摘要:2018年8月30~31日一条超长生命期的中尺度线状对流（线状β-MCS）引发了华南沿海一次极端降水事件[1056.7 mm（24 h）?1]，刷新广东省24小时雨量历史纪录，造成严重洪涝并引起社会极大关注。文章采用多源观测资料与NCEP/NCAR_FNL分析资料，首先从观测分析角度提炼该次过程的降水特点与卫星、雷达的基本演变特征，然后分析了极端降水的天气尺度背景与中尺度环境条件，最后从中尺度大气动力学角度探讨超长生命期线状β-MCS的触发演变与海陆边界风向旋转维持的关系，并揭示线状对流组织与维持的可能物理机制。结果表明：季风低压作为稳定的天气尺度背景，大气层结表现为深厚暖湿与持续不稳定，季风云团北推上岸造成华南沿海大范围暴雨，一条超长生命期、准静止、低顶高、低质心并具备后向传播特征的线状β-MCS造成高潭持续性强降雨，降水强度大、持续时间极长是累积雨量破纪录的主要原因。对流触发及线状β-MCS组织发展与地面风场有密切关系，海陆边界风向旋转率方程定性分析发现地面风场受多尺度调节影响，季风低压的天气尺度项、局地地形摩擦项与中尺度气压梯度项对地面风场协同形成的反向强迫及平衡机制，是偏南气流长时间维持的关键。斜坡地形与黄江河谷一侧偏南气流增强并建立“暖脊”，致使山脉一侧冷池出流边界无法向南扩展，形成强烈的水平温度梯度，基于中尺度动力学方程定量诊断表明β-MCS的线状组织过程及对流维持的动力机制来源于局地垂直风切变，这种局地垂直风切变有别于环境垂直风切变，其显著增强是对地面强烈纬向水平温度梯度响应的结果。
关键词:极端降水事件/
线状β-MCS（中尺度线状对流）/
对流组织化/
温度梯度/
局地垂直风切变
Abstract:An extreme rainfall event [maximum of 1056.7 mm (24 h)?1] induced by an ultra-long-duration, linearly-shaped mesoscale convective system (β-MCS) occurred over Gaotan town (GT) of the Guangdong Province on 30–31 August 2018. This event broke the highest record for the Guangdong Province, caused a severe flash flood, and created social concern. Using multiple observations and NCEP/NCER_FNL data, we performed an evaluation of the precipitation, convection, and environmental conditions as well as initiation and maintenance for the β-MCS. The data showed that the tropical cloud clusters moved northward and induced large-scale heavy rainfall, with a background of a monsoon depression in a mesoscale favorable environment. A linearly-shaped β-MCS, characterized with a back-building, ultra-long-duration, quasi-stationary, low echo-top-height, and low echo-convective-centroid was responsible for the record-breaking rainfall over GT. Analysis using a rotation rate equation of sea and land breezes indicated the convection initiation and organization are closely related to the near-surface flow affected by multiscale systems. Southerly flow was sustained for a long period that was determined by reverse forces between the monsoon depression, local terrain, and barometric gradient, while all three came to a balance. Strengthening of the southerly flow on the HJ river valley side over the terrain slope helped warm-ridge development of the temperature field, blocking cold pooling. The outflow boundary moved southeastward on the mountain side over the terrain slope, leading to a sharp temperature gradient in that region. Quantitative diagnosis using a mesoscale atmospheric dynamics equation demonstrated the dynamic mechanism sustaining the convection maintenance and β-MCS organization to local vertical wind shear, causing a sharp temperature gradient.
Key words:Extreme rainfall/
Linearly-shaped β-MCS (Mesoscale Convective System)/
Convection organization/
Temperature gradient/
Local vertical wind shear
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