5:15 PM - 6:45 PM
[AAS04-P05] Frequency of heavy rainfall and characteristics of mesoscale and synoptic-scale environmental fields in Sanyo
Keywords:Sanyo, heavy rain, moist absolutely unstable layer, topography effect
Sanyo (Hiroshima and Okayama prefectures) is surrounded by the Chugoku and Shikoku Mountain Ranges, and has the characteristics of a Seto Inland Sea climate with low precipitation. The region is vulnerable to landslides and flooding, and in recent years has experienced damage from flooding every year. In particular, the 2014 Hiroshima torrential rains and the heavy rain event 2018 West Japan caused strong heavy rainfall in Sanyo. The case analysis of these event pointed out the existence of characteristic moisture transport at the synoptic-scale. In recent years, heavy rainfall in East Asia has been evaluated based on the existence of a moist absolutely unstable layer (MAUL), which is pointed out to be related to topography. However, a comprehensive analysis based on multiple cases, focusing on strong heavy rainfall in the Sanyo region, has not progressed. In particular, there are few examples evaluating the relationship between the effect of topography and precipitation at mesoscale, such as mountainous areas and the Seto Inland Sea. In this study, we summarized the characteristics of the synoptic-scale in heavy rainfall days in the mountains and the Seto Inland Sea during the 39-year warm season from 1979 to 2015 by month in relation to water vapor transport. We also evaluated the effect of the topography of Sanyo and the usefulness of MAUL by clarifying the characteristics of the synoptic-scale and mesoscale characteristics of heavy precipitation days in the Chugoku, Shikoku, and Kyushu regions, especially in Sanyo.
A monthly composite analysis of heavy rainfall days in Sanyo was conducted for geopotential height and vertical integrated water vapor transport, and so on, using Japanese 55-years Reanalysis (JRA55). It was found that the heavy rainfall occurred in different environmental fields in each month between June and September. June and September are characterized by stationary fronts and water vapor transport and instability in the upper atmosphere that enhance precipitation. The north-south pattern is characterized by the co-movement of the sea-level corrective pressure over the tropical western North Pacific and the geopotential height over Japan, and the baroclinic structure causes instability in the upper atmosphere, which is a factor contributing to precipitation.
While It is well known the characteristics of the pattern of heavy rainfall throughout western Japan, the pattern of precipitation maxima days in Sanyo had not been elucidated. Therefore, we investigated the characteristics of such days. As a result, it was found that there are two major types of cases: (1) cases in which a low-pressure system exists in the south of western Japan and water vapor flows northward by a strong pressure gradient with the Pacific High, and (2) cases in which water vapor transport from a tropical cyclone over the South China Sea flows from western China via the southeastern part of China. In the former case, precipitation was generated by the lower-level water vapor flowing into the Kii and Bungo Channels and being lifted by the Sanyo mountains and fronts. In addition, MAUL developed in coincidence with the precipitation distribution. In the latter case, the convergence and divergence of the thick water vapor transport by the complex topography of the Sanyo region is considered to have led to precipitation. Comparing with the former cases, MAUL didn’t develop. Although there were some cases MAUL formed, it couldn’t explain the rainfall distribution. By focusing on the total magnitude pattern of precipitation maxima in Sanyo identified in this study, it may be possible to predict the risk of heavy rain and landslide disasters in Sanyo several days in advance. In addition, the suggestion that the precipitation distribution associated with MAUL varies depending on the characteristics of the total magnitude and moisture transport is useful for the evaluation of short duration strong precipitation forecasts.
A monthly composite analysis of heavy rainfall days in Sanyo was conducted for geopotential height and vertical integrated water vapor transport, and so on, using Japanese 55-years Reanalysis (JRA55). It was found that the heavy rainfall occurred in different environmental fields in each month between June and September. June and September are characterized by stationary fronts and water vapor transport and instability in the upper atmosphere that enhance precipitation. The north-south pattern is characterized by the co-movement of the sea-level corrective pressure over the tropical western North Pacific and the geopotential height over Japan, and the baroclinic structure causes instability in the upper atmosphere, which is a factor contributing to precipitation.
While It is well known the characteristics of the pattern of heavy rainfall throughout western Japan, the pattern of precipitation maxima days in Sanyo had not been elucidated. Therefore, we investigated the characteristics of such days. As a result, it was found that there are two major types of cases: (1) cases in which a low-pressure system exists in the south of western Japan and water vapor flows northward by a strong pressure gradient with the Pacific High, and (2) cases in which water vapor transport from a tropical cyclone over the South China Sea flows from western China via the southeastern part of China. In the former case, precipitation was generated by the lower-level water vapor flowing into the Kii and Bungo Channels and being lifted by the Sanyo mountains and fronts. In addition, MAUL developed in coincidence with the precipitation distribution. In the latter case, the convergence and divergence of the thick water vapor transport by the complex topography of the Sanyo region is considered to have led to precipitation. Comparing with the former cases, MAUL didn’t develop. Although there were some cases MAUL formed, it couldn’t explain the rainfall distribution. By focusing on the total magnitude pattern of precipitation maxima in Sanyo identified in this study, it may be possible to predict the risk of heavy rain and landslide disasters in Sanyo several days in advance. In addition, the suggestion that the precipitation distribution associated with MAUL varies depending on the characteristics of the total magnitude and moisture transport is useful for the evaluation of short duration strong precipitation forecasts.