The Increasing Trend of Intense Precipitation in Japan Based on Four-hourly Data for a Hundred Years

Long-term changes of precipitation intensity were analyzed using a dataset which was recently compiled by the Japan Meteorological Agency (JMA). After some quality check, data of four-hourly, daily, and hourly precipitation at 46, 61, and 8 stations, respectively, were used for the period 1898-2003 on the condition that data for at least 80 years were usable in each month. As the measure of precipitation intensity, ten categories were defined so as to equate the total precipitation amount in each month at each station. The result is characterized by increase of precipitation in high categories, namely intense precipitation, and decrease in low categories. The linear trend for the highest and lowest categories is ±20-30% per century. This feature is found invariably for four-hourly, daily, and hourly precipitation, and qualitatively for all the seasons and regions.