An extreme rainfall event occurred over western Japan and the adjacent Tokai region mainly in early July, named "the Heavy Rain Event of July 2018", which caused widespread havoc. It was followed by heat wave that persisted in many regions over Japan in setting the highest temperature on record since 1946 over eastern Japan as the July and summertime means. The rain event was attributable to two extremely moist airflows of tropical origins confluent persistently into western Japan and largescale ascent along the stationary Baiu front. The heat wave was attributable to the enhanced surface North Pacific Subtropical High and upper-tropospheric Tibetan High, with a prominent barotropic anticyclonic anomaly around the Korean Peninsula. The consecutive occurrence of these extreme events was related to persistent meandering of the upper-level subtropical jet, indicating remote influence from the upstream. The heat wave can also be influenced by enhanced summertime convective activity around the Philippines and possibly by extremely anomalous warmth over the Northern Hemisphere midlatitude in July 2018. The global warming can also influence not only the heat wave but also the rain event, consistent with a long-term increasing trend in intensity of extreme precipitation observed over Japan.

This study investigates the influence of strong southerly moisture flux on an extreme rainfall event over western Japan in early July 2018, by using a global atmospheric reanalysis dataset. During its peak period from 5 to 7 July, extensive and unprecedented rainfall observed along the well-defined quasi-stationary Baiu front was attributed to two branches of extremely moist inflow from the southern confluence into western Japan. One was a shallow southerly airstream enhanced by the surface North Pacific Subtropical High, and the other was a deeper southwesterly airstream accompanying enhanced convection over the East China Sea. Both the vertically integrated moisture flux from the south and its convergence into western Japan reached the highest levels for 60 years due to an overwhelming contribution from the intensified southerlies. Anomalous diabatic heating associated with the active convection over the East China Sea acted to maintain the southwesterly moisture flux by inducing low-level cyclonic potential vorticity anomalies. During the rainfall event, a strong meander of the upper-level subtropical jet associated with the intensified surface North Pacific Subtropical High accompanied an amplified upper-level trough over the Korean Peninsula, which acted to induce ascent dynamically along the Baiu front.

This study investigates future changes in temperature and precipitation extremes over Japan by the end of the 21st century using ensemble simulations under the Representative Concentration Pathway 8.5 scenario projected by a convection-permitting regional climate model with a grid spacing of 2 km.For all ensemble members, the projected future climate indicates robust increases in the 99th percentile of hourly temperature over all regions of Japan. In contrast, the 99th percentile of hourly precipitation increases over the northern and part of the eastern regions of Japan, particularly on the Sea of Japan side of northern Japan in July. A couple of local-scale areas in Hokkaido are identified as a cause of significant increases in rainfall over this region.Increases in horizontal convergence near the surface and in the middle troposphere are responsible for increased heavy precipitation over a local-scale area of Hokkaido in July. The enhanced convergence near the surface can be attributed to strengthened westerly wind, whereas that in the middle layer can be explained by vertically unstable layers.(Citation: Murata, A., H. Sasaki, H. Kawase, M. Nosaka, T. Aoyagi, M. Oh'izumi, N. Seino, F. Shido, K. Hibino, K. Ishihara, H. Murai, S. Yasui, S. Wakamatsu, and I. Takayabu, 2017: Projection of future climate change over Japan in ensemble simulations using a convection-permitting regional climate model with urban canopy. SOLA, 13, 219−223,

Abstract:To facilitate accurate assessments of the regional impacts of global warming, and make informed decisions about appropriate measures to mitigate them, detailed global warming projections with uncertainties are needed. The Ministry of Environment of Japan and the Japan Meteorological Agency performed 21 different multi-scenario and multiensemble experiments in Japan using the regional climate model MRI-NHRCM with a horizontal resolution of 20 km.To estimate the total range of uncertainty due to natural fluctuations and the variety of experimental runs by a single climate model with multi-physics and multi-SST ensembles under each greenhouse gas emission scenario, a unique statistical method that combined a mixture distribution and bootstrap resampling was adopted. Based on three models that adopted the Yoshimura scheme as a cumulus convection parameterization, annual mean temperatures in Japan were projected to rise significantly by 1.1 ± 0.4°C, 2.0 ± 0.4°C, 2.6 ± 0.6°C, and 4.4 ± 0.6°C under the RCP2.6, RCP4.5, RCP6.0, and RCP8.5 scenarios, respectively, at the end of the 21st century relative to the end of the 20th century (ensemble means ± standard deviations). In contrast, changes in future annual precipitation over Japan were projected to be statistically insignificant.

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