Mesoscale Studies of Convection, Tornadogenesis, and Fronts

9801720 Wakimoto首席调查员将从事与严重的局地风暴和中尺度锋面结构有关的三个研究课题：1)龙卷风和龙卷风发生，2)弓形回声锋线结构和锋线起始，以及3)冷暖锋面边界的中尺度记录。在基于数值模拟和观测研究的基础上，对超级单体风暴中的中层中气旋的理解已经取得了重大进展。然而，关于龙卷风的起源和动力学的知识并没有那么快地取得进展。这在一定程度上可以归因于在这些强烈涡旋附近收集高分辨率数据的困难。利用在雷暴实验中验证旋转起源期间收集的独特数据集，首席调查者将探索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发生研究的焦点：i)什么是导致超级单体龙卷风发生的过程？2)中气旋和龙卷风之间有什么关系？3)超级单体龙卷风是从云底下降还是从边界层向上累积？对一个没有产生龙卷风的强烈超级单体风暴的分析目前正在进行中。初步结果表明，一个明确的低层中气旋发展了几次，但并没有导致龙卷风的启动。超级单体机载多普勒雷达的25个通道目前正在合成中。一个有待检验的假设是，中气旋内部的涡旋破裂导致了一场弱龙卷风的发展。目前正在进行两项单独的研究，重点是狂风线。首先，关于弓形回声结构的全面数据集还处于分析的早期阶段。弓形回波是一种雷达信号，通常与下击暴流和反回波联系在一起。目前的情况提供了一个机会来研究这种回声从线性结构到弓形的整个演变。另一项研究考察了强冷锋和干线合并后引发的强烈狂风线的形成。虽然后一次事件被认为是一种共同的启动机制，但似乎还没有一项研究提供这一过程的详细测量。1997年冬季，组织了一次名为FASTEX(锋面和大西洋风暴路径实验)的国际野外实验，以研究海洋气旋发生。主要的数据平台是几架研究飞机，它们发射落差探测仪，记录飞行水平的现场测量，并使用机载雷达收集高分辨率雷达反射率和多普勒速度信息。初步雷达综合揭示了冷锋和暖锋前所未有的运动结构。利用FASTEX数据集，我们建议使用多普勒风场和部署在高海拔的多个下沉探测仪相结合的方法来检查沿锋结构和评估锋生。这项研究的成功完成将大大增加对强局地对流风暴的认识和冬季锋面结构的中尺度方面的知识。

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.***