A Study On MCS In Cold Vortex Over Northeastern China

It is always difficult to forecast severe convection accurately in cold vortex over northeaster China(CVNE).The similarities and differences of synoptic scale structure for three type of CVNE,named as longitudinal type,latitudinal type and moving type were analyzed.The concept model of CVNE system structure and its difference with extratropical cyclone were summarized.The 3- dimension dynamic structure and evolvement of MCS in CVNE was simulated with MM5 and the effects of solar radiation and topography on convection were simulated with sensitive experiments.The characteristic of planetary boundary layer of MCS in CVNE and its effects on the evolvement of severe convection were studied.Finally the forecasting respects for severe convection in CVNE were summarized.The main research fruits include:The precipitation of developing phase was produced by moisture baroclinic instability in westerly frontal zone which belongs to regional mixing precipitation.The frontal structure in mature phase of CVNE is not distinct and the rainfall mainly is scattered and convective with obvious daily variety.There is instable energy area within CVNE in all phases,but it has a huge difference.Basically the instable energy area matches with the south flow in front of 925hPa convergent line.The instable energy area is located in southeastern cold vortex far from center in developing phase,located in southeast of cold vortex more near center in mature phase.There is wet cold tongue which extends from northwestern Pacific in boundary layer of CVNE and it is located in Northern low circulation.The tongue extends westward and southward to the rear of low in mature phase and has a bigger area.The cold air occupies the whole low circulation in weakening phase.There is active baroclinic system which perhaps brings rainfall in frontal zone in the south of CVNE at the mature phase.The air below 850hPa level is nearly saturate and the T-Td of 700hPa level has a big variety in developing phase of convection.The vapor convergent center in the west couples with the vapor divergent center in the east while the convection is going on and the convection occur in the top of vapor flux axis.The circulation of CVNE consists of three air flows.Warm and moist southwest flow climb on the energy frontal zone in low level through temperature frontal zone in middle level to upper troposphere and outflow with upper jet.The dry and cold northwest air sinks from the rear of upper vortex to middle level and one part combine with ascending air and one part descend to low level then outflow with northeast flow.The eastern flow in the top of vortex move westward and one part merge into block high and one part enter northwestern flow in the rear of vortex.The warm and moisture condition of low level play key role to the occurrence of convection for the dry and cold condition in middle level of CVNE.Dry cold air from middle level adiabatic subsiding is important mechanism for establishment and maintenance of 700hPa dry-warm lid.Warm and moist airflow climbing and dry warm lid restraining are the important mechanism for convective instable energy accumulation.In the mature phase of MCC,surface pressure field presented strong thunderstorm high and weak leading low and wake low.Combination of leading low and 800～700hPa warm core low,which is low pressure disturbance caused by surface convergence and updraft pump and potential heat warming,play important role to maintaining and movement of convective system.In initial stages,the convergence of low layer is stronger than upper layer of boundary layer,and the swirl is weaker,so convergence line of boundary layer is one of important factor to trigger convection.The strong convergence in boundary layer moved upwards and swirling eddy is more strong,so the meso-scale vortex in upside of boundary layer is the important source of energy and vapor for storm,which appear accompanying strong convection development and play important feedback role to maintain and development of convection,and it is main entry of warm and wet airflow to maintain storm system.Atmospheric solar radiation can trigger severe convection by inspiring meso-scale circulation but not by accumulating instable energy in the background of synoptic-scale system of cold vortex.It is important condition by which triggering this severe convection in cold vortex that surface shortwave radiation heat up low layer of troposphere atmosphere,and it product strong meso-scale convergence and convective instable layer to urge convection breaking out and maintaining.