5—9月不同类型东北冷涡的统计特征及成因

利用NCEP （2.5°×2.5°）逐6 h再分析资料及中国气象局提供的MICAPS观测资料，对1989—2018年5—9月生成并维持的东北冷涡进行统计分析。结果表明，近30年东北冷涡出现频率逐年上升；年平均冷涡过程为7.4次，维持时间为3～5 d；5月冷涡出现频率最高，8、9月较低；5—7月平均每年受冷涡影响天数分别为9.9、8.8、7.0 d，受东北冷涡影响最长时间可达19 d以上。按照不同特性将其分为北、中、南涡及强、弱冷涡。北涡较少出现在7月，中涡很少出现在6月，南涡集中在5、6月。弱冷涡出现频率约为强冷涡的1.2倍。春末（5月）、秋初（9月）出现强北涡的频率较高，而夏季（6—8月）出现弱中涡的频率较高。北涡出现在春末秋初时，偏强的高空急流加强了对流层上层的辐散，与中下层环流场配合，使冷涡得以维持。此外，冷涡中心位势高度较低，配合有明显冷槽或冷核和强上升运动，干侵入有较强的促进作用，有利于冷涡的发展及加强；中涡出现在夏季时，高空急流及干侵入偏弱，冷涡中心附近各要素场不利于冷涡的加强。

Statistical characteristics and causes of different types of northeast cold vortex from May to September

The method of the subjective and objective combination was used to detect and track Northeast China cold vortexes (NECVs) in May-September, based on 30-year NCEP/NCAR reanalysis data and MICAPS observation data provided by China Meteorological Administration.Over the past 30 years, the number of NECVs has steadily increased.NECV processes lasted 3 to 5 days on average in northeast China each year.In May, the frequency was highest, while in August and September, it was lowest.May-July, the longest time influenced by NECVs can exceed 19 days, with an annual average of 9.9 days, 8.8 days, and 7.0 days, respectively.NECVs can be categorized according to their characteristics:north, middle, and south cold vortices (NCV, MCV, and SCV) as well as strong and weak cold vortices.SCVs mostly occur in May and June.Weak NECVs occurred about 1.2 times as often as strong NECVs.The NCVs in late spring and early autumn, as well as the weak MCVs in summer, occurred frequently.Researchers found that when NCVs occur in late spring and early autumn, the upper-level westerly jets are stronger, strengthening the divergence in the upper troposphere and the vortex circulation.During the upper and lower levels of circulation, strong jets cooperated with the circulation fields, enabling the cold vortices to develop and grow constantly.Other than the jets and circulation, the lower central potential height coupled with the obvious cold-core and stronger ascending motions were conducive to the development of NCV.Furthermore, the dry invasion was strongly promoted by lower-level cold advection and the downward intrusion of high potential vorticity (PV).The MCVs happened in the summer when the opposite was true.