A study of afternoon heavy rainfall in Taiwan during the mei-yu season

Abstract

This study investigates how sounding characteristics affects afternoon rainshowers on days during which the rainfall amount exceeded 100 mm, as observed at least at 1 out of 232 to 326 automatic rainfall and meteorological telemetry system stations and 22 conventional surface stations in Taiwan during the mei-yu (monsoon rain) season between 1993 and 1997. In the first group of three out of five events (Group A), the West Pacific Subtropical High was just to the east of Taiwan, and a southerly wind beneath 700 hPa was observed with a speed ranging from 4 to 13 m/s. In the other two events (Group B), the ridge axis of the West Pacific Subtropical High was over southern Taiwan and the wind was southwesterly with a more uniform speed of below 700 hPa. In Group A, most of the precipitation was observed in low sloping and mountainous areas over northern and central Taiwan while in Group B, most of the precipitation occurred at elevations over 500 m in central Taiwan. Based on the sounding characteristics of these two groups, the Penn State/NCAR MM5 Mesoscale model was employed in 31 experiments to investigate the impact of wind direction, speed and relative humidity on the distribution of heavy precipitation during the mei-yu season. The predominant southerly or southwesterly wind converged in northern Taiwan over the lee side of the north–south-orientated central mountain range (CMR). Meanwhile, surface heating resulted in a northerly upslope wind in northern Taiwan. The lee side convergence and upslope wind accompanied by onshore flow from northern coastal areas, promoted simulated precipitation systems in northern Taiwan. In the simulation, an upslope wind over the windward side of western Taiwan, intensified precipitation systems over sloping areas. A larger wind component perpendicular to CMR was produced in predominantly southwesterly flow than in predominantly southerly flow. Subsequently, a larger wind shear was generated between the boundary layer and the free atmosphere over the western slope, resulting in a high entrainment to inhibit the upward motion and the development of precipitation systems. Consequently, the simulated rainfall accumulation over Taiwan in predominantly southwesterly wind was less than in predominantly southerly wind. The increase in the speed of the predominant wind also increased wind shear over the slope, reducing the vertical velocity and the intensity of the precipitation. Accordingly, less rainfall accumulated over the island in simulations with higher predominant wind speeds. These results are consistent with observation. Simulated rainfall fell over low land in high low-level relative humidity experiments, with a low level of free convection (LFC).