Mesoscale Studies of Convection, Tornadogenesis, and Fronts

主要研究方向为：1)龙卷风和龙卷风形成；2)弓形回波飑线的结构和飑线的形成；3)冷暖锋边界的中尺度记录。基于数值模拟和观测研究，对超级单体风暴中的中层中气旋的认识取得了重大进展。然而，关于龙卷风的成因和动力学的知识并没有取得如此迅速的进展。这可以部分归因于在这些强烈涡旋附近收集高分辨率数据的困难。利用在验证雷暴旋转起源实验中收集的独特数据集，首席研究员将继续研究三个悬而未决的问题，这些问题仍然是大量龙卷风和龙卷风形成研究的焦点：1)导致超级单体龙卷风形成的过程是什么？2)中气旋和龙卷风有什么关系？3)超级单体龙卷风是从云底下降还是从边界层向上形成？目前正在对一场没有形成龙卷风的强烈超级单体风暴进行分析。初步结果显示，一个定义明确的低层中气旋在若干场合发展，但没有导致龙卷风的开始。目前正在合成超级单体的25次机载多普勒雷达信号。一个有待验证的假设是，中气旋内部的涡旋破裂导致了弱龙卷风的发展。目前正在进行两项针对飑线的独立研究。首先，弓形回波结构的综合数据集正处于分析的早期阶段。弓形回波是一种雷达信号，通常与下爆和回波有关。本案例提供了一个机会来检查这种回声从线性结构到弓形结构的整个演变。第二项研究考察了强冷锋和干线合并后强烈飑线的形成。虽然后一事件被认为是形成飑线的一种常见的起始机制，但似乎没有一项研究对这一过程提供了详细的测量。1997年冬季，为研究海洋气旋的形成，进行了一项国际野外试验，称为FASTEX（锋和大西洋风暴路径试验）。主要数据平台是几架研究飞机，这些飞机释放了落风探空仪，记录了飞行水平的原位测量数据，并使用机载雷达收集了高分辨率雷达反射率和多普勒速度信息。初步的雷达合成揭示了冷锋和暖锋前所未有的运动结构。研究人员建议使用FASTEX数据集，结合多普勒风场和部署在高海拔的多个下投探空仪，对沿锋面结构进行检查，并对锋生进行评估。这项研究的成功完成将大大增加对强地方性对流风暴和冬季锋面结构中尺度方面的认识

9801720Wakimoto The Principal Investigator will pursue three research topics associated with severe local storms and mesoscale frontal structures: 1) tornadoes and tornadogenesis, 2) structure of bow-echo squall lines and squall-line initiation, and 3) mesoscale documentation of warm and cold frontal boundaries. There have been major advances in understanding of mid-level mesocyclones within supercell storms based on numerical simulations and observational studies. However, knowl-edge concerning the genesis and dynamics of tornadoes has not progressed as rapidly. This can be, in part, attributed to the difficulty of collecting high resolution data in the vicinity of these intense vortices. Using unique data-sets collected during the Verification of Origins of Rotation in Thunderstorm Experiment, the Principal Investigator will pursue three outstanding questions that remain the focus of a considerable amount of research on tornadoes and tornadogenesis: I) What are the processes that lead to supercell tornadogenesis? 2) What is the relationship between the mesocyclone and the tornado? 3) Do supercell tornadoes descend from cloud base or build upward from the boundary layer?An analysis of an intense supercell storm which did not spawn a tornado is currently underway. Preliminary results suggest that a well-defined low-level mesocyclone developed on several occasions but did not lead to the initiation of a tornado. Twenty-five passes of an airborne Doppler radar by the supercell are currently being synthesized. A hypothesis to be tested is that vortex breakdown within the mesocyclone led to the development of an weak tornado. Two separate studies focusing on squall lines are currently underway. First, a comprehensive data set on the structure of a bow echo is in the early stages of analysis. The bow echo is a radar signature that is often associated with downbursts and derechos. The present case provides an opportunity to examine the entire evolution of this echo from a linear structure to a bow-shaped. A second study examines the initiation of an intense squall line as a result of the merger between a strong cold front and the dryline. Although the latter event is known to be a common initiation mechanism for the development of squall lines, there does not appear to be a study that has provided detailed measurements of this process.During the winter of 1997, an international field experiment called FASTEX (Fronts and Atlantic Storm Track Experiment) was organized to study oceanic cyclogenesis. The primary data platforms were several research aircraft which released dropwindsondes, recorded in situ measure-ments at flight level, and collected high-resolution radar reflectivity and Doppler velocity informa-tion using on-board radars. Preliminary radar syntheses have revealed unprecedented kinematic structure of cold and warm fronts. Examination of the along-frontal structure and an assessment of frontogenesis using a combination of the Doppler wind field combined with numerous dropsondes deployed at high altitudes are proposed using the FASTEX data set.Successful completion of this research will add substantially to knowledge of severe local convective storms and to the mesoscale aspects of winter-time frontal structures.***