Ensemble-based sensitivity analysis and predictability of an extreme rainfall event over northern Taiwan in the Mei-yu season: The 2 June 2017 case

In the present study, an ensemble-based sensitivity analysis (ESA) on the extreme-rainfall event along the northern coast of Taiwan on 2 June 2017 in the Mei-yu season is performed using the results from 45 forecast members with a grid size of 2.5–5 km. An quasi-stationary rainband associated with the front produced localized rainfall up to 635 mm in 12 h (0000–1200 LST 2 June), causing serious flooding and inundation near the northern tip of Taiwan. With a relatively large spread (i.e., standard deviation or σ), small ensemble mean (~130 mm), and low probability of heavy rainfall in northern Taiwan, the ensemble indicates a lower predictability there, compared to the topographic rainfall over the mountains. However, the ESA allows for identification of several contributing factors to heavy rainfall in northern Taiwan in a quantitative manner as given below. With their impact given in change of (areal-mean) 6-h rainfall amount per one σ increase, these factors include: (1) surface frontal position and moving speed (−16.00 mm per 5 km h⁻¹), (2) position of 700-hPa wind-shift line (+12.59 mm per 0.4° latitude), (3) environmental moisture amount near the surface front (+11.73 mm per 0.92 g kg⁻¹ in mixing ratio), (4) timing and location of frontal low-pressure disturbance (+11.03 mm per 1.38° longitude), and (5) frontal intensity (+9.58 mm per 3 K in equivalent potential temperature difference across 0.5°). While many of the factors identified are interconnected, they tend to increase the local rainfall through lengthening the duration and enhancing near-surface convergence along the northwestern coast of Taiwan over the area immediately upstream.