Long-term changes of heavy precipitation and dry weather in Japan (1901-2004)

Long-term changes in the intensity and frequency of heavy precipitation in Japan were analyzed using quality checked daily precipitation data at 51 stations from 1901 to 2004. The analysis is based on ten categories defined from precipitation intensity and frequency, and some indices of heavy precipitation, such as ! 100 mm days, the annual maximum, and the top 100 cases during the 104 years. The result indicates that heavy precipitation based on these indices has increased during the 104 years. The linear trend of precipitation corresponding to the upper 10% is 2.3% per decade, and that of the number of top 100 cases is 2.6% per decade on the average over the stations. The increase is most pronounced in western Japan and in autumn, while weak, but similar signals are found in other regions and seasons as well. However, no increasing trend is found for less intense precipitation, such as >= 50 mm days and the number of top 1000 cases. Analysis was also made for 5, 11, and 31 day precipitations, and some indices of dry weather. It is found that the frequency of dry weather has increased during the 104 years. The number of days with precipitation less than 1 mm has increased in all the seasons and regions, with a trend of 0.4-0.7% per decade on the average, while the lower 1% of cases of 31-day precipitation have doubled with a trend of 10% per decade.