A modeling study of an extreme rainfall event along the northern coast of Taiwan on 2 June 2017

In this study, the extreme rainfall event on 2 June 2017 along the northern coast of Taiwan is studied from a modeling perspective. While a peak amount of 645 mm was observed, two 1 km experiments produced about 400 and 541 mm, respectively, using different initial and boundary conditions, and thus are compared to isolate the key reasons for a higher total amount in the second run. While the conditions in the frontal intensity and its slow movement are similar in both runs, the frontal rainband remains stationary for a long period in this second run due to a frontal disturbance that acts to enhance the prefrontal southwesterly flow and focuses its convergence with the postfrontal flow right across the coastline. Identified as the key difference, this low-pressure disturbance is supported by the observation, and without it in the first run, multiple slow-moving rainbands pass through the coastal region and produce more widely spread but less concentrated rainfall, resulting in the lower peak amount by comparison. To explore and test the effects of Taiwan's topography in this event, two additional 1 km runs are also used. It is found that the removal of the terrain in northern Taiwan allowed the postfrontal cold air to move more inland and the rainfall became less concentrated, in agreement with a recent study. Also, when the entire island topography of Taiwan is removed, the result showed significant differences. In this case, the blocking and deflecting effects on the prefrontal flow are absent, and the heavy rainfall in northern Taiwan does not occur.