【作者】 张立祥；

【导师】 李泽椿；

【作者基本信息】 南京信息工程大学 ， 气象学， 2008， 博士

【摘要】 东北冷涡是造成东北地区低温冷害、持续阴雨洪涝、突发性强对流天气的重要天气系统,对东北地区的天气气候有重大影响。东北冷涡强对流是预报中的难点,并常常造成业务预报的失败,2002年7月11至15日发生在辽宁的一次东北冷涡天气过程,引发了沈阳连续5天出现强对流天气,过程雨量239mm,降水非常集中,1小时雨量达56.9mm,伴有大风等强对流天气,最大风力达10-12级,造成严重损失。业务预报仅1天报了大雨,其余为雷阵雨。因此,在东北冷涡强对流天气预报方面,不论降水强度、还是落区,均达不到社会对天气预报精度的需求。究其原因主要是缺乏对东北冷涡诱发中尺度系统的机制及其演变规律的认识,导致预报员缺乏相应的预报技术。为此,首先按照东北冷涡的天气尺度结构,将其分为经向型、纬向型、移动型三类,分析东北冷涡系统结构的特点及其与典型温带气旋的差别,给出影响东北冷涡中尺度对流系统的天气背景。以此为基础,利用中尺度数值模式,同化应用地面加密观测资料和多普勒雷达资料,模拟了东北冷涡中尺度系统的动力结构及演变,分析了中尺度系统的边界层特征及其对强对流演变的影响,并通过敏感性试验研究太阳辐射、地形等对冷涡对流系统的影响。主要研究结果与结论有:东北冷涡发展阶段降水主要由其南部锋区湿斜压不稳定产生,一般属于大范围混合型降水,影响系统为对流层低层的温带气旋;成熟阶段冷涡环流锋面结构不明显,降水以分散性对流降水为主,具有明显的日变化。东北冷涡在各发展阶段均存在对流不稳定区,然而不稳定区的分布有很大差异,但基本对应925hPa辐合线前偏南气流区,冷涡发展阶段,对流不稳定能量分布在冷涡中心东南部,与冷涡南部西风锋区诱发的低层气旋相配合;冷涡成熟阶段对流不稳定能量的分布接近冷涡中心。中层干冷空气绝热下沉是东北冷涡700hPa附近干暖盖形成和维持的重要因素。低层暖湿气流爬升及干暖盖的抑制作用是东北冷涡强对流不稳定能量积累的重要机制。不稳定能量的积累是一个较长的过程,而能量的释放是一个非常短暂的过程。东北冷涡中尺度对流系统成熟阶段地面气压场表现为强的中高压,并有弱的前导低压和尾随低压配合。由地面辐合、上升气流抽吸、潜热增温共同形成的低压扰动,对对流系统的维持和移动有重要作用。前导低压是下沉气流外流与环境气流辐合的产物,它只在风暴最强时出现,其他阶段表现为低压带,其辐合中心对应风暴未来的移动方向,引导风暴移动。中高压形成后,其对应的下沉气流外流与环境气流辐合,形成强的中尺度辐合区(阵风锋),是风暴形成后大气边界层主要的气流辐合源。同化多普勒雷达资料后,虽然背景场只有天气尺度系统,同化场表现出明显的中尺度环流,且与实况场接近,分辨率更高,可以清晰地分辨地面辐散外流、中层涡旋等中尺度对流系统。强对流形成初期边界层低层的辐合强于边界层高层辐合,边界层辐合线是对流触发的重要因素之一。冷涡成熟阶段边界层顶的中尺度辐合涡旋是强对流重要的能量、水汽输送系统,是风暴系统维持的入流的主要入口。雷暴对应的边界层冷丘可以影响中尺度温度场和湿度场,其产生的强烈的温度和湿度梯度,对应很强的中尺度湿斜压作用,对对流系统的垂直环流产生影响,进而影响对流系统的演变。冷丘的形成改变了风暴发生区的大气层结条件,这样不但影响自身的移动,而且对其他雷暴单体的移动、发展产生影响。大气中短波辐射影响强对流的触发时间,可使强对流提前爆发,但对强度影响不大。地面短波辐射对大气的加热作用是触发东北冷涡强对流的重要条件。在东北冷涡天气尺度环流背景下,大气中短波辐射通过激发中尺度环流触发强对流的发生;地面短波辐射加热在对流层低层产生强的中尺度辐合及对流不稳定层结共同促使了对流的爆发和维持。在以上研究基础上,总结出冷涡强对流的预报着眼点:1.不同冷涡环流型、发展阶段及预报区域所处部位对应不同的天气。2.东北冷涡条件下,与暖湿输送和辐合有关的边界层不连续边界对对流的形成非常重要,例如暖湿切变线、θe锋、偏南风急流等。3.中高压形成后,形成强的中尺度辐合区(阵风锋),其强烈的动力上升作用可以诱发潜在不稳定区新对流的发生或产生更强的对流。更多还原

【Abstract】 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.更多还原