The Japan Meteorological Agency defines extreme climate events as phenomena occurring once every 30 years or less. To prepare for disasters, we investigated future projections of 30-year return values of heavy precipitation in the Tohoku District of northern Japan based on a large number of ensemble warming projections that are dynamically downscaled to 5-km grids under the Social Implementation Program on Climate Change Adaptation Technology. The 30-year return values of daily and hourly precipitation are projected to significantly increase under global warming, indicating the strengthening of extremely heavy rainfall. Their averaged ratios across the Tohoku District in the 2-K and 4-K warmer climates to those in the present climate are 1.12 and 1.30 times for daily precipitation and 1.18 and 1.45 times for hourly precipitation, respectively. In particular, the 30-year return values are enormous on the eastern slopes of the Kitakami and Abukuma Mountains and the Ou Mountain Range. The rates of increase in the 30-year return values are pronounced in the northern part of Tohoku, where the surface air temperature increase is more significant than in other regions. These results suggest the need to upgrade disaster prevention measures for heavy rainfall as a climate change adaptation in the Tohoku District. We also examined the regional dependency of seasonal variation in the occurrence rates of daily and hourly extreme events. The results indicated that the occurrence rates tend to be relatively high on the Sea of Japan side of the Tohoku District in July due to the Baiu front, over the entire Tohoku District in August due to strong convective instability, and on the Pacific Ocean side due to September typhoons and autumn stationary fronts. This seasonality is projected to remain almost unchanged under warmer conditions.

1. [1] Intergovernmental Panel on Climate Change, “Climate change 2021: The physical science basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change,” Cambridge University Press, 2021. https://doi.org/10.1017/9781009157896
2. [2] “Paris agreement/CP/2015/L.9/Rev.1,” United Nations Framework Convention on Climate Change, 2015.
3. [3] C. Li, F. Zwiers, X. Zhang, G. Li, Y. Sun, and M. Wehner, “Changes in annual extremes of daily temperature and precipitation in CMIP6 models,” J. of Climate, Vol.34, No.9, pp. 3441-3460, 2021. https://doi.org/10.1175/JCLI-D-19-1013.1
4. [4] F. Fujibe, “Clausius–Clapeyron-like relationship in multidecadal changes of extreme short-term precipitation and temperature in Japan,” Atmospheric Science Letters, Vol.14, Issue 3, pp. 127-132, 2013. https://doi.org/10.1002/asl2.428
5. [5] Japan Meteorological Agency (JMA), “Climate change monitoring report 2020,” 2022. https://www.jma.go.jp/jma/en/NMHS/ccmr/ccmr2020.pdf [Accessed July 31, 2024]
6. [6] Y. Imada, H. Kawase, M. Watanabe, M. Arai, H. Shiogama, and I. Takayabu, “Challenges of risk-based event attribution for heavy regional rainfall events,” npj Climate and Atmospheric Science, Vol.3, Article No.37, 2020. https://doi.org/10.1038/s41612-020-00141-y
7. [7] H. Kawase, M. Yamaguchi, Y. Imada, S. Hayashi, A. Murata, T. Nakaegawa, T. Miyasaka, and I. Takayabu, “Enhancement of extremely heavy precipitation induced by Typhoon Hagibis (2019) due to historical warming,” SOLA, Vol.17A, Special Edition, pp. 7-13, 2021. https://doi.org/10.2151/sola.17A-002
8. [8] JMA, “Global warming projection Vol.9,” 2017 (in Japanese).
9. [9] Sendai District Observatory, “Global warming projection in Tohoku District,” 2019 (in Japanese).
10. [10] R. Mizuta, A. Murata, M. Ishii, H. Shiogama, K. Hibino, N. Mori, O. Arakawa, Y. Imada, K. Yoshida, T. Aoyagi, H. Kawase, M. Mori, Y. Okada, T. Shimura, T. Nagatomo, M. Ikeda, H. Endo, M. Nosaka, M. Arai, C. Takahashi, K. Tanaka, T. Takemi, Y. Tachikawa, K. Temur, Y. Kamae, M. Watanabe, H. Sasaki, A. Kitoh, I. Takayabu, E. Nakakita, and M. Kimoto, “Over 5,000 years of ensemble future climate simulations by 60-km global and 20-km regional atmospheric models,” Bulletin of the American Meteorological Society, Vol.98, No.7, pp. 1383-1398, 2017. https://doi.org/10.1175/BAMS-D-16-0099.1
11. [11] M. Fujita, R. Mizuta, M. Ishii, H. Endo, T. Sato, Y. Okada, S. Kawazoe, S. Sugimoto, K. Ishihara, and S. Watanabe, “Precipitation changes in a climate with 2-K surface warming from large ensemble simulations using 60-km global and 20-km regional atmospheric models,” Geophysical Research Letters, Vol.46, Issue 1, pp. 435-442, 2019. https://doi.org/10.1029/2018GL079885
12. [12] M. Ishii and N. Mori, “d4PDF: Large-ensemble and high-resolution climate simulations for global warming risk assessment,” Progress in Earth and Planetary Science, Vol.7, Article No.58, 2020. https://doi.org/10.1186/s40645-020-00367-7
13. [13] R. Mizuta, H. Yoshimura, H. Murakami, M. Matsueda, H. Endo, T. Ose, K. Kamiguchi, M. Hosaka, M. Sugi, S. Yukimoto, S. Kusunoki, and A. Kitoh, “Climate simulations using MRI-AGCM3.2 with 20-km grid,” J. of the Meteorological Society of Japan, Vol.90A, pp. 233-258, 2012. https://doi.org/10.2151/jmsj.2012-A12
14. [14] H. Sasaki, A. Murata M. Hanafusa, M. Oh’izumi, and K. Kurihara, “Reproducibility of present climate in a non-hydrostatic regional climate model nested within an atmosphere general circulation model,” SOLA, Vol.7, pp. 173-176, 2011. https://doi.org/10.2151/sola.2011-044
15. [15] A. Murata, H. Sasaki, M. Hanafusa, and K. Kurihara, “Estimation of urban heat island intensity using biases in surface air temperature simulated by a nonhydrostatic regional climate model,” Theoretical and Applied Climatology, Vol.112, pp. 351-361, 2013. https://doi.org/10.1007/s00704-012-0739-2
16. [16] M. Ohba and S. Sugimoto, “Differences in climate change impacts between weather patterns: Possible effects on spatial heterogeneous changes in future extreme rainfall,” Climate Dynamics, Vol.52, pp. 4177-4191, 2019. https://doi.org/10.1007/s00382-018-4374-1
17. [17] Y. Osakada and E. Nakakita, “Future change of occurrence frequency of baiu heavy rainfall and its linked atmospheric patterns by multiscale analysis,” SOLA, Vol.14, pp. 79-85, 2018. https://doi.org/10.2151/sola.2018-014
18. [18] K. Yoshida, M. Sugi, R. Mizuta, H. Murakami, and M. Ishii, “Future changes in tropical cyclone activity in high-resolution large-ensemble simultaions,” Geophysical Research Letters, Vol.44, Issue 19, pp. 9910-9917, 2017. https://doi.org/10.1002/2017GL075058
19. [19] D. Hatsuzuka, T. Sato, K. Yoshida, M. Ishii, and R. Mizuta, “Regional projection of tropical-cyclone-induced extreme precipitation around Japan based on large ensemble simulations,” SOLA, Vol.16, pp. 23-29, 2020. https://doi.org/10.2151/sola.2020-005
20. [20] M. Nosaka, M. Ishii, H. Shiogama, R. Mizuta, A. Murata, H. Kawase, and H Sasaki, “Scalability of future climate changes across Japan examined with large-ensemble simulations at +1.5-K, +2-K, and +4-K global warming levels,” Progress in Earth and Planetary Science, Vol.7, Article No.27, 2020. https://doi.org/10.1186/s40645-020-00341-3
21. [21] T. Miyasaka, H. Kawase, T. Nakaegawa, Y. Imada, and I. Takayabu, “Future projections of heavy precipitation in Kanto and associated weather patterns using large ensemble high-resolution simulations,” SOLA, Vol.16, pp. 125-131, 2020. https://doi.org/10.2151/sola.2020-022,2020
22. [22] JMA, “Kanto-Tohoku heavy rainfall in September, 2015 and heavy rainfall associated with T1518,” 2015. https://www.jma.go.jp/jma/kishou/books/saigaiji/saigaiji_2015/saigaiji_201501.pdf [Accessed August 2, 2024]
23. [23] Y. Nihei, K. Ohtsuki, H. Nagano, T. Hattori, T. Sakuraba, Y. Kurakami, M. Tanaka, and K. Tomita, “Field investigation on flood disaster in Kinugawa river basin: Kanto-Tohoku flood hazard, 2015,” Advances in River Engineering, Vol.22, pp. 321-326, 2016 (in Japanese). https://doi.org/10.11532/river.22.0_321
24. [24] D. Komori, V. S. A. Hendrawan, A. Ichiba, K. Yamada, and A. Goda, “Mechanism of rainfall inundation caused by the typhoon Hagibis in Iwate Prefecture coastal zone, Japan,” J. of JSCE, Vol.10, pp. 195-205, 2022. https://doi.org/10.2208/journalofjsce.10.1_195
25. [25] JMA, “Heavy rainfall from 1 to 6 August 2022 caused by frontal activity,” 2023. https://www.jma.go.jp/jma/kishou/books/saigaiji/saigaiji_2022/saigaiji_202301.pdf [Accessed August 2, 2024]
26. [26] Y. Matsubayashi and N. Konno, “Report on survey of flood damage of August 2022 in Ichinohe town,” Tohoku J. of Natural Disaster Science, Vol.59, pp. 81-86, 2023 (in Japanese).
27. [27] M. Sasaki, T. Takeuchi, S. Takase, and Y. Mitobe, “Investigation and research on downpour disaster in August, 2022 in Aomori Prefecture,” Tohoku J. of Natural Disaster Science, Vol.59, pp. 93-98, 2023 (in Japanese).
28. [28] JMA, “Heavy rainfall due to the Baiu front in 2023,” 2023. https://www.data.jma.go.jp/obd/stats/data/bosai/report/2023/20230808/jyun_sokuji20230628-0716.pdf [Accessed December 1, 2023]
29. [29] H. Matsumori, T. Kamataki, and F. Konno, “On inundation in the central district of Akita city caused by the Akita torrential rainfall in July 2023,” Tohoku J. of Natural Disaster Science, Vol.60, pp. 67-70, 2024 (in Japanese).
30. [30] T. Sasai, H. Kawase, H. Kanno, Y. J. Yamaguchi, S. Sugimoto, T. Yamazaki, H. Sasaki, M. Fujita, and T. Iwasaki, “Future projection of extreme heavy snowfall events with a 5 km large ensemble regional climate simulation,” J. of Geophysical Research: Atmospheres, Vol.124, Issue 24, pp. 13975-13990, 2019. https://doi.org/10.1029/2019JD030781
31. [31] H. Kawase, T. Sasai, T. Yamazaki, R. Ito, K. Dairaku, S. Sugimoto, H. Sasaki, A. Murata, and M. Nosaka, “Characteristics of synoptic conditions for heavy snowfall in western to northeastern Japan analyzed by the 5-km regional climate ensemble experiments,” J. of the Meteorological Society of Japan, Vol.96, Issue 2, pp. 161-178, 2018. https://doi.org/10.2151/jmsj.2018-022
32. [32] T. Hoshino and T. J. Yamada, “Analysis of spatiotemporal characteristics of annual maximum rainfall using a large-ensemble dataset (d4PDF): Application to Tokachi river basin,” J. of Japan Society of Civil Engineers, Ser. G (Environmental Research), Vol.74, Issue 5, pp. I_25-I_31, 2018 (in Japanese). https://doi.org/10.2208/jscejer.74.I_25
33. [33] H. Kawase, M. Nosaka, S. I. Watanabe, K. Yamamoto, T. Shimura, Y. Naka, Y.-H. Wu, H. Okachi, T. Hoshino, R. Ito, S. Sugimoto, C. Suzuki, S. Fukui, T. Takemi, Y. Ishikawa, N. Mori, E. Nakakita, T. J. Yamada, A. Murata, T. Nakaegawa, and I. Takayabu, “Identifying robust changes of extreme precipitation in Japan from large ensemble 5-km-grid regional experiments for 4K warming scenario,” J. of Geophysical Research: Atmospheres, Vol.128, Issue 18, Article No.e2023JD038513, 2023. https://doi.org/10.1029/2023JD038513
34. [34] K. Hirano, “Relationship between rainfall return period and disaster-hit region during the heavy rain event of July 2018 in Japan,” National Disaster Research Report, Vol.53, pp. 59-66, 2019 (in Japanese). https://doi.org/10.24732/nied.00002171
35. [35] N. Tanaka, T. Ohta, and Y. Makihara, “Flood warning/advisory improvement based on JMA runoff index,” Sokko-Jihou, Vol.75, pp. 35-69, 2008 (in Japanese).
36. [36] H. Terada and H. Nakaya, “Operation methods of critical rainfall for warning and evacuation from sediment-related disasters,” Technical Note of National Institute for Land and Infrastructure Management, Vol.5, pp. 1-58, 2001 (in Japanese).
37. [37] R. Mizuta and H. Endo, “Projected changes in extreme precipitation in a 60-km AGCM large ensemble and their dependence on return periods,” Geophysical Research Letters, Vol.47, Issue 13, Article No.e2019GL086855, 2020. https://doi.org/10.1029/2019GL086855