Mountainous areas along the Kaoping River (高屏溪) are most likely to see torrential rainfall after a typhoon leaves Taiwan, a study showed yesterday.
The study was conducted by the National Science and Technology Center for Disaster Reduction and was presented at the Conference on Weather Analysis, Forecasting and Seismic Observation at the Central Weather Bureau in Taipei.
The study found that typhoons making landfall in Hualien, crossing the Central Mountain Range and leaving via the west coast can trigger torrential rainfall both before and after they make landfall.
The research analyzed 112 typhoons that hit the nation between 1992 and last year, and it identified five regions in which torrential rain was most likely to occur — mountainous areas in Yilan County, the north side of Hualien and along the Kaoping River, as well as Snow Mountain (雪山) and Alishan (阿里山).
Mountainous areas along the Kaoping River were identified as the most susceptible to torrential rain, the study showed, adding that the area had accumulated rainfall of 3,000mm when Typhoon Morakot hit in August 2009.
The study also found that although the Central Weather Bureau lifted the alert for Morakot within five days of first issuing a sea alert, the frequency for the occurrence of torrential rainfall, in which the hourly rainfall exceeded 40mm, was above 40 percent.
Meanwhile, another study conducted by the bureau showed that average temperatures have risen by 0.8oC in the past 100 years. In particular, the average temperature in metropolitan areas has risen by 1.4oC.
While annual rainfall has not varied much in the past century, the study showed that the number of days when it rains have gradually decreased. The decrease was more obvious in the spring and the summer, it showed.
However, the number of days when it rains in the fall has actually increased, as has the intensity of the rainfall.
Among the 10 times where single-day rainfall exceeded 350mm, eight of them occurred after 1991. The occurrence of extreme torrential rainfall became more frequent after 1996, the study showed.