Analysis of Remote Precipitation Caused by Typhoon Khanun (2017)
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摘要: 台风远距离暴雨(TRP)是极端降水事件的重要成因,但业务预报对其量级和落区的系统性低估亟需进行深入的机制研究。以2017年台风“卡努”(登陆广东)在浙江引发的TRP为例,基于地面观测、雷达资料及NCEP CFSR再分析数据(0.5 °×0.5 °),揭示了暴雨的三维动力-热力耦合机制。结果表明:(1) 台风外围环流与低空急流协同构建水汽通道,暖平流显著增强暴雨区大气不稳定能量;(2) 低层台风倒槽北伸至浙江后,在涡度垂直输送(水平辐合辐散与扭曲项主导)作用下演变为切断低压,成为暴雨的直接触发系统;(3) 500 hPa副高边缘突发短波槽,其与低层切断低压的垂直耦合进一步强化上升运动。涡度收支分析表明,近地层涡度通过垂直运动向中高层(850~500 hPa)输送,形成“低层辐合-涡度增长-上升运动加强”的正反馈循环,最终导致极端降水。研究提出,TRP预报需重点关注低层暖平流强度、涡度垂直输送效率及高低空系统配置的协同作用,为改进业务预报模型提供理论依据。Abstract: TRP (Tropical cyclone Remote Precipitation) often produces extreme precipitation events, the actual magnitude of which is larger than forecasted. Therefore, it is necessary to carry out research to improve the forecast accuracy of such pecipitation. This paper analyzed a TRP event in Zhejiang caused by the NO.20 typhoon "Khanun" (which made landfall in Guangdong Province) in 2017. The evolution and characteristics of the precipitation system were analyzed using ground observation and radar data, while the specific causes were investigated using NCEP CFSR reanalysis data (horizontal resolution 0.5 °×0.5 °). The results show that the rainstorm process had the following characteristics. (1) Under the guidance of the typhoon peripheral circulation, there were obvious water vapor channels in the lower layer to transport water vapor to the rainstorm area, accompanied by strong warm advection; (2) The tip of the low-level typhoon trough extended to Zhejiang and developed into a cut-off depression; (3) The rainstorm area was originally controlled by the subtropical high at 500 hPa, and a short-wave trough developed at the edge of the subtropical high during the precipitation process, accompanied by strong upward motion. Vorticity budget analysis showed that the initial disturbance for the development of the cut-off low was mainly provided by the horizontal convergence divergence term and the distortion term in the surface layer. Vorticity was then transmitted upward from the near-surface layer to 850 - 500 hPa, resulting in the development of depression in the low-level layer and the emergence of short-wave trough in the middle layer, while strengthening the upward movement and surface cyclone. The key mechanism for the strong precipitation was the transformation of the typhoon trough into a strong cut-off low due to intense warm advection and vertical vorticity transport at low level. For forecasting, it is crucial to monitor the intensity of warm advection at low level and the coupling of vorticity and vertical movement at low level.
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图 3 2017年10月15日08时(a)、15日20时(b)和16日08时(c)850 hPa高度场(黑色实线,单位:dagpm,下同)、温度场(红色实线,单位:℃,下同)、风场(棕色风羽,单位:m·s-1,下同)和温度平流(填色,单位:10-4 ℃·s-1,下同),15日08时—16日20时远距离降水区域平均的850 hPa温度平流(红色圆点)和涡度(蓝色方块,单位:10-5 s-1)随时间变化曲线(d),15日08时(e)和16日08时(f)500 hPa高度场、温度场、风场和850 hPa水汽通量(填色,单位:g·cm-1·s-1·hPa-1),15日08时假相当位温(填色,单位:K,下同)、垂直速度(黑色实线,单位:10-1 hPa·s-1,下同)、风矢量(垂直速度放大10倍)沿122 °E剖面分布(g),16日02时假相当位温、垂直速度、风矢量(垂直速度放大10倍)沿123 °E剖面分布(h)
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