[A-AS04] Extreme Events and Mesoscale Weather: Observations and Modeling

[AAS04-P10] Attribution of historical increase of stationary short-duration heavy rainfall in Kyushu considering interannual variation of moisture transport fields

*Kazuya Wakao¹, Tomonori Sato² (1.Graduate School of Environmental Science, Hokkaido University, 2.Hokkaido University)

Keywords:Heavy Rainfall, Mesoscale Meteorology, Local Climate, Climate Change

Organized mesoscale convective systems that appear stagnant over Kyushu region, southwestern Japan, in early July 2020 caused extreme rainfall for several hours and resulted severe flood. It has been pointed out that short-duration heavy rainfall has increased in western Japan. Researchers have found that the increase in water vapor content associated with rising temperature is the key to understand the reason for increasing frequency of heavy rainfall. However, the amount of water vapor during heavy rainfall generally reflects synoptic-scale environments, such as the position of atmospheric rivers, which are different depending on the disturbance types and atmospheric circulations. It means that the mechanism of recent increase in heavy rainfall should be investigated considering the interannual variations in the atmospheric circulation field that drives moisture transport. This study focuses on the interannual variations in the moisture transport to elucidate the spatial characteristics and the mechanism of recent increase in stationary heavy rainfall occurring in the Kyushu region. We defined heavy rainfall areas (HRAs) as a representation of stationary heavy rainfall events using radar/raingauge-analyzed precipitation amounts produced by the Japan Meteorological Agency (JMA) for the period 2006-2022 (April-November). In order to consider moisture transport associated with synoptic environment, atmospheric circulation fields around Japan were classified using self-organization maps. The classification was based on the sea level pressure which governs the horizontal moisture transport. The spatial characteristics and interannual variation of HRAs were investigated for each classified pressure pattern. Results show that the location and shape of HRAs are different according to the pressure pattern. It is found that the number of HRAs has been increasing since 2015 for HRAs occurring with North Pacific High (NPH) extending to the south of the Kyushu region (NPH type). The interannual variation of HRAs that occurred in the NPH type was further decomposed into three factors: the number of NPH-type pressure pattern, the probability of HRAs occurrence under a given NPH-type pressure pattern, and the number of simultaneous HRAs occurrence, and their interannual variations were investigated. The results show that the interannual variation of HRAs is most significantly correlated with the probability of HRAs occurrence. Similar results are confirmed in the surface rain gauge dataset over 41 years (1980-2020). These results suggest that the recent increase in heavy rainfall is caused by increased probability of heavy rainfall in a specific pressure pattern with dominant North Pacific high.