Orographic Controls on Extreme Precipitation associated with a Mei-yu Front

Taiwan regularly receives extreme rainfall due to seasonal Mei-yu fronts that are modified by Taiwan’s complex topography. One such case occurred between 1-3 June 2017 when a Mei-yu front contributed to flooding and landslides from over 600 mm of rainfall in 12 hours near 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 duration over the CMR are partially explained by a lack of orographic enhancements to Mei-yu frontal convergence near the terrain. A prominent feature missing with the reduced terrain is a redirection of postfrontal westerly winds attributed to orographic deformation, i.e., the redirection of flow due to upstream topography. Orographically deforming winds converge with prefrontal flow 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 Pacific (PRECIP) field campaign.