Characteristics of Rainfall Distributions over Taiwan during the Taiwan Area Mesoscale Experiment (TAMEX)

Abstract

Based on 808 daily rainfall stations, rainfall distributions over Taiwan during TAMEX (Taiwan Area Mesoscale Experiment) (10 May?27 June 1987) were analyzed. The total rainfall had a maximum on the windward slopes under the prevailing southwesterly monsoon flow, indicating that orographic lifting plays an important role in determining rainfall distribution. About 50% of the rainfall occurred as scattered orographic showers during nonfrontal periods. Along the northwestern coastal plain, about 80% of the rainfall was associated with the passage of surface fronts. Along the eastern coastal, mountainous region, which is on the lee side under the prevailing southwesterly monsoon flow, a large portion of the rainfall was produced by upper-level, cold-core lows and a midlevel vortex event. For these cases, low-level flow was southerly or southeasterly because Taiwan was on the western or southwestern periphery of the western Pacific subtropical high. In this situation, eastern Taiwan was on the windward side. Rainfall occurred on the western side of the mountainous slopes during the late season (1?27 June) twice as much as during the early season (10?31 May). This is consistent with a more persistent southwesterly monsoon flow with a higher moisture content in June than in May. On the windward slopes, the enhancement of orographic lifting by anabatic winds was important for the development of rain showers in the afternoon. The timing of maximum hourly rainfall incidences on the lower windward slopes occurred 1?2 h earlier than on the upper slopes and the mountain interior. Within the Taipei Basin, the hourly rainfall maximum occurred during 1500?1600 LST because of the convergence of surface winds from onshore flow. In the northern and northeastern coastal mountains, the hourly rainfall incidences had an afternoon maximum (1600 LST) due to anabatic winds. In the eastern coastal mountainous region, hourly rainfall also had a maximum during 1700?2000 LST. Along the western coastal regions with flat terrain the diurnal variations of hourly rainfall and hourly rainfall incidences were less important than in other regions.