.5%, respectively, while reducing the root mean square error (RMSE) by 16.3%. Compared to the original scheme, using the modified STOFD scheme further improves the TS (CC) by 6.4% (1.3%) and decreases the RMSE by 6.3%, indicating that winter precipitation simulation in the western TP is further enhanced. Mechanism analysis indicates that the suppression of overestimated precipitation results from the weakened simulation of vertical motion and water vapor transport, which are caused by the attenuated westerly winds due to the STOFD scheme. The significant reduced precipitation on the windward slope is attributed to the combined effects of anomalous downward motion and negative water vapor transport anomalies. Conversely, changes in precipitation simulation on the leeward slope are not significant, which results from the mutually offsetting influences of anomalous upward motion and negative water vapor transport anomalies. This study indicates the importance of considering the anisotropic topographic effects in the STOFD scheme.