Presentation Information
[O12-P110]Characteristics and Mechanism of Multi-Peak Rainfall During the Rainy Season
*Leyang Lin1, *Amane Takeuchi1, *Yorito Uchida1, *Natsumi Yamamoto1, *Koki Shinagawa1 (1. MAKUHARI JUNIOR AND SENIOR HIGH SCHOOL SHIBUYA KYOIKU GAKUEN)
Keywords:
meteorology,precipitation,rainy season
Rainfall during the rainy season does not change monotonically over time, and it fluctuates with repeated increases and decreases, exhibiting a multi-peak structure with several maxima. In this study, we focus on peaks in daily precipitation during the rainy season in Japan (hereafter referred to as “peaks”) and aim to clarify their characteristics. First, to examine the basic tendencies in the number of peaks, we used daily precipitation data during the rainy season from 1995 to 2024 at four locations: Wajima, Toyama, Osaka, and Shimizu. The rainy season periods were determined based on announcements by the Japan Meteorological Agency, and years without clearly defined periods were excluded from the analysis. In addition, precipitation associated with typhoons was excluded to focus on rainfall related to the seasonal rain front. To define peaks objectively, several criteria were introduced. Specifically, a peak was defined as a day that satisfies all of the following conditions: (1) the daily precipitation is within the top 25% of that year, (2) the interval between adjacent peaks is at least 5 days, and (3) the precipitation shows a deviation exceeding one standard deviation relative to surrounding days. Based on this definition, the number of peaks in each year was calculated, and their temporal variations and regional differences were compared. As a result, it was found that approximately three to four peaks per year were observed at most locations. This suggests that rainfall during the rainy season has a relatively stable multi-peak structure. Furthermore, even when the thresholds used in the peak definition were slightly varied, no significant changes in the number of peaks were observed. This indicates that the results are not highly sensitive to the definition and can be considered robust. Next to investigate longer-term variations, we analyzed the temporal changes in peak numbers using data from 1960 to 2024 at Wajima. Although there is some year-to-year variability, the regression line is nearly horizontal, and no clear long-term increasing or decreasing trend was identified. In addition, we examined the relationship between the number of peaks and the length of the rainy season. For this purpose, the number of peaks in each year was divided by the duration of the rainy season. The regression line for this ratio was also nearly horizontal, indicating that the frequency of peak occurrence per unit time remains approximately constant. This suggests that there is a proportional relationship between the number of peaks and the length of the rainy season, which means years with longer rainy seasons tend to have more peaks. In conclusion, daily precipitation during the rainy season exhibits a stable multi-peak structure, and the number of peaks is not strongly affected by differences in the definition. It is also related to the length of the rainy season. For future work, we aim to investigate the atmospheric conditions under which each peak occurs to further understand the mechanisms behind this multi-peak structure. We also plan to examine temporal variations in precipitable water around precipitation areas to explore its relationship with moisture transport.