FORECAST UNCERTAINTY OF A MESO-β-SCALE VORTEX ON JULY 17, 2021 IN THE MIDDLE AND LOWER REACHES OF THE YANGTZE RIVER
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摘要: 2021年7月16—17日，在大尺度鞍型背景场中长江中下游地区生成了准静止的β中尺度低涡系统，造成苏皖地区出现局地特大暴雨及雷暴大风天气。欧洲中心（EC）控制预报对低涡位置的描述较实况明显偏北，由此在降水预报中也呈现出较大偏差，给预报决策带来较大误导。采用EC控制和集合预报产品，并基于“预报挑战度（MFC）”和“可预报性演变指数（PHDX）”等客观方法对低涡及降水预报不确定性进行分析，并在此基础上探讨模式偏差成因，得到以下结论：（1）对流层低层低涡东侧西南气流和东南气流的辐合以及低空急流的水平涡度输送是低涡发展的主要动力因素，而低涡东侧和南侧降水的潜热释放则构成低涡发展的热力因素；（2）EC控制预报不同起报时次均出现低涡位置偏北及雨带预报偏北现象，其集合预报产品离散度无法覆盖实况降水，揭示了此次过程的低可预报性，MFC和PHDX则能够客观指示此次过程低可预报性；（3）前期模式对低涡南侧西南气流南风分量预报偏大及对东侧辐合区刻画偏北造成前期东段降水偏北，而后在潜热释放、低空急流与低涡正反馈机制影响下，偏北的降水区造成低涡进一步预报偏北，最终导致整个时段预报较观测呈现巨大差异。Abstract: From July 16 to 17, 2021, a quasi-stationary meso-β-scale vortex (MβV) was developed in the middle and lower reaches of the Yangtze River, causing localized heavy rainfall, thunderstorms, and gales. The ECMWF control forecast exhibited obvious northward bias of the MβV as well as large errors in the precipitation forecast. In this study, two recently proposed metrics-the Measure of Forecast Challenge and the Predictability Horizon Diagram Index-were applied to analyze the forecast uncertainty of the MβV and its associated precipitation, and the reason for model bias was discussed thereby. The results show that: (1) The convergence of the low-level southwest and southeast streams on the east side of the vortex and the horizontal vorticity transport of the low-level jet are the main dynamical factors for the development of the vortex, while the latent heat release of MβV associated precipitation is the main thermal factor. (2) EC control forecasts with different initialization time show northward bias of the MβV as well as the rainbelt, and the dispersion of EC ensemble forecasts is not consistent with observation, indicating the low predictability of this event. (3) The overestimation of v component of the low-level southwest stream south of the MβV and the northward error of the convergence zone east of the MβV lead to a northward bias of precipitation forecast. After that, the positive feedback mechanism between latent heat release, low-level jet, and the MβV causes a further northward bias and finally leads to the marked error in the forecast by EC during this event.
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Key words:
- forecast error /
- forecast challenge degree /
- predictability
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图 1 7月16日20时（a1~a3）、17日02时（b1~b3）、17日08时（c1~c3）、17日14时（d1~d3）500 hPa高度场和风场（第一行）、700 hPa流场（第二行）和850 hPa流场和相当位温（第三行）天气形势演变
图 4 7月17日02时（a1~d1）、08时（a2~d2）、14时（a3~d3）的850 hPa涡度收支情况（单位：PUV）
a1~a3.水平平流项（HADV）；b1~b3.垂直平流项（VADV）；c1~c3.散度项（CONV）；d1~d3.倾斜项（TILT）。
图 6 15日20时（a1~d1）、16日08时（a2~d2）、16日20时（a3~d3）起报的EC控制预报6 h累积降水
a1~a3. 17日02时；b1~b3. 17日08时；c1~c3. 17日14时；d1~d3. 17日20时。单位：mm。
图 7 15日20时（a1~d1）、16日08时（a2~d2）、16日20时（a3~d3）起报的预报挑战度（MFC，填色）与集合离散度（SPRD，等值线范围为5~30 mm，间隔为5 mm）
图 8 15日20时（a1~b1，a2~b2）、16日08时（c1~d1，c2~d2）、16日20时（e1~f1，e2~f2）起报的850 hPa水平风场U分量（第一行）和V分量（第二行）的预报挑战度（MFC，填色）与集合离散度（SPRD，等值线，间隔为5 mm）
图 9 17日02时（a1~d1）、17日08时（a2~d2）、17日14时（a3~d3）预报偏北区域（红框）的平均6 h累积降水与前期（6 h前）环境场的ESA
a1~a3. 500 hPa位势高度；b1~b3. 850 hPa位势高度；c1~c3. 850 hPa纬向风；d1~d3. 850 hPa经向风。各子图中的等值线或风矢量为对应时次的集合平均场，阴影区域表示通过0.05显著性检验。
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