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

Abstract. 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 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 pre-frontal southwesterly flow and focus its convergence with the post-frontal 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, three 3-km runs are also used. It is found that the removal of the terrain in northern Taiwan makes only minor differences, in contrast to the result of a recent study. Only when the entire island topography of Taiwan is removed, does the result show significant differences. In this case, the blocking and deflecting effects on the pre-frontal flow are absent, and the heavy rainfall in northern Taiwan does not occur.