Analysis To The Impacts Of The Northeast Cold Vortex (NECV) Process On The Rainfall During Meiyu Period Over The Yangtze-Huaihe River

Based on the NCEP reanalysis,the 160-station monthly mean precipitation an temperature and 740-station day-to-day precipitation over China from 1954-2003,a study is investigated about the atmospheric heat sources of the region of the Tibetan Plateau(TP) impacts on the Northeast Cold Vortex(NECV) by composite analysis of the selected strong and weak years. We also choose ten strong NECV processes from six years which Meiyu were very strong to analyse the NECV impacts on the rainfall during Meiyu period over Yangtze-Huaihe river. The main results are as follows:(1) The strong heating over the west part of the TP causes the anticyclonic circulation appeared in the vicinity of Ural Mountains and Okhotsk sea and the cyclonic circulation appeared in Baikal, so there is "two bridge and one trough" which reflects that the interaction between heating of the TP and the circulation in mid-high latitude. Furthermore, the northeastward propagating Rossby wave rain mduced by the anomalous heating over the TP reach far distance areas, which will influence the NECV during Meiyu period. As a result, when the heatmg over the west part of the TP in May will be more stronger , and the NECV will be also more strong during Meiyu period.(2) The possible mechanism of the influence of NECV on Meiyu is that when the NECV moves eastward and southward, the western Pacific subtropic high extends westward correspondingly in the middle troposphere. This interaction between the NECV and western Pacific subtropical high leads to the reinforcement of pressure gradient over the southern part of Yangtze-Huaihe River, which results in the increase of the kinetic energy. Subsequently, the downward transportation of the kinetic energy causes the formation of the southwest low level jet (LLJ) in the low troposphere. The convergence between the northward warm and wet southwesterly flow brought by LLJ and the southward dry and cold air by NECV is favorable to the formation and persistence of the Meiyu front, and also can strengthen the convective instability of the inferior layer in the Yangtze and Huaihe River basin, result in the ascending motion and the activity of the Meiyu rainfall.(3) The heat source of Qinghai-Tibet Plateau was stronger in May, 1983, the height departure was negative, the vortex departure was positive in 500hPa from Mongolia to northeast area during the Meiyu period, and the ground temperature departure was negative, the rainfall departure was positive in northeast area, all these weather elements indicate that the northeast vortex was active. But the distribution of all these weather elements in 1999 was just the reverse of 1983, which means the northeast vortex during the Meiyu period was weaker in 1999.(4) The frequency of the northeast vortex during the Meiyu period in 1983 was higher than 1999, the west expanding of the ridge of the West Pacific Subtropical High Pressure was more obviously, there was more mid-top troposphere kinetic energy transferring down procedure and cold air south moving procedure, the north transfer of the water vapor was stronger in 1983, so the rainfall in Yangzi-Huai River area during the Meiyu period of 1983 was obviously more than 1999.