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Lance F. Bosart, Anton Seimon, Kenneth D. LaPenta, and Michael J. Dickinson

Abstract

The process of tornadogenesis in complex terrain environments has received relatively little research attention to date. Here, an analysis is presented of a long-lived supercell that became tornadic over complex terrain in association with the Great Barrington, Massachusetts (GBR), F3 tornado of 29 May 1995. The GBR tornado left an almost continuous 50–1000-m-wide damage path that stretched for ∼50 km. The apparent rarity of significant tornadogenesis in rough terrain from a supercell well documented in operational Doppler radar motivated this case study. Doppler radar observations showed that the GBR supercell possessed a midlevel mesocyclone well prior to tornadogenesis and that the mesocyclone intensified as it crossed the eastern edge of New York’s Catskill Mountains and entered the Hudson Valley. Tornadogenesis occurred as the GBR mesocyclone crossed the Hudson Valley and ascended the highlands to the east. Subsequently, the mesocyclone weakened as it approached the Taconic Range in western Massachusetts before it intensified again as it moved downslope into the Housatonic Valley where it was associated with the GBR tornado. Because of a dearth of significant mesoscale surface and upper-air observations, the conclusions and inferences presented in this paper must be necessarily limited and speculative. What data were available suggested that on a day when the mesoscale environment was supportive of supercell thunderstorm development, according to conventional indicators of wind shear and atmospheric stability, topographic configurations and the associated channeling of ambient low-level flows conspired to create local orographic enhancements to tornadogenesis potential. Numerical experimentation is needed to address these inferences, speculative points, and related issues raised by the GBR case study.

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Arthur Witt, Donald W. Burgess, Anton Seimon, John T. Allen, Jeffrey C. Snyder, and Howard B. Bluestein

Abstract

Rapid-scan radar observations of a supercell that produced near-record size hail in Oklahoma are examined. Data from the National Weather Radar Testbed Phased Array Radar (PAR) in Norman, Oklahoma, are used to study the overall character and evolution of the storm. Data from the nearby polarimetric KOUN WSR-88D and rapid-scanning X-band polarimetric (RaXPol) mobile radar are used to study the evolution of low- to midaltitude dual-polarization parameters above two locations where giant hailstones up to 16 cm in diameter were observed. The PAR observation of the supercell’s maximum storm-top divergent outflow is similar to the strongest previously documented value. The storm’s mesocyclone rotational velocity at midaltitudes reached a maximum that is more than double the median value for similar observations from other storms producing giant hail. For the two storm-relative areas where giant hail was observed, noteworthy findings include 1) the giant hail occurred outside the main precipitation core, in areas with low-altitude reflectivities of 40–50 dBZ; 2) the giant hail was associated with dual-polarization signatures consistent with past observations of large hail at 10-cm wavelength, namely, low Z DR, low ρ HV, and low K DP; 3) the giant hail fell along both the northeast and southwest edges of the primary updraft at ranges of 6–10 km from the updraft center; and 4) with the exception of one isolated report, the giant hail fell to the northeast and northwest of the large tornado and the parent mesocyclone.

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