摘要:2016年6月30日至7月4日出现了当年入汛以来最强降雨过程，然而数值预报却出现了明显误差。为此，本文首先对比和分析了当今预报性能最优越的欧洲中期天气预报中心（European Centre for Medium Range Weather Forecasting，简称ECMWF）和美国环境预报中心/全球预报系统（National Centers for Environmental Prediction / Global Forecast System，简称NCEP）的确定性和集合预报差异及误差原因。结果显示，针对雨带位置，NCEP模式预报较为准确，ECMWF模式存在明显的偏北误差。基于集合预报的进一步分析表明，前者预报偏差小，源于误差在逐日变化中呈现偏北、偏南交替出现所致。而后者偏北误差却呈稳定维持的特征。接着，将集合预报成员划分为了准确组和偏差组，通过二者合成分析揭示出，当雨带位置的预报偏北时，对应降雨较强，反之亦然。最后，讨论了500 hPa层西风槽与雨带位置之间的联系。结果表明，当西风槽强度预报偏强时，雨带位置偏北、降雨强度偏强。ECMWF模式的西风槽持续偏强，因而雨带位置稳定偏北。NCEP模式的西风槽偏强、偏弱交替出现，导致了雨带位置的偏北、偏南误差。研究结果对在实际业务中开展误差订正具有很好的参考意义，也有助于提高梅汛期预报准确率。
关键词:ECMWF模式/
NCEP模式/
集合预报/
误差
Abstract:From June 30 to July 4, 2016, an extreme precipitation event occurred in Yangtze-River valley, which is the strongest rainfall process since the flood season. Obvious errors appeared in predicting the location of the rain band in operational forecast. Based on the deterministic and ensemble model data obtained from the European Centre for Medium-Range Weather Forecasting (ECMWF) and the National Centers for Environmental Prediction Global Forecast System (NCEP-GFS), the difference of the model performance between them is compared, and the causes of forecast error are analyzed. The results show that the forecast of NCEP is more accurate and that there are obvious northward errors in the rain band location forecasted by the ECMWF deterministic model. Further analysis from ensemble data reveals that despite the smaller forecast error of NCEP, which is caused by the obvious instability of its errors, it appears alternately northerly and southerly error from day to day. Although the ECMWF model predicts the rain band with further north location, this northerly error is stable. In addition, by dividing the ensemble members into accurate and bias groups, composite analysis between them reveals that the member with further north rain band location will have stronger precipitation and vice versa. Finally, the connection between the westerly trough on 500-hPa level and the position of rain band is discussed in this paper. The results show that when the strength of the westerly trough is stronger, the rain belt is located further north, and the intensity of rainfall is more obvious and vice visa. The ECMWF model forecasted that the westerly trough continued to be strong. Owing to this reason, this model gives a stable northerly position of the rain belt forecast. Because the NCEP model predicts stronger and weaker westerly troughs alternately, its forecast of the rain belt location shows corresponding northerly and southerly errors. This conclusion can serve as an important reference for the forecast errors correction of the rain belt location in the forecast operation and can also help to improve the forecast accuracy of rain band location in Meiyu season.
Key words:ECMWF model/
NCEP model/
Ensemble forecast/
Error
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