THE DIFFERENCE OF RAINFALL DISTRIBUTION IN RELATION TO TIME-SCALE

Abstract

The purpose of this study is to investigate the difference of rainfall distribution in relation to time-scale, in other words to clarify the difference between rainfall distribution for a short time-scale and that for a longer time-scale. The heavy rainfall of September 8_??_13, 1976, in the Shikoku island caused by typhoon 7617 Fran is chosen as a case study for this purpose. Hourly precipitation data were collected from 363 raingauge stations in the Shikoku island (Fig. 1). Typhoon 7617 Fran affected western Japan from September 8 to 13, 1976 (Fig. 2), and more than 1, 000 mm of rain fell on the Shikoku island in a period of six days (Figs. 4 and 5, the recorded maximum total precipitation for six days was 2, 781 mm at Hiso, Tokushima prefecture). This typhoon lingered over the sea to the southwest of Kyushu from September 10 to 12 (Fig. 2), when no appreciable change in synoptic conditions took place (Fig. 3). Therefore, the period from 9 a. m. on September 10, to 9 a. m. on September 12 was chosen for this study. Rainfall distribution maps for hourly, 3 hours, 6 hours, 12 hours and 24 hours were delineated. All maps except the hourly map were compiled by moving averages based on hourly precipitation. Axes are drawn in areas of heavy rainfall, with precipitation exceeding one-half of the maximum precipitation, which are illustrated in each chart. Summaries of the axes are presented on maps for each time-scale (Fig. 6). The axes in a shorter time-scale (1 hour or 3 hours) are widely and randomly distributed. Any close relationship is not found between the distribution of the axes and the topography. This shows that the distribution of rainfall in this time-scale markedly changes with time, and that heavy rainfall was observed in wider areas. On the other hand, the axes in longer time-scales (12 hours or 24 hours) are unevenly distributed, and there exists an obvious relationship between the distribution of the axes and landforms. The heavy rainfall areas in such time-scales always appear along the windward side of the crests. To conclude from the results described above, the regionality of rainfall distribution becomes clear as the time-scale increases. The distribution of rainfall in a short time-scale is not related to synoptic condition or topography; however, those in longer time-scales are controlled by synoptic condition.