MESOSCALE CHARACTERISTIC ANALYSIS AND PRIMARY DISCUSSION ON THE FORMATION OF THE 7 MAY 2017 TORRENTIAL RAINFALL IN GUANGZHOU
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摘要: 2017年5月7日发生在广州北部的特大暴雨，局地性强，最大雨强达184.4 mm/h，3 h雨量突破了广东省历史极值，强降水持续时间长，具有明显的中尺度特征。特大暴雨有A区（花都）和B区（增城、黄埔）两个中心，它们在降水特点、地面中尺度特征及触发、对流的发展演变等方面各有特点。由于天气尺度强迫背景弱，数值模式无明显反映，给预报带来了很大的挑战。利用常规及加密自动站、多普勒雷达、风廓线、地基GPS等非常规观测资料，结合ERA-Interim 0.125 °×0.125 °逐6 h再分析资料重点分析和讨论了此次过程的中尺度特征、对流的触发与演变，以期为今后这类暴雨预报提供着眼点。结果表明：（1）此次过程突发性强，降水强度大，A区降水开始时间早，范围较B区小，但B区小时雨强更强，强降水持续时间更长；（2）次天气尺度边界层“7”字型的风压场形势下，脊后回流并加强的偏南风使暖层和湿层增厚，“下密上疏”的温度垂直结构，为强降水的发生提供了有利的环境条件。进入对流云中水汽质量无异常但产生了大量降水，极高的降水效率很可能是对流系统内部云水高效转化的结果，云的微物理过程在形成此次高强度的降水发挥着重要作用；（3）A区强降水发生前暖空气在山前堆积造成升温升压，东、西两支绕流广州城区的气流汇合并在工业区暖中心、山前暖空气堆积具有较高的对流边界层位置触发了对流；（4）B区强降水发生前持续降压并形成中尺度低压槽，A区中尺度对流系统前方入流造成的负变压，与地形强迫造成的风速辐合共同作用触发了B区对流。中尺度反气旋底部的偏北风与偏南、东南两支气流辐合稳定，使强降水长时间维持；（5）回波具有后向传播，垂直顶高低、质心低的热带对流回波特征，降水效率高。降水的拖曳下沉及蒸发冷却使边界层形成冷池，并与前侧暖湿空气相互作用，不断激发新的对流，冷池出流是持续抬升机制，是强降水持续时间长的重要原因。B区冷池厚度、暖湿气流爬升的高度与坡度比A区更大，冷池出流与暖湿气流辐合强度也比A区更强，造成B区雨强更强、持续时间更长，累积雨量更大。Abstract: A torrential rainfall that occurred on 7 May 2017 in Guangzhou was only in a small range, but the strongest rainfall intensity reached 184.4mm/h, and 3h precipitation broke the historical record of Guangdong province. The torrential rainfall lasted for a long time, with obvious mesoscale characteristics and two centers in region A(Huadu) and region B(Huangpu, Zengcheng). They had their own features in precipitation, surface mesoscale field, triggering mechanism and convection evolution. Due to unfavorable weather conditions and unsatisfactory capabilities of numerical models, forecasters were faced with a great challenge. With analysis and discussion of mesoscale convective system environment, triggering conditions and evolution to the torrential rainfall by using conventional data, encrypted automatic meteorological observations, Doppler weather radar, wind profile, ground-based GPS and ERA-Interim 0.125 °×0.125 ° 6h reanalysis data, it is shown as follows. Firstly, the torrential rainfall is characteristic of suddenness and high intensity. Precipitation started initially in region A, but hourly precipitation intensity in region B is stronger and precipitation duration is longer than that of region A. Secondly, atmospheric circulation in boundary layer of sub-synoptic scale is similar to the shape of '7', and vertical temperature gradient is dense at the low level but sparse at the upper level, a southerly thickens the warm layer and humid layer, being favorable for the torrential rainfall. Highly efficient precipitation is closely associated with efficient condensation inside of convective systems, despite lacking considerable water vapor inflow. Micro-physical processes of cloud played an important role in it. Thirdly, warm air accumulated in front of the hill lead to temperature and pressure rising before the rainfall in region A, and two warm air flows around Guangzhou city became confluent and were uplifted by the two warm centers with high convective boundary layer height, triggering the convection. Fourthly, pressure kept dropping and a mesoscale trough formed before rainfall in region B. The inflow to MCS of region A caused the pressure to drop in region B, and local wind speed convergence by the forcing of complex underlying surface was the main triggering mechanism in region B. Northerly wind from the mesoscale high pressure in region B was convergent with the southeasterly and southerly wind for a long time. Fifthly, backward propagation, low-echo top and low-echo-centroid resulted in highly efficient precipitation. A mesoscale cold pool was formed due to the drag and descent of rainfall and cooling from evaporation, and the outflow from the cold pool was the main uplifting mechanism for long-lived precipitation, which continuously generated convective cells. In region B, the cold pool is remarkably thicker and the warm-humid flow ascended higher on a larger slope than in region A, which is the reason why the duration was longer and the accumulated rainfall was larger in region B.
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Key words:
- torrential rainfall /
- mesoscale characteristics /
- triggering and evolution /
- β-MCS /
- cold pool
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图 1 2017年5月7日00—14时广州地区累积降水量(a)及花都区花山镇两龙墟(711059)、花都区花东镇花都大道北(713024)、增城区新塘镇永宁街荔湖高尔夫球场(713322)、增城区中新镇政府(711022)、黄埔区九龙镇金坑村(713229)、黄埔区九龙镇洋田村(713160)逐小时雨量(b)
图 3 6日20时925 hPa形势和地面准静止锋(a)，7日08时925 hPa 12 h、24 h变温(单位: ℃)、变高(单位: dagpm)和比湿变化(单位: g/kg)(b), 6日20时—7日04时深圳风廓线(c)及7日00时—08时花都风廓线(d)
等值线为等位势高度线(单位:dagpm), 圆点线区域内T-Td≥8 ℃, 点划虚线为高压脊, 双实线为暖式切变线, 方框为发生特大暴雨的位置。深圳风廓线经度114.00 °E, 花都风廓线经度113.23 °E。
图 4 2017年5月6日20时(a)和5月7日02时(b)950 hPa假相当位温(单位: K)和风及6日20时—7日20时A区(黑线)、B区(灰线)950~700 hPa(虚线)和700~500 hPa(实线)温度差随时间变化(c, 单位: ℃)
图 5 6日20时(a)和7日02时(b)清远站探空及6日08时—7日14时珠海地基GPS反演TPW（整层可降水量）与ERA-Interim再分析资料深圳TCWV整层气柱水汽含量(c，单位:mm)，6日08时—7日08时逐6小时22 ℃等露点线位置(d，单位: ℃)
方框为A、B区特大暴雨发生时段。
图 6 7日02时925 hPa风场与低层(1 000~700 hPa)积分水汽通量(a，单位：kg/(m·s))，7日05—06时B区新塘永宁街荔湖高尔夫球场（713322）垂直积分液态水含量(b，单位: 10 kg/m2)与分钟雨量(单位: mm)
图 7 A、B区地形位置和地面辐合线位置(a)，7日00—08时逐小时地面温度场、风场和流场(b~i)，02时地面气压场和风(j)，A区地面自动站1 h变压(k, 单位: hPa)及自动站温度(l, 单位: ℃)
等值线为等温线，蓝色箭头线为流线, 黄色箭头为反气旋。褐色线为等压线，黄箭头为风向。
图 9 B区南部地面自动站逐5 min气压(a，单位：hPa), B区④⑤⑥⑦特大暴雨站逐20 min风向风速(单位：m/s)及气温(单位：℃)(b)，05时40分B区中尺度反气旋示意图(c)
灰色阴影区为地形，双实线为地面辐合线。
图 12 A区02:18雷达强度回波(a)，过AB线雷达强度剖面(b)，速度剖面(c)、B区05:30雷达强度回波(d)，过CD线雷达速度剖面(e)，过EF线雷达强度剖面(f)和速度剖面(g)
白色虚线框为王子山所处位置。
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