Diurnal variation in the frequency of heavy precipitation in Japan

By using data for 18 years, diurnal variation in the frequency of heavy precipitation in Japan was analyzed. Diurnal variation patterns at each station were obtained for three durations (one, three and six hours), and several thresholds ranging from moderate to very high intensities (5-80 mm/h, 10-160 mm/3 h and 20-320 mm/6 h). Then their regional characteristics were examined by using the fuzzy c-means method (FCM) in a search for representative patterns. The results are summarized as follows: (a) In coastal regions a morning maximum prevails, especially on the western side of land, (b) A broad maximum around midnight is found in many regions, especially on the southern and eastern sides of mountainous regions, as well as a part of the inland region, (c) A sharp maximum in late afternoon is found in part of the inland regions, especially in the eastern part of mountain regions and the adjacent plain, and (d) A maximum around midday is found over the Nansei Islands. The midnight maximum is more conspicuous for heavier precipitation on longer time scales (three- and six-hourly), in agreement with the empirical knowledge among forecasters that disastrous rainfall is more often encountered in the nighttime.