Recent positive trend in heavy rainfall in eastern Japan and its relation with variations in atmospheric moisture

Abstract

A recent trend in heavy rainfall in eastern Japan was studied using 31 years' worth of AMeDAS (1976–2006) hourly rainfall data having fine spatial resolution. The non-parametric Wilcoxon rank-sum test was applied to the heavy rainfall in two 10-year periods (1980–1989 and 1997–2006), and the trends determined were compared with recent variations in atmospheric moisture. Two regions were found to be representative of the significant positive trend in heavy rainfall. One is the region around the southeastern foot of the mountains, where the evening convective maximum was naturally dominant. Heavy rainfall increased in the evening on days when no synoptic-scale disturbances passed over the region and thermally induced local circulation toward the mountainous region prevailed. In addition, the specific humidity at 850 hPa also increased on convective days at the nearest upper air observation site. It is reasonable to suggest that the decrease in atmospheric stability due to the increase in specific humidity enhanced the existing mechanism for the evening convective maximum that is specific to the southeastern foot of the mountains. The other representative region is the region around the Pacific coastline, where the early morning convective maximum was naturally dominant. Heavy rainfall increased in the early morning on days when synoptic-scale disturbances passed over the region. Cumulonimbus clouds became active in the months when atmospheric stability was low; however, the atmospheric stability was not controlled by the water vapour in the lower atmosphere. It was notable that the variations in the water vapour made different contributions to the recent trends in these two regions, even though the distance between the two regions is 150 km. Copyright © 2011 Royal Meteorological Society