From the perspective of mesoscale climatology, the present study utilized hourly surface rainfall data in May-June, 1993-1998 to analyze characteristics of precipitation and heavy rainfall events over central Taiwan (forecast areas 5-10 in the 1992 Post-TAMEX experiment), including their geographical distribution, diurnal variation, and differences between the plain and mountain areas. Since the data are much improved in horizontal resolution than before, this study provides a more accurate and complete rainfall-related climatology. Results suggest that significant diurnal and regional variations exist in precipitation during the Mei-yu season over central Taiwan. Rainfall intensity (or heavy rainfall frequency) is higher in afternoon-evening hours and lower in late night-morning hours, and higher over mountains than plains, suggesting the important roles of both topography and daytime solar heating. Peaking at 1700 LST, primary rainfall (and frequency) maximum occurs over the southwestern slope of the Snow Mountain Range (SMR). The Ali-shan area, previously thought to have the highest heavy rainfall frequency over Taiwan in Mei-yu season, is only identified here to be the secondary maximum that is not apparent in June. Averaged rainfall intensity exhibits a greater diurnal variation over mountains than plains, more so in June than May, while afternoon maximum occurs on windward side slopes, rather than near mountain ridges, indicating the role of forced uplift by terrain. In June, afternoon rainfall near foothills also increases, which should be related to local circulation (upslope winds). Over the plains, convection development is inhibited in June, and rainfall intensity remains low during the entire day. Type B heavy rainfall events are the most commonly observed type (about 1/2), while types A and C are fewer (each about 1/4). In May, heavy rainfall events occur twice as many times as in June, on at least 33% and about 25% of the days, respectively. In general, the frequency is higher during daytime and lower during nighttime, higher during afternoon and lower before noon, and low from late night to next morning. Regional heavy rainfall events (with larger rainfall area) have similar diurnal variation characteristics, but the decrease after evening hours tends to be slower. Compared to other types, type A heavy rainfall events also have a higher percentage that qualify as regional events. Characteristics of heavy rainfall events over the plains (＜200m) are also compared with those over the mountains (≥200m) in this study. Except for a stronger tendency to concentrate in the afternoon, events over mountains also show a higher frequency before noon (more so in June) most likely linked to local circulations. On the other hand, events over plains exhibit higher frequency near 0400 LST, which should be related to land breeze. In June, it is uncommon for heavy rainfall to occur in the morning over plains, for type A events to occur over mountains, or for regional events to occur anywhere, particularly over mountains. This indicates firstly the role of inhibition played by the subtropical high in June, and secondly, that the increase in averaged rainfall intensity in June afternoon is largely attributed to frequent but short-lived showers from air-mass thunderstorm development.
|Number of pages||28|
|Publication status||Published - 2005|