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[ACG30-P05] The Increase in "widespread and long-lasting heavy rainfall" in summer and The meandering of westerlies.
Keywords:widespread and long-lasting heavy rainfall, Baiu front, The meandering of westerlies
In recent years, there have been many cases of "long-lasting" rainfall over a "wide area" as examples of remarkable heavy rainfall. Previous studies have investigated the occurrence sites of extreme "widespread heavy rainfall" (Shibuya et al., 2021). However, there has been no statistical investigation of "widespread" and "long-lasting" heavy rainfall events, characteristics of atmospheric fields, and reasons for long-lasting rainfall events. This study aims to investigate the characteristics of the atmospheric field at the time of "widespread and long-lasting heavy rainfall", the causes of its persistence, and the changes over recent years.AMeDASwas used for precipitation data, and JRA-55 was used for atmospheric field data. Daily data were used for both data sets, and the analysis period was June, July, and August from 1982 to 2021. First, we defined a "widespread rainfall day" as a day when more than 50% of all stations observed daily precipitation of 1.0 mm or more. The "widespread long-duration rainfall" is defined as either 1) a series of three or more widespread rainfall days or 2) a series of three or more consecutive rainfall days, excluding days that do not fall into the category of widespread rainfall days but are covered on the days before and after the widespread rainfall day. Next, we defined "widespread long-term heavy rainfall" as those with a daily average of 10,000 mm or more among the widespread long-term rainfall cases. Tsuguchi and Kato 2014 investigated the factors that bring about long-lasting heavy rainfall over a wide area. Finally, Based on the results, we categorized the long-lasting heavy rains brought by the rainy season fronts by westerly wind meandering patterns and conducted composite map analysis for each pattern to compare the characteristics of atmospheric fields and heavy rains, and to investigate the causes of prolonged rainfall and changes over the recent years. First, 554 days of rainfall were extracted from 91 cases of widespread long-term heavy rainfall over a wide area. Of these, about 83% (464 days) were attributed to the baiu front. suggesting that the rainy season front is the main cause of the "widespread long-term heavy rainfall".Next, westerly wind meandering patterns were classified into (1) the west trough type, (2) the east trough type, and (3) the mixed trough, and a composite map analysis was conducted. In the case of (1), warm air advection in the lower troposphere was dominated by a trough west of Japan. This enhanced the north-south gradient of the 850-hPa equivalent potential temperature, which in turn enhanced convection in the baiu frontal zone. The persistence of this effect is considered to have enhanced convection in the frontal zone of the baiu season, resulting in widespread and long-lasting heavy rainfall. In case (2), cold air advection by a trough extending from east to north of Japan and warm air advection by the overhang of the Pacific High was dominant. In the case of (3), both (1) and (2) were characterized, and cold and warm air advection was predominant. Finally, to investigate the cause of the longer duration, a wide area, short duration heavy rainfall was extracted and compared to the longer duration cases.
The results show that the westerly wind meandering is not persistent in the widespread short-duration heavy rainfall events, which results in the absence of temperature advection and water vapor supply by the overhang of the Pacific High. These two factors are thought to contribute to the prolonged duration. In addition, the prolongation, the increase in outbreaks, and the amount of precipitation was observed for the (3) over recent years. This suggests that the persistence of westerly meandering patterns such as the (3) may have an influence on the increasing tendency of long-lasting heavy rainfall over a wide area.
The results show that the westerly wind meandering is not persistent in the widespread short-duration heavy rainfall events, which results in the absence of temperature advection and water vapor supply by the overhang of the Pacific High. These two factors are thought to contribute to the prolonged duration. In addition, the prolongation, the increase in outbreaks, and the amount of precipitation was observed for the (3) over recent years. This suggests that the persistence of westerly meandering patterns such as the (3) may have an influence on the increasing tendency of long-lasting heavy rainfall over a wide area.