基于CMA-BJ的北京地区短时强降水预报试验

A Probabilistic Forecast Experiment of Short-duration Heavy Rainfall in Beijing Based on CMA-BJ

  • 摘要: 基于2019—2021年4—9月北京快速更新数值预报系统(CMA-BJ)产品以及北京地区地面气象站逐时降水实况, 从表征水汽条件、热力和能量条件以及动力条件的多个物理量中筛选出在有无降水、是否强降水情形中有显著差异的物理量作为因子, 采用配料法和模糊逻辑算法构建北京地区0~12 h时效逐小时短时强降水概率预报模型。以2019—2021年4—9月最优TS评分和偏差评分的概率值和组合反射率因子为确定性预报的概率阈值和消空处理阈值, 运用该预报模型对2022年4—9月每日4次0~12 h预报时效北京地区短时强降水产品进行预报和检验。结果表明:北京地区短时强降水TS评分和偏差评分分别为0.104和1.341, 预报效果明显优于CMA-BJ预报产品。概率预报模型能够有效提升强降水高发地区, 即山前及平原地区的短时强降水预报技巧, 获得较为平衡的命中率和空报率, 但对山区预报技巧的提升有限。

     

    Abstract: Based on numerical prediction products from China Meteorological Administration Beijing model (CMA-BJ), precipitation observation of ground weather stations in Beijing and ECMWF ERA5 dataset, the hourly rainfall samples from April to September during 2019-2021 are divided into short-duration heavy rainfall (SDHR, greater than 20 mm·h-1), ordinary rainfall (between 0.1 and 19.9 mm·h-1) and no rainfall (less than 0.1 mm·h-1). The probability density distribution characteristics of physical parameters are comparatively analyzed, including moisture conditions, thermal and energy conditions, and dynamic conditions for three categories. Monthly predictors are selected from these parameters by comparing their ability to discriminate among SDHR, ordinary rainfall and no rainfall weather. It is found that the distributions and thresholds of physical parameters differ between months to some extent. Among that, moisture conditions, thermal and energy conditions, and dynamic conditions are relatively stronger but with less discrimination degrees among SDHR, ordinary rainfall and no rainfall weather in July and August. The background circulation and the distributions of physical parameters show obvious monthly differences, so the forecast model is established for each different period. After that, forecast model of SDHR for 0-12 h at 1 h intervals in different periods is established by using the ingredients-based method and fuzzy logic algorithms. When probabilistic and composite reflectivity thresholds are 0.6 dBZ and 15 dBZ, threat score (TS) and bias of SDHR are 0.14 and 1.14 in Beijing from April to September during 2019-2021, showing relatively better forecasts. Therefore, the probabilistic and composite reflectivity thresholds corresponding to the optimal TS and bias for 2019-2021 are taken as the forecast probability and eliminating false thresholds of SDHR, and 0-12 h hourly forecast products of SDHR four times a day are tested from April to September of 2022. Results show that TS and bias of SDHR are 0.104 and 1.341, respectively, indicating that the probability prediction products are better than that of CMA-BJ. SDHR products achieve greater improvement, and balance hit rate and false alarm rate well in the piedmont and plain areas with high SDHR frequency. But performances in mountainous areas are not as good as that in plain areas, which may be related to less stations in the mountainous areas of the forecast model. In addition, the result based on case analysis show the predicted area of the products is relatively larger, but high probability area has a good indication for SDHR in Beijing.

     

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