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

In this study, the extreme rainfall event on 2 June 2017along the northern coast of Taiwan is studied from a modeling perspective.While a peak amount of 645 mm was observed, two 1 km experiments producedabout 400 and 541 mm, respectively, using different initial and boundaryconditions, and thus are compared to isolate the key reasons for a highertotal amount in the second run. While the conditions in the frontal intensityand its slow movement are similar in both runs, the frontal rainband remainsstationary for a long period in this second run due to a frontal disturbancethat acts to enhance the prefrontal southwesterly flow and focuses itsconvergence with the postfrontal flow right across the coastline.Identified as the key difference, this low-pressure disturbance is supportedby the observation, and without it in the first run, multiple slow-movingrainbands pass through the coastal region and produce more widely spread butless concentrated rainfall, resulting in the lower peak amount bycomparison. To explore and test the effects of Taiwan's topography in this event, twoadditional 1 km runs are also used. It is found that the removal of theterrain in northern Taiwan allowed the postfrontal cold air to move moreinland and the rainfall became less concentrated, in agreement with a recentstudy. Also, when the entire island topography of Taiwan is removed, theresult showed significant differences. In this case, the blocking anddeflecting effects on the prefrontal flow are absent, and the heavyrainfall in northern Taiwan does not occur.