Structural Features Of Cold Vortex And Its Formation Mechanism Of Heavy Rainfall Over Northeast China

The cold vortex is the main weather systems in summer over northeastern China, its generation and evolution could not only cause microtherm and flood events over northeastern China, but also have significant impact on the circulation over northern China, Meiyu period over southern China and Eastern Asia. Thereby the research on the cold vortex over northeastern China (CVNE) has great significance for us. Several issues are promoted after reviewing and summarizing the past groundwork on the CVNE. Aiming to these issues, this paper first classifies the different duration and velocity of CVNE, and then composite analysis of CVNE is conducted. Furthermore, the evolution characteristics, triggering mechanism of convective systems and the terrain effects are analyzed using encrypt observational data and mesoscale model. Finally, the configuration model and tri-dimensional conception model of heavy rainfall related to CVNE are studied. The major results are listed as follows:1. The durative and slow CVNEs, which circulation of the development and mature state are zonal distribution, are mainly situated at the eastern Neimenggu municipality, the middle and west of Jilin and Heilongjiang province. Continuous cold air and positive vorticity advection are transported to the CVNEs from the West and northwest low trough, and the strong high level Jet favorably strengthen the cyclonic circulation and the divergent field at the east of CVNEs, and it accelerates the updraft motion in the east and sinking motion in the west of CVNESs. The brief and fast CVNEs which circulation of the entire state are meridional distribution and has weaker high level Jet and warm high ridge in the west side of CVNE, mainly situated at the northeastern Neimenggu municipality, the northwest of Heilongjiang province.Different from the heavy rainfall over southern China, the rainstorm during the development of the CVNE have its particular characteristics. The cold vortex locates at the left side of exit area of high level Jet, which strengthens the cyclonic circulation and the divergent field, and it could trigger upward flow and cause convection. At the west side and south side of the CVNE, there is circumrotate dry incursion because of the high rotation of the CVNE, which could bring rainstorm at the east and north of the CVNE. 2. Budgets of vorticity and divergent and rotational components kinetic energy (KD and KR) are examined. The vorticity perturbation brings by average field of westerly causes the enhancement of local vorticity. The high level vorticity of the CVNE mainly decided by the divergent perturbation item which caused by the high level Jet and middle level cold air. Although KD is minimum, it has great impact on the kinetic energy transport between KD and KR. Source of KD is provided by the kinetic generation item GD and horizontal convergent item HFD of divergent wind via baroclinic process, which makes available potential energy convert to KD then provides kinetic energy source to the KR via transport item C(KD,KR), and it's one of the main kinetic energy source of low level KR. The sources of divergent and rotational components kinetic energy have various characteristics at different level and different state of CVNE. At the development state, the generate rate of KR, KD and C(KD,KR) are mainly concentrate at the lower troposphere, while at the mature state, the generate rate of KR and KD is convert to the mid-troposphere, and generate rate of KR and KD in the lower troposphere is turn into negative. At the weaken state, kinetic energy diminish remarkably in the mid-troposphere, then in the upper troposphere. The convert item C(KD,KR) which serve as the― bridge‖effect during the evolution of the CVNE, has some prediction meaning to the rainfall related to the CVNE.3. The vertical structure of convective clouds at different state and mesoscale and microscale structure of convective system are analyzed, and the results show that, at the beginning of development stage, the structure of the warm front is characterized with isolated and deep convective systems; at the development and matured stage, the intensity of radar echo is weaker than its development stage, and the convective systems are shallow. The echo top of occlusion under the cold vortex system presents some special characters: at the southeast of occlusion, its echo area drive by the cold-dry air; at the middle area of occlusion is the main convective system and northwest area with echo caused by the lifting effect of warm front confront the cold front. There is delamination between ice water and liquid water at the tail of the occlusion front. The height of incursion layer of dry-cold air is about 5km, above is the weak echo consist of ice water, while below is the echo with liquid water; At the mature stage of the cold vortex, the convective systems are located mainly outside the cold vortex, and with isolated convective systems, which contains plenty of ice water content mainly locates at the north of the cold vortex, while liquid water content exists below the 0℃layer of the cold vortex center.4. The terrain of northeast China has great impact on the circulation and rainfall of CVNE, while the Greater Higgnan Mountains (GHM) account for the primary effect, which caused by the by-pass-hill airflow and climbing-hill ascending flow. Convergence, vorticity, vertical motion, temperature advection, and rainfall of north and east side of GHM are corresponding changing with the terrain height. XiaoHingganLing (XHL) which has direct impact on the rainfall over Heilongjiang province mainly causes local circumfluence and rainfall. Changbai Mountain (CM) has great impact on the rainfall over the middle and east of northeastern China. Its impact extent is larger than GHM and XHL, while Jilin province accounts for the main effect of CM.5. Structure model and heavy rainfall model of CVNE are summarized. At the middle and upper troposphere, high PV pool in the center of CVNE stretches and invades into lower troposphere, which caused upward flow ahead of the direction of CVNE. Warm and high moisture air from lower latitude transports to the east side of CVNE, and it bring strong convection. At the development state, the CVNE locates at the exit area of north side of high level Jet and entrance part of east gale center, causing divergent and triggering upward flow of the east part of CVNE. There are two vapor transporting ways at the lower troposphere: one is warm-wet airflow brought by southwest wind, the other is eastward wind. High moisture and high energy tongue extend to heavy rainfall places when local rainstorms of CVNE take place. At the weakening state, high level Jet and low level jet weaken and the CVNE locates over north of entrance area and south of exit area of west gale center, which cause corresponding convergence ahead of the CVNE, the PV at the center of CVNE diminishes and the CVNE wear off eastward.