Secular variation in the occurrence rate of convective rainfall across the Kanto District during Summer

Abstract

This study was undertaken to clarify the secular variation in the occurrence rate of convective rainfall across the Kanto district during summer. The data used for this study were hourly precipitation (108 locations) from AMeDAS observation sites in July and August between 1980 and 2009, and daylight hours and wind direction/speed (73 locations). First, 850 sunny days were selected for which synoptic-scale disturbances in and around the target study area were not analyzed with the data from the 00UTC weather chart and AMeDAS daylight hour data. The rainfall occurring on these sunny days was considered to be due to a convective system accompanying the solar heating of the ground surface. There are three types of secular variation in the occurrence of convective rainfall. When there was an increase in the occurrence rate of type-I convective rainfall, there was a secular decrease in the occurrence of type III. Consequently, a negative correlation between types I and III was confirmed. When the geographic distribution of each type was compared by quantity, type I was distributed in the northern Kanto mountain ranges, while types II and III were distributed the foothills of the northern mountains and southern Kanto. An increase in the occurrence rate of convective rainfall and secular tendencies were confirmed for type I. In contrast, for type III, while the intensity and frequency of convective rainfall both decreased, the intensity of overall rainfall increased. In conclusion, this study highlights the importance of discussing the secular variability in the occurrence of rainfall in southern Kanto including the Tokyo metropolitan area in relation to secular variation on the Kanto district scale. Furthermore, the variability should be analyzed while taking the speed of land and sea breezes into account, along with synoptic-scale disturbances in rainfall intensity.