Effect of Low-Level Jets on the Movement of the Mei-Yu Front and Heavy Rainfall

On 2 June 2017, a slow-moving Mei-Yu front produced extreme rainfall along the northern coast of Taiwan, with a maximum observed daily accumulation of 645.5 mm. This study employs Weather Research and Forecasting (WRF) Model simulations to investigate how variations in barrier jet intensity influence frontal movement and rainfall distribution during this event. The control simulation (CTRL) successfully reproduces the quasi-stationary Mei-Yu front, a pronounced barrier jet along the northwestern coast of Taiwan, and a maximum daily rainfall of about 680 mm.

A series of sensitivity experiments was designed to systematically modify barrier jet intensity while retaining the interaction between the front and northern Taiwan’s terrain. The results reveal a clear dependence of frontal propagation on barrier jet strength. When the barrier jet is weakened, the front advances southward more rapidly, shortening its residence time over northern Taiwan and leading to reduced rainfall accumulation. In contrast, a stronger barrier jet maintains a more northward frontal position, enhances low-level convergence and upward motion, and shifts the rainfall maximum northward, producing rainfall amounts comparable to those in CTRL.

The low-level equivalent potential temperature () gradients are similar across all experiments, indicating that the contribution of the large-scale environment to the frontal system is comparable among cases. Consequently, differences in frontal evolution and rainfall distribution can be attributed primarily to variations in barrier jet intensity. Vorticity budget analyses further demonstrate that a stronger barrier jet enhances low-level convergence and moisture transport, thereby slowing frontal propagation and resulting in increased rainfall accumulation over northern Taiwan.