中国大范围持续性极端低温事件的一类平流层前兆信号

本文利用NCEP/NCAR逐日再分析资料,研究了中国大范围持续性极端低温事件(EPECE)的平流层前兆信号及其对对流层环流异常的影响。结果表明,在52个EPECE中,有17个EPECE具有一类共同的平流层前兆信号:(1)在EPECE发生前10天左右,在巴伦支海一带的100 hPa位势高度场呈现较强的正距平特征;(2)随后,该正异常环流逐渐向东移动,并在EPECE发生的前5天左右,使贝加尔湖西北侧200 hPa纬向风显著减弱。本文据此提出了关于EPECE平流层前兆信号的两个判别条件,并以这两个判别条件对1949～2009年冬季(11月至次年3月)所有逐日低频场进行了后查和检验。结果表明,上述两个判别条件对EPECE的发生日期具有一定的预测能力。本文进一步通过位涡(PV)反演探讨了平流层前期异常环流如何影响EPECE发生的物理机制。分析表明,巴伦支海附近的平流层中低层PV异常有利于该地区对流层中上层正高度异常的维持,后者强度的25%来源于前者的作用。

A Specific Stratospheric Precursory Signal for the Extensive and Persistent Extreme Cold Events in China

On the basis of daily NCEP/NCAR data sets, this study investigates a stratospheric circulation precursor for the extensive and persistent extreme cold events (EPECEs) in China. Results show that 17 out of 52 EPECEs are characterized by a specific stratospheric circulation precursor. The precursor exhibits itself clearly in two aspects. (1) Strong positive geopotential height anomalies occur around the Barents Sea in the lower stratosphere (100 hPa) 10 days prior to the occurrence of the EPECE. (2) In relation to the eastward movement of the positive height anomalies, the zonal wind over the northwest of Lake Baikal is weakened significantly at 200 hPa about 5 days before the occurrence of an EPECE. According to these stratospheric precursor circulation features, two initial assessment conditions are proposed. To test prediction accuracy, the two assessment conditions are applied to the daily low-frequency data of all winter seasons (November to March) from 1949 to 2009. Results show that our two proposed assessment conditions, at least to some extent, can enable the prediction of the occurrence date of an EPECE. Regarding the physical mechanism of the influence of the stratospheric circulation precursor on the EPECE, potential vorticity (PV) inversion analysis shows that the mid and lower stratospheric PV anomalies around the Barents Sea can induce the geopotential height anomalies at the upper and midtroposphere. In particular, 25% of the real amplitude of the latter originates from the influence of former.