Increasing trends in heavy rain during the warm season in eastern Japan and its relation to moisture variation and topographic convergence

Abstract

ABSTRACTUsing 34 years of hourly observational data collected from 1976 to 2009, the frequency of hourly heavy‐rain events during the warm season in eastern Japan was studied. A significant regional trend of heavy rain was not recognized over the area as a whole; however, stations with increasing trends in heavy rain were detected when daytime convergences towards the central mountains were strong. These stations were concentrated in a mesoscale region at the southern foot of the mountains in the northern Kanto district. In addition, the surface‐specific humidity (qsfc) in the mountainous region nearby also displayed an increasing trend, which was correlated with the increasing frequency of hourly heavy‐rain events. It is thought that the mechanism driving the recent increasing trend in heavy rain is an increase in qsfc in the mountainous region under the conditions that mesoscale convergence of surface winds developed.