Orographic effects on heavy rainfall events over northeastern Taiwan during the northeasterly monsoon season

The effects of orographic lifting and blocking on a heavy rainfall event with an accumulation of 631.5 mm on 11 October 2009 over the Lan-Yang Plain (LYP) in northeastern Taiwan during the northeasterly monsoon season were studied by performing observational data analyses and numerical simulations using the Weather Research and Forecasting (WRF) model. The synoptic environment included a low-level easterly wind over the East China Sea and a southeasterly wind over the western North Pacific Ocean which produced convergence areas leading to the heavy rainfall event. The mesoscale features and the orographic lifting and blocking effects on the production and maintenance of the heavy orographic rainfall without the direct influence of the typhoonʹs circulations during the northeasterly monsoon season in fall were first investigated here. Due to orographic blocking on the prevailing easterly wind over the western LYP, the induced near-surface northeasterly flow containing moist airstream was lifted over the windward (south) side of the LYP and rainfall was enhanced in situ. Meanwhile, the precipitating system was embedded in a weak middle-level flow with the wind reversing its direction over the windward side of the LYP, resulting in a quasi-stationary system over the slope area. Furthermore, the prevailing easterly wind ascended over the coastal slope south of the LYP and enhanced the rainfall there. In addition, the approaching east–west oriented rainband from southeast Taiwan also strengthened the rainfall intensity over northeastern Taiwan. Two sensitivity tests were performed to examine the effect of the orographic lifting of the moist airstream on the production of heavy rainfall. The sensitivity experiment with Taiwanʹs topography removed (the NT run) shows that the simulated accumulated rainfall over northeastern Taiwan was less than 50 mm in one day, much less than in the control run (CR run). In the NT run, the low-level convergence over northeastern Taiwan produced by the synoptic circulations and the simulated rainband still approaching northeastern Taiwan, are similar to the CR run. Another sensitivity experiment replacing the LYP with a plateau (the PL run) shows that the accumulated daily rainfall over the slope south of the LYP was reduced by ~ 250 mm compared to the CR run. The reduction of rainfall was caused by lifting relatively less moist air over the slope south of the LYP. These sensitivity tests indicate that the amount of low-level moisture and the orographic effects are equally important for the formation and maintenance of heavy rainfall over northeastern Taiwan under a favorable environment.