The Diurnal Cycle of Winds, Rain, and Clouds over Taiwan during the Mei-Yu, Summer, and Autumn Rainfall Regimes

Abstract

The diurnal variations in surface winds, rain, and clouds over Taiwan are presented for three rainfall regimes: the mei-yu (16 May?15 June), summer (16 July?31 August), and autumn (16 September?15 October). Though the magnitude of diurnal island divergence and convergence is similar under each regime, the diurnal variations of rain and clouds vary considerably between the regimes. These differences are related to the seasonal changes in environment winds, stability, moisture, and weather systems. In addition to orographic lifting on the windward side, rainfall occurrences for all three rainfall regimes are strongly modulated by the diurnal heating cycle with an afternoon maximum. The largest day?night differences in rainfall occur in summer and the smallest differences occur in autumn. The upper-level high cloud (<235 K) frequencies have a pronounced afternoon maximum over the mountainous areas in the afternoon because of combined effects of orographic lifting and solar heating. These clouds are advected downstream by the upper-level winds in late afternoon and early evening. The highest afternoon high cloud frequencies occur in summer (>30%) with the lowest upper-level cloud cover in autumn (?10%). In autumn, most of the orographic showers on the eastern and northeastern windward side in the late afternoon and early evening are not from deep clouds. The weak early-morning rainfall maxima for all three seasons are related to the localized boundary layer convergence due to the orographic blocking of the prevailing winds and their interactions with the offshore/land breeze. During disturbed, prefrontal periods in the mei-yu, bands of high clouds and rain tend to develop in the early morning in the convergence zone off the northwest coast. These rainbands are responsible for the early-morning rainfall maximum on the northwest coast. They do not occur in summer or autumn.