Physical responses of Baiu extreme precipitation to future warming: Examples of the 2018 and 2020 western Japan events

Baiu (Meiyu) is a continuous and large-scale rainfall phenomenon in East Asia, whose extreme events have caused many disasters in recent years. Global warming will accelerate the global water cycle and tend to increase the risks of extreme precipitation events. However, to date, the response of Baiu extreme rainfall to future climate warming is still not well investigated. In this study, by conducting the ensemble experiments with a very high resolution (2 km) regional climate model and pseudo warming technique, we reproduced two Baiu extreme rainfall events in 2018 and 2020 that caused severe losses in western Japan, and investigated its response to a +4K warmer climate. We found that climate warming will apparently increase the intensity of the peak Baiu precipitation, and the increasing rates can be much larger than the well-known Clausius-Clapeyron (C–C) scaling (∼7% K−1) with the maximum rate reaching 16.7% K−1 for Shikoku in the 2018 event. This increase in peak precipitation is jointly caused by the increased atmospheric moisture and the accelerated vertical air motion. However, both super and under C–C scaling of changes are expected for total precipitation, with the range from 1.5% K−1 to 17.4% K−1 for different regions and events. The rainfall change rates are jointly influenced by moisture transport direction and elevation, topography, and vertical air motion. The moisture inflow with lower elevation tends to cause a larger rainfall increase. It should also be noted that our regional simulations underestimate the extreme precipitation when reproducing the two events, which is caused by the bias in the GCM data. Implementing bias correction methods or future improvement of GCM simulations could be the solutions for future study. Overall, this study points out the risk of extreme Baiu rainfall under future global warming, and highlights the different increasing rates of extreme Baiu rainfall due to specific climate conditions.