一次梅汛期强降雨过程的中期时效预报误差分析

Analysis of the forecast error of rainband position in medium-range during a Meiyu heavy rainfall event

2016年6月30日至7月4日,中国长江流域发生了入汛以来最强的一次极端降雨过程,但对雨带位置的预报却出现了显著误差。为此,本文基于欧洲中期天气预报中心的预报资料,利用天气学诊断方法,分析了确定性和集合预报的基本情况,讨论了预报误差产生原因及其演变特征。结果表明:梅雨锋上次天气尺度波动在中国黄淮—辽东半岛到朝鲜半岛—日本东部一带呈“负—正—负”的分布,它的强弱对雨带位置的变化起着重要影响。当该波动偏强时,有利于低层季风向北伸展,加之冷空气强度偏弱,进而造成雨带位置偏北,反之亦然。此外,通过对比集合预报成员中的准确和偏北成员组,发现该次天气尺度波动来源于青藏高原东北部的初始误差场。伴随着中纬度西风波动的向东传播,该误差在中低层沿着梅雨锋向东移动、并不断增强,最终造成中国长江中下游地区雨带位置明显偏北。

From June 30 to July 4,2016,the strongest extreme rainfall hit the Yangtze basin since the flood season.However,in the operation,the prediction of the location of rain belts has a significant error.Therefore,based on the ensemble forecast data from the European Medium-Range Weather Forecast Center(ECMWF),this study analyzed the basic situation of the deterministic and ensemble forecasting using the method of synoptic diagnosis and discussed the causes of forecasting error.The results show that the sub-synoptic scale disturbance along the Meiyu front is dominant in the variability of the Meiyu rain band position,which appears a negative-positive-negative distribution in the region from Huanghuai-Liaodong Peninsula to the Korean Peninsula-eastern Japan.Its strength has an important influence on the change of rain belt position.When the disturbance is strong,it is favorable for the low-level monsoon to extend to the north,and the cold air intensity is weak,thus causing the location of the rain belt to be north,and vice versa.In addition,it is found that the synoptic-scale fluctuation originates from the initial error field in the northeast of Qinghai-Tibetan Plateau by comparing the accurate and northerly members of the ensemble forecast group.Along with the eastward propagation of mid-latitude westerly wind fluctuation,the error moves eastward along the Meiyu front in the middle and lower layers,and increases continuously,finally causing the rain belt in the middle and lower reaches of the Yangtze River to be significantly northerly.