Quasi-stationary Band-Shaped Precipitation Systems, Named "Senjo-Kousuitai", Causing Localized Heavy Rainfall in Japan
Quasi-stationary Band-Shaped Precipitation Systems, Named "Senjo-Kousuitai", Causing Localized Heavy Rainfall in Japan
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DOI：
10.2151/jmsj.2020-029
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发表时间：
2020-06-01
影响因子：
3.1
通讯作者：
Kato, Teruyuki
中科院分区：
文献类型：
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作者：
Kato, Teruyuki
日本经常出现局部强降雨，三小时累积降雨量超过200毫米，引发严重山体滑坡和洪水。此类事件主要是由准稳态带状降水系统带来的，日语称为“senjo-kousuitai”。千叠高水台被定义为长50-300公里、宽20-50公里的带状强降雨区，由对流单体相继形成和发展，排列成多单体团，并在几乎同一地点经过或停滞数小时。千叠香水队的形成过程主要分为两种类型：折线型是指由于暖湿空气的流入而在准静止局地锋面上同时形成的对流单体；后建型是指在低层风上游陆续形成的新对流单体与已有的对流单体呈线性排列。在2014年8月20日日本西部观测到的广岛强降雨案例中，千叠高水台的再现需要至少2 km的水平分辨率，足以粗略地解析对流单元的形成和发展过程，而准确再现其内核结构则需要250-500 m的分辨率。当使用事件发生前10小时的初始条件时，2公里模型定量地再现了广岛的情况，但随着初始时间越来越接近千叠高水台的发生时间，最大累积降水量的预测量显着减少。这种减少是由于低空干燥空气过量流入，改变了新的多细胞团的发生区域。根据以往局地性强降雨事件的环境大气场，统计构建了千叠高水台事件诊断预报的六种有利发生条件。根据500米高度数据判断的低空水汽场，代表性地选择了（1）水汽通量大（>150 g·m(-2)·s(-1)）和（2）距离自由对流水平较近（<1000 m）两种条件。选择其他四个有利条件； (3) 中层相对湿度较高（500 hPa 和 700 hPa 时> 60 %），(4) 根据风暴相对环境螺旋度估计的大垂直切变（> 100 m(-2) s(-1)），(5) 天气尺度上升区（700 hPa 时平均场为 400 km），以及 (6) 排除经常出现在700-850 hPa 并抑制对流发展（即平衡水平 > 3000 m）。
In Japan, localized heavy rainfall events producing accumulated three-hour precipitation amounts larger than 200 mm are often observed to cause severe landslides and floods. Such events arc mainly brought by quasi-stationary band-shaped precipitation systems, named "senjo-kousuitai" in Japanese. Senjo-kousuitai is defined as a band-shaped heavy rainfall area with a length of 50-300 km and a width of 20-50 km, produced by successively formed and developed convective cells, lining up to organize multi-cell clusters, and passing or stagnating at almost the same place for a few hours. The formation processes of senjo-kousuitai are categorized mainly into two types; the broken line type in which convective cells simultaneously form on a quasi-stationary local front by the inflow of warm and humid air, and the back building type in which new convective cells successively forming on the upstream side of low-level winds linearly organize with pre-existing cells.In this study, previous studies of band-shaped precipitation systems are reviewed, and the numerical reproducibility of senjo-kousuitai events and the favorable conditions for their occurrence are examined. In a case of Hiroshima heavy rainfall observed in western Japan on 20 August 2014, the reproduction of the senjo-kousuitai requires a horizontal resolution of at least 2 km, which is sufficient to roughly resolve the formation and development processes of convective cells, while a resolution of 250-500 m is necessary to accurately reproduce their inner core structures. The 2-km model quantitatively reproduced the Hiroshima case when initial conditions 10 hours before the event were used, but the predicted amounts of maximum accumulated precipitation were considerably reduced as the initial time became closer to the occurrence time of the senjo-kousuitai. This reduction was brought from the excessive inflow of low-level dry air that shifted occurrence areas of new multi-cell clusters.Six favorable occurrence conditions of senjo-kousuitai events for their diagnostic forecasts were statistically constructed from environmental atmospheric fields in previous localized heavy rainfall events. Two conditions of (1) large water vapor flux amounts (> 150 g m(-2) s(-1)) and (2) short distances to the level of free convection (< 1000 m) were chosen representatively for the low-level water vapor field that is judged based on 500-m height data. Four other favorable conditions are selected; (3) high relative humidity at midlevels (> 60 % at 500 hPa and 700 hPa), (4) large vertical shear estimated from the storm relative environmental helicity (> 100 m(-2) s(-1)), (5) synoptic-scale ascending areas (400 km mean field at 700 hPa), and (6) the exclusion of warm air advection frequently appearing at 700-850 hPa and inhibiting the development of convection (i.e., an equilibrium level > 3000 m).