Study the Moist Flow Regime Transition and Heavy Orographic Rainfall During Typhoon Passage over Taiwan's Central Mountain Range

Super Typhoon Nepartak, the third most intense global tropical cyclone in 2016, made landfall on 8 July in Taitung County, Taiwan, making it one of the most devastating and deadly storms to make landfall in Taiwan in the past decade. As Nepartak moved towards and passed over Taiwan’s Central Mountain Range (CMR), heavy orographic rainfall occurred along its movement, with the amount of rainfall increasing as it approached landfall. This study aims to investigate the moist flow regime transition and formation and enhancement mechanisms of orographic rainfall during the passage of Nepartak over the CMR, using both real case and idealized case (2D and 3D) Weather Research and Forecasting (WRF) simulations. We differentiate the contribution of different forcings leading to the formation of orographic rainfall, such as orographic lifting, the release of instabilities, and the enhancement of a pre-existing system. Our results show that the moist flow regime transition significantly influenced the formation of orographic rainfall. In particular, when Nepartak was approaching the eastern flank of Taiwan’s CMR, the moist flow regime continued changing, dramatically impacting the upslope rainfall. The study also found that upslope rainfall over Yu-Shan Mountain was mainly produced by orographic lifting of the outer circulation, while heavy orographic rain production over Tai-Tung resulted from a low-CAPE and high-wind environment. Our study provides valuable insights into the orographic impacts on precipitation during the passage of a tropical cyclone over an isolated, 3D mountain range.