Plain Language Summary

Abstract

Taiwan regularly receives extreme rainfall due to seasonal mei-yu fronts that are modi ed by Taiwan’s com- plex topography. One such case occurred between 1 and 3 June 2017 when a mei-yu front contributed to ooding and land- slides from over 600 mm of rainfall in 12 h near the Taipei basin, and over 1500 mm of rainfall in 2 days near the Central Mountain Range (CMR). This mei-yu event is simulated using the Weather Research and Forecasting (WRF) Model with halved terrain as a sensitivity test to investigate the orographic mechanisms that modify the intensity, duration, and location of extreme rainfall. The reduction in WRF terrain height produced a decrease in rainfall duration and accumulation in northern Taiwan and a decrease in rainfall duration, intensity, and accumulation over the CMR. The reductions in northern Taiwan are linked to a weaker orographic barrier jet resulting from a lowered terrain height. The reductions in rainfall intensity and dura- tion over the CMR are partially explained by a lack of orographic enhancements to mei-yu frontal convergence near the ter- rain. A prominent feature missing with the reduced terrain is a redirection of postfrontal westerly winds attributed to orographic deformation, i.e., the redirection of ow due to upstream topography. Orographically deforming winds converge with prefrontal ow to maintain the mei-yu front. In both regions, the decrease in mei-yu front propagation speed is linked to increased rainfall duration. These orographic features will be further explored using observations captured during the 2022 Prediction of Rainfall Extremes Campaign in the Paci c (PRECIP) eld campaign.