The environmental characteristics of a heavy rainfall event caused by a linear-stationary heavy rainfall area (LS-HRA) that occurred in Yamagata–Niigata prefectures (YNP), northeastern Japan, on 3-4 August 2022 were examined and compared with similar LS-HRAs previously occurring in those prefectures using Japanese Meteorological Agency mesoscale analysis data. During the heavy rainfall event, a low-level meso-α scale low that continued to exist over the Sea of Japan contributed to maintaining strong low-level moisture flux and its convergence around the event region. Low-level, high-equivalent potential temperature provides unstable conditions in the area. Moreover, six favorable conditions for the LS-HRA occurrence were satisfied in upwind areas over the Sea of Japan. A comparison of the environment with five previous events in YNP showed similarities and differences among the events. Common features are the presence of meso-α lows and associated intense low-level moisture flux toward YNP. Meanwhile, the thermodynamic atmospheric conditions differed among the events; the upper-level cold temperature destabilized two cases, while the other three cases, including the present event, were destabilized by the low-level high equivalent potential temperature.

“Senjo-kousuitai” is a quasi-stationary band-shaped precipitation system (QSBPSs). Its frequency of occurrence over Japan during the 2023 rainy season was higher than usual, especially in Kyushu. This paper evaluates the impact of historical anthropogenic global warming and natural variability on the frequency of QSBPSs using risk-based event attribution based on a 100-ensemble regional climate simulation with 5-km grid spacing. In the historical ensemble experiments, the frequency of QSBPSs during the 2023 rainy season exceeded that expected in a typical year. The re-analysis and the ensemble experiment showed a westward extension of the Pacific subtropical high that led to an enhanced water vapor flux over Kyushu, indicating that this synoptic condition was forced by the global distribution of sea surface temperature during the 2023 rainy season. A comparison between historical and non-warming experiments demonstrated that historical anthropogenic global warming increases the occurrence probability of QSBPSs. The rising temperature results in a higher frequency of inflow of large amounts of water vapor, which facilitates the development of QSBPSs. In addition, the decrease in atmospheric stability at low levels, caused by the increase in sea surface temperature, is likely to contribute to an increasing probability of QSBPSs.

Two quasi-stationary quasi-linear convective systems caused local heavy rainfall on 9 August 2013 in the northern Tohoku region of Japan. We investigate this rare event in the region using a numerical simulation and examine airflow structures and mechanisms. The amount and locations of the simulated precipitation agree well with the observed values. The environment is favorable to convective systems, and analyses based on forward trajectories and composites of convective cells clarify airflow structures. Mountains upstream of the extreme precipitation areas trigger the back-building convective systems, whereas humid low-level inflows converging below the convective systems play a role in maintaining the convective systems downstream where there are no significant mountains.