Causes of an East Asian Winter Cold Surge and Its Impact on a Flash Flood Event in Malaysiaia
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摘要: 利用卫星观测、ERA5和GDAS再分析数据等对东亚冬季风冷涌引发的一次马来西亚极端暴雨洪水事件进行了分析。结果表明：在东亚冬季风背景下，东亚大槽重建、地面冷高压加强以及日本岛-西北太平洋爆发性气旋发展等为冷涌形成提供了有利的环流背景；热带气旋“雷伊”影响及中高纬度冷空气的持续补充进一步延长了冷涌维持时间。冷涌与热带气旋外围环流、热带东风气流、“海洋性大陆”(Maritime Continent)西部地形等之间的相互作用导致了冷涌涡旋(Borneo Vortex)的产生和维持，为强降水提供了持续的涡旋动力条件。冷涌气流与赤道偏东气流之间的辐合以及与马来半岛海岸线地形的相互作用进一步促进低层动力条件的增强，导致多阶段动力辐合条件的接连作用。冷涌气流所构建的从南海到马来半岛的水汽通道是降水的主要水汽源，其中偏东分量的水汽输入最强、其次为偏北分量，导致对流层低层强水汽辐合条件的产生。冷涌气流南侵过程中，受广阔海洋影响，温湿条件不断增加，且越低层越明显，从而导致不稳定条件的产生和加强。在对流层低层高温、高不稳定能量以及整层高湿环境下，有利于中尺度对流系统发展和高效暖云降水的产生。Abstract: This study analyzed the impact of a cold surge during the East Asian winter monsoon on an extreme flash flood event in Malaysia using satellite observations and reanalysis data including ERA5 from the European Centre for Medium-Range Weather Forecasts and data from the Global Data Assimilation System. The results indicate that, with the East Asian winter monsoon in the Northern Hemisphere, the redevelopment of the East Asian trough, the strengthening of surface cold high pressure, and the development of explosive cyclones over Japan and Northwestern Pacific jointly provided favorable largescale circulation background for the cold surge formation. The impact of Tropical Cyclone Rai and the continuous influx of cold air from mid and high latitudes prolonged the duration of the cold surge. Interactions between the cold surge, the peripheral circulation of the tropical cyclone, the easterly airflow in tropical regions, and the topography of the western Maritime Continent led to the formation and maintenance of the Borneo Vortex, providing continuous dynamic conditions for heavy precipitation. The convergence between the cold surge and the equatorial easterly airflow, along with the interaction between the cold surge and the topography of the Malay Peninsula's coastline, further enhanced low-level dynamic conditions, which then led to the continuous influence of multi-stage dynamic convergence. The water vapor channel established by the cold surge from the South China Sea to the Malay Peninsula was the main source of precipitation moisture, with the easterly component contributing the strongest water vapor input, followed by the northerly component. This pathway created strong water vapor convergence conditions in the lower troposphere near the Malay Peninsula. As the cold surge moved southward, its temperature and humidity conditions are intensified by the vast ocean surface, particularly in the lower layers, leading to the generation and strengthening of unstable conditions. The warm, energetic, and highly humid environment in the lower troposphere was conducive to the development of mesoscale convective systems and the generation of efficient warm-cloud precipitation.
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Key words:
- cold surge /
- Borneo Vortex /
- flash flood /
- water vapor
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图 2 冷涌指数的时间演变(单位：m·s-1)
红色：CSI-C(110.0~117.5 °E沿15 °N平均经向风速)，黑色：CSI-V (105.0~ 112.5 °E沿12°N平均经向风速)，绿色：CSI-Wind(105.0~ 112.5 °E沿12 °N平均全风速)。
图 3 2021年12月17日00时(a、c、e)、12时(b、d、f)大尺度环流形势
a.、b.500 hPa位势高度场(蓝色等值线，单位：dagpm)，风场(风向杆)及水平涡度平流(填色，单位：10-5 s-2); c.、d.海平面气压(填色，单位：hPa)及24 h变压(红色等值线，单位：hPa); e.、f. 925 hPa位势高度场(蓝色等值线，单位：dagpm)，及风场(填色为风速≥8 m·s-1)。
图 4 2021年12月15日00时—18日18时冷涌涡旋演变
a.中心位置(红色，间隔3 h)及12月16日18时850 hPa风场和位势高度场(蓝色等值线，单位：dagpm); b.移动速度(单位：km·h-1)。
图 5 2021年12月16日00时(a、c)、18日00时(b、d)的冷涌风场及动力条件特征
a.、b.925 hPa风场(红色等值线为风速，间隔4 m·s-1)和水平风散度(填色，单位：10−6s−1); c.、d.10 m风场(填色为风速，单位：m·s-1)和海平面气压(蓝色等值线，单位：hPa)。
图 6 冷涌影响期间动力条件的垂直剖面，马来半岛附近(101~104 °E，2~6 °N)最强垂直上升速度(填色, 单位: Pa·s-1)、最强水平散度(黄色虚线, 单位: 10-4 s-1)和平均水平风的时间-垂直剖面(a)，以及2021年12月16日17时(b)沿5 °N和17日17时(c)、18日16时(d)沿4 °N的水平风场(红色等值线为风速，单位：m·s-1)，垂直上升运动(填色，单位: Pa·s-1)纬向-垂直剖面
图 7 2021年12月17日12时500 hPa(a)、850 hPa(b)、925 hPa(c)风场、比湿(蓝色等值线，单位：g·kg-1) 和气温(填色，单位：℃)；925 hPa水汽通量(填色，单位：10-4 g·cm-1·hPa-1·s-1)和水汽通量散度(红色虚线，单位：10-9 g·cm-2·hPa-1·s-1) (d)
图 8 利用NOAA ARL的HYSPLIT迹线模式计算的马来半岛冷涌气流空气质点的后向传播轨迹(a)及其相对湿度(单位：%)(b)和温度(单位：K)(c)演变
图中不同颜色线条代表来源于不同纬度的空气质点。
图 9 马来半岛四周(100~104 °E，4~6 °N)不同方向的水汽收支(等值线，单位：10-4 g·cm-1·hPa-1·s-1)及风场的时间-垂直剖面
a.南侧(4 °N)；b.北侧(6 °N)；c.西侧(100 °E)；d.东侧(104 °E)。
图 10 马来半岛四周(100~104 °E，4~6 °N)不同方向的整层水汽收支垂直积分(单位：10-4 g·cm-1·hPa-1·s-1) 时间演变
天蓝色：南侧(4 °N); 红色：北侧(6 °N); 蓝色：西侧(100 °E)；绿色：东侧(104 °E); 黑色：四个方向总和。
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