Identification of key factors in future changes in precipitation extremes over Japan using ensemble simulations

Abstract

Key factors in future changes in wintertime precipitation extremes over an area of the Pacific Ocean side of Japan (the Tokai region) at the end of the 21st century are identified using ensemble simulations projected by a non-hydrostatic regional climate model (NHRCM), driven at the lateral boundaries by an atmospheric general circulation model forcing under the Representative Concentration Pathway 8.5 scenario. The 99th percentile of projected daily precipitation, a measure of precipitation extremes, noticeably increases over the Tokai region in winter. Differences in meteorological variables related to precipitation between the present and future climates are investigated. It is found that a key factor in changes in precipitation extremes is the increases in specific humidity over a deeper layer (from the lower to middle troposphere). The low-level moist flow associated with an extratropical cyclone is southerly and impinges on coastal mountains in the Tokai region, leading to enhanced convergence and precipitation. This orographically induced precipitation can be enhanced when mid-level specific humidity increases in the future climate, as well as at lower levels.