The predictability of atmospheric blocking in global ensemble prediction systems

阻塞是一种对当地天气有重大影响的大气现象。由于长时间几乎静止的高压系统，它可能导致极高或极低的温度。此外，一个块可以施加强烈的影响，上游，在现场和下游的天气型，通过扰乱中纬度西风气流。因此，阻塞可能是高压系统周围截止低压中严重降水异常的原因。最先进的集合预报系统（EPS），这是互动的全球大包围（TIGGE）的一部分，在预测大西洋，欧洲和太平洋阻塞的频率表现良好，但在预测阻塞的开始和衰减有困难。发作的可预测性比在初始条件下已经指定阻断的可预测性差约1天。此外，TIGGE集合预报系统（EPS）在模拟格陵兰和乌拉尔阻塞频率方面存在问题。本研究致力于调查选定的阻塞事件在中期，每月和季节ECMWF EPS的动力学。两种不同的聚类方法的基础上，ESTA/PC 3分析和一种方法的基础上集成分析协方差将被应用到EPS。将研究不同类型的块形成或衰变与大尺度事件（如破碎的Rossby波）或小尺度非绝热过程（如湿空气团平流引起的降水）之间的联系。将比较它们在阻塞事件生命周期中的作用。同样的研究方法也将应用于TIGGE总体，并推广仅针对中程ECMWF EPS所获得的结果。此外，将计算两个冬季和一整年的ECMWF分数和技能分数，并评估EPS在阻滞方面的预测技能。

Blocking is an atmospheric phenomenon which has major implications for local weather. It can lead to extremely high or low temperatures due to the long lasting almost stationary high pressure system. Furthermore, a block can exert a strong impact on upstream, in-situ and downstream synoptic weather patterns by disrupting the midlatitude westerly flow. Thereby, blocking can be the cause for severe precipitation anomalies in cut-off lows surrounding the high pressure system. The state of the art ensemble prediction systems (EPSs) which are part of the THORPEX Interactive Grand Global Ensemble (TIGGE) perform well in predicting the frequencies of Atlantic European and Pacific blocking but have difficulties in predicting the onset and the decay of blocking. The predictability of the onset is about 1 day worse than the predictability if the blocking is already specified in the initial conditions. Furthermore, the TIGGE ensemble prediction systems (EPSs) have problems in simulating the frequencies of Greenland and Ural blocking. This study is dedicated to investigate the dynamics of selected blocking events in the medium range, the monthly, and the seasonal ECMWF EPS. Two different clustering methods based on EOF/PC 3 analysis and one method based on ensemble analysis covariance will be applied to the EPSs. Links between different types of block formation or decay and large scale events like breaking Rossby waves or small scale diabatic processes like precipitation resulting from the advection of moist air masses will be investigated. Their roles in the life cycle of a blocking event will be compared. The same investigation methods will be applied to the TIGGE ensembles and to carry forward the results obtained for the medium range ECMWF EPS alone. Additionally, for two winter seasons and for a whole year scores and skill scores used at the ECMWF will be calculated and the predictive skill of the EPSs will be assessed with regard to blockings.