The impact of topography on the storm characteristics and precipitation associated with the june 2017 Meiyu front in Taiwan

Abstract The Meiyu front is a large-scale pre-monsoon rainfall event that impacts the moisture-rich region of Taiwan, where interactions with the Central Mountain Range (CMR), a north–south mountain range in the middle of Taiwan, can produce extreme rainfall from synoptic-scale, mesoscale, and orographic contributions. We use the Weather Research and Forecasting (WRF) model to investigate the record-breaking Meiyu event of 1–3 June 2017, in which Taipei received more than 600 mm of rain in 12 h. A control simulation with full topography (CTRL) is compared with a sensitivity experiment in which the CMR height is reduced by 50% (TMOD); the 50% reduction is chosen to drive the orographic flow from a strongly blocked regime toward a flow-over regime so that the role of the barrier in shaping storm modes can be isolated. In both simulations, the Deep and Wide Convective Core (DWCC) storm mode is the dominant rainfall contributor over Taiwan owing to its high rain rate (CTRL mean of 12.1 mm h⁻ 1 ) and frequency. In TMOD, DWCC mean intensity drops to 11.6 mm h⁻ 1 and average storm area decreases by about 17%, reducing the DWCC rainfall contribution by roughly half. Broad Stratiform Regions (BSRs) and non-extreme rainfall both increase their contributions but do not offset the DWCC loss, resulting in 31% less rainfall over Taiwan in TMOD relative to CTRL, with the largest reductions over the CMR. An ingredient-based framework links these changes to shifts in maximum upward velocity, vertical moisture flux ( wq ), and low-level shear. On the windward side of the CMR, all three ingredients weaken with reduced terrain, while on the lee side, reduced blocking and a higher effective Froude number allow updrafts and moisture transport to extend eastward. Consequently, DWCC locations shift from over the CMR in CTRL to off the east coast of Taiwan in TMOD, and BSRs shift westward over the island. The extremity of the 1–3 June 2017 event arises from the simultaneous co-location of these three ingredients with the CMR; the experiments demonstrate that this extreme outcome is contingent on the full terrain, not merely a linear orographic enhancement. The results provide a mechanistic explanation that connects mountain-flow dynamics to the storm-mode balance that controls rainfall accumulation during Meiyu front events.