Characteristics of Persistent Activity and Strength of Cold Vortex During May and June in Northeast China

Using the NCEP/NCAR daily reanalysis data from 1960 to 2012, processes of the northeast cold vortex (NECV) are identified objectively. Combined with the characteristic of precipitation, a process standard of NECV persistent activities is proposed, and quantitative analysis is done using the NECV intensity index which has great significance on its persistence. If the NECV is stronger, the activity of the blocking situation over the Ural Mountains and the Lake Baikal in May, and blocking situation over the Okhotsk Sea in June is more frequent. The composition of high and low index year circulation is calculated including the geopotential height, temperature and wind between 1000 hPa and 200 hPa, and two features in the identification zone of NECV are revealed. First, the NECV converge in middle to upper troposphere (from 500 hPa to 200 hPa), having a deep and cold structure. The low pressure system exists form lower troposphere (1000 hPa) to upper troposphere (200 hPa) and a strong baroclinic component exists in geopotential height field. There is cold center in upper troposphere and thermal trough in lower troposphere. In middle to upper troposphere, westerlies have obvious shunt and confluent, and the blocking situation between them shows higher in northeast and lower in southwest. Second, the NECV converge in middle to lower troposphere (from 850 hPa to 1000 hPa), the cold center is shallow and the structure is not obvious. It is straight in the westerlies in middle to upper troposphere (from 500 hPa to 200 hPa) and the blocking situation has vanished. The weak cold center only exists in upper level, and the lower troposphere is controlled by low pressure and warm ridge. This definition of cold vortex persistent activities provides basis and reference for further study on its climate effect and short-term climate prediction.