The predictability of atmospheric blocking in global ensemble prediction systems

阻塞是一种大气现象，对当地天气有重大影响。由于持续时间长，几乎是静止的高压系统，可能会导致极高或极低的温度。此外，一个区块可以通过扰乱中纬度西风气流对上游、原地和下游天气形势产生强烈影响。因此，阻塞可能是高压系统周围截止点低压出现严重降水异常的原因。作为THORPEX交互大全球预报系统(TIGGE)的一部分，集合预报系统(EPSS)在预报大西洋、欧洲和太平洋阻塞的频率方面表现良好，但在预报阻塞的开始和衰减方面存在困难。如果在初始条件中已经指定了阻塞，则开始的可预报性比可预报性差约1天。此外，TIGGE集合预报系统(EPSS)在模拟格陵兰和乌拉尔阻塞频率方面存在问题。这项研究致力于研究选定的阻塞事件在中期、月和季节ECMWF EPS中的动力学。两种不同的基于EOF/PC3分析的聚类方法和一种基于集成分析协方差的方法将被应用于EPSS。不同类型的块体形成或衰变与大尺度事件(如Rossby波破裂)或小尺度非绝热过程(如潮湿气团平流引起的降水)之间的联系将被研究。我们将比较它们在阻塞事件生命周期中的作用。同样的研究方法将应用于TIGGE系综，并推广仅针对中程ECMWF EPS所获得的结果。此外，将计算两个冬季赛季和一整年在ECMWF使用的分数和技能分数，并评估EPS关于堵塞的预测技能。

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.