Temporal and Spatial Characteristics of Typhoon Extreme Rainfall in Taiwan

We studied the temporal and spatial characteristics of extreme typhoon rainfall in Taiwan using Central Weather Bureau hourly Precipitation data from 21 surface stations during the past 51 years (1960-2010). Extreme rainfall is defined as 95th percentile intensity of total rain events, or equivalently, rain events greater than 9 mm hr(-1) which contribute 40% to the total rain amount in Taiwan. It was found that approximately 70% (20%) of extreme rain is in the typhoon season (Mei-Yu) from July to October (from May to June). There are significant variations of typhoon extreme rainfall over the annual and decadal time scales, with larger extreme rainfall values and events in the periods of 1960-1976 and 1994-2010, and less in the 1977-1993 period. The recent 1994-2009 period has the most extreme rainfall and events, as well as, inter-annual variability. In contrast, there are strong inter-annual variations of Mei-Yu extreme rainfall, but no significant decadal variations. The averaged typhoon rain intensity, however, is about the same, being 19 mm hr(-1) in all these three periods. Our analysis indicates that the typhoon extreme rainfall spatial pattern is phased locked with the Central Mountain Range, Taiwan. In general, the amount of extreme rainfall was related to the typhoon translation speed and duration time, but not typhoon intensity. Slower speeds and longer duration time lead to larger extreme rainfall values. Our analysis also indicate that the mean duration time of Taiwan landfall typhoons with northern tracks (tracks north of 23 degrees latitude) is about 3 hours longer than that of southern track typhoons in the last 51 years, and is more likely to produce three times as much extreme rainfall. The interactions of summer or winter monsoons with typhoons are also important factors that may contribute to the extreme rainfall in Taiwan. Examples of extreme rainfall due to typhoon circulation interaction with summer and winter monsoon flows are presented. Monsoon water vapor supply, typhoon slow translation speed, and mesoscale convection due to typhoon-monsoon flow interactions are the key factors in extreme precipitation events.