东北夏季极端低温天气事件的定义及其冷空气路径分析

采用东北三省150 个测站1961~2010 年的逐日温度资料和同期美国环境预报中心（NCEP）以及国家大气研究中心（NCAR）2.5°×2.5°分辨率的全球再分析资料，定义了东北夏季极端低温天气事件（extreme low-temperatureevent 以下简称ELTE事件），统计给出了ELTE事件的发生、峰值和持续时间的日历表。根据该日历分析，发现了影响东北地区夏季ELTE事件的最主要的2 条冷空气路径（西北路径和东北路径），1961~2010 年期间，影响东北地区的ELTE事件最主要是受西北路径的冷空气的影响，其次是东北路径。同时也发现，ELTE事件20 世纪80 年代发生频率最高，90 年代后开始减少，但是东北路径的ELTE事件在90 年代以后开始明显的增加。还发现，ELTE事件发生的前4 d，贝加尔湖地区若是被低压槽覆盖，易发生西北路径的ELTE事件，若该区域被一个东北-西南走向的高压斜脊控制，同时鄂霍次克海区域存在一个低压中心，形成西北“+”东南“-”的跷跷板形态，易发生东北路径的ELTE事件。

Definition of Extreme Low-temperature Events over Northeastern China in Summer and the Related Cold Air Path

In this article, the 1961-2010 global reanalysis data was acquired from the National Centers for Environmental Prediction/National Centers for Atmospheric Research (NCEP/NCAR), which has a horizontal resolution of 2.5° Lon. ×2.5° Lat. The daily temperature data of 150 observatory stations in Northeast China was employed in the article too. A definition of extreme low-temperature event (ELTE) was given in the article, and the calendar of ELTE'occurrence, peak value time and time-duration were investigated. It unveiled that: the occurrence of ELTE were mainly influenced by two major cold air tracks (i.e., north-west cold path ELTE and north-east cold path ELTE). Focusing on the tracks of ELTE events, it was found that during the period of 1961 to 2010, the north-west cold path was dominated compared with north-east cold track. Meanwhile, it should also be noted here that the maximum frequency was occurred in 1980's, and after 1990's it was declining significantly. However, it was clear that the north-east cold path ELTE was increasing obviously after 1990's. Additionally, once the district of Baikal was dominated by low pressure trough, four days before ELTE occurrence (i.e., -4 d), north-west cold path ELTE was easy to take place. Moreover, while a blocking high in Baikal area and matched a low-pressure center in Okhotsk, a dipole of "+, -" wave train, north-east cold path ELTE was prone to take place.