Temporal variability of daily precipitation concentration in Japan for a century: Effects of air temperature rises on extreme rainfall events

Abstract

Climate change induced by air temperature rises may have a significant impact on change in rainfall patterns. Indeed, flood disasters due to downpours have increased over the last few decades in Japan. This study considers the Lorenz Curve, a concept used in economic theory, to describe spatial-temporal variability in the daily time series of precipitation concentration. The results reveal that annual average air temperature has risen at 10 rural sites and 5 urban sites over the period from 1901 to 2020 (120 years). The obtained trends are positive, and statistically meaningful at the 1% level. Whereas, annual precipitation has not changed significantly in the period (1901–2020) since obtained trends at all 15 sites are non-statistically meaningful (p > 0.01). In the meantime, annual maximum daily precipitation has increased for 1 of 15 sites, i.e. Ishigaki, while annual maximum hourly precipitation has increased for 4 of 15 sites, i.e. Choshi, Sapporo, Tokyo and Fukuoka. Calculated Gini coefficient, i.e. the precipitation concentration index (CI) has increased at all 15 sites in the period (1901–2020). The trends are positive, and statistically significant (p < 0.01). This indicates that rainfall has become more concentrated into a fewer rainy days throughout a year as the air temperature has risen for the 120 year period. The correlation between the precipitation CI and warming trends of individual monthly average air temperatures are weak, i.e. almost no correlation, for April, May, June, and November. Whereas, the obtained Spearman's rank correlation coefficient ρs = 0.78 and 0.72 indicate a strong correlation between the trend in air temperature rises and the trend in precipitation CI for July and August. This suggests that air temperature rises during summer (July and August) play an important role in the change in precipitation characteristics throughout Japan.