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Leslie R. Lemon

Abstract

The radar and surface structure of a severe thunderstorm's wake vortex on 25 June 1969 are examined. Two probable aerodynamic causes for wake vortex formation are Kármá vortices and starting vortices to the updraft lee. Kármán vortices would form, as do those observed, within the echo core at the updraft lee “edges” and move downstream with the ambient flow. However, in contrast to Kármán theory, radar reflectivity distributions consistently suggest anticyclonic flow. Starting vortices should develop as observed, during transition of a thunderstorm updraft from non-rotational to rotational state, as a vortex of comparable strength but opposite circulation. Four other thunderstorms examined also produced severe weather, increased intensity rapidly, and turned right or formed hook echoes contemporaneously with vortex shedding.

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Leslie R. Lemon

Abstract

An organized persistent severe thunderstorm on 25 June 1969 blends elements of the classic supercell (Browning and Donaldson, 1963) and the multi-cell storm (Marwitz, 1972), as revealed by detailed surface and radar data. Changes in supercell strength reflect contributions of cells from a flanking line that overtake and combine with the main storm. One cell contributed to an intensified mesodepression and increased surface convergence as the cell merged with the supercell weak echo (updraft) region. Cells move both to the right and left as well as with the mean winds. Surface data reveal distinctively arranged surface discontinuities, a large persistent mesodepression, an associated convergence area, and one principal downdraft.

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Leslie R. Lemon

Abstract

Recently, a rare radar artifact called the “flare echo” or “three-body scatter signature” has been examined by several researchers. Here, this midlevel storm signature is called the “three-body scatter spike” (TBSS) and is examined in detail for some severe storms scanned by operational WSR-88Ds. The TBSS is a generally 10–30-km long region of echo aligned radially downrange from a highly reflective (>63 dBZ) echo core. It is found almost exclusively aloft and is characterized by low reflectivity and is usually characterized by near-zero or weak inbound velocities. Spectrum widths are very broad and often noise like. The aforementioned research concluded that it is caused by non-Rayleigh radar microwave scattering (Mie scattering) from a region of large hydrometeors; most likely large, wet hail. This conclusion is supported and expanded upon.

WSR-88D data are presented concerning a storm attended by a TBSS that produced giant (>5 cm) hail and violent surface winds. In this case, the three-body signature was found in midlevels, down radial from a 65-dBZ reflectivity core in the storm echo overhang. It was most pronounced during 2.5-cm surface hail occurrence and preceded by ∼25 min “baseball-size” hail. The signature is shown in base reflectivity and velocity products and in cross sections of reflectivity, velocity, and spectrum width. TBSS theory of production, its appearance, and characteristics are discussed as well as its operational use. Other examples are also examined briefly. Time and location of surface severe weather reports are also examined and related to three-body scattering. The operational interpretation and application to the severe storm warning program are emphasized. It is suggested that the TBSS is a sufficient but not necessary condition for large hail detection. Although all storms examined in this study produced hail >6.5 cm, it is concluded that surface hail of at least 2.5-cm diameter should be expected with artifact-bearing storms. It is further shown that recognition of the TBSS can provide a warning lead time because it typically precedes the largest surface hailfall (and very often violent surface winds) by 10–30 min.

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Leslie R. Lemon and Charles A. Doswell III

Abstract

Severe thunderstorm evolution is synthesized, using published and unpublished studies of radar, instrumented aircraft, visual and surface observations. These observations reveal the existence of a downdraft (originating at 7–10 km AGL) on the relative upwind side of the updraft. Air decelerates at the upwind stagnation point, is forced downward and mixes with air below which then reaches the surface through evaporative cooling and precipitation drag. The initially rotating updraft is then transformed into a new mesocyclone with a divided structure, in which the circulation center lies along the zone separating the rear blank downdraft from the updraft. This process appears to result, in part, from tilting of horizontal vorticity into the vertical. It is proposed that the zone of strong vertical velocity gradient across which the mesocyclone comes to be positioned is also characterized by a strong temperature gradient and is the genesis region of strong tornadoes. Although no direct observations are available yet, we further propose that the strong temperature contrast plays a potential modulating role in tornadogenesis by solenoidal generation of vorticity, in analogy with the extratropical cyclone, to which the transformed mesocyclone bears a striking resemblance.

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Leslie R. Lemon, Donald W. Burgess, and Rodger A. Brown

Abstract

Single-Doppler Velocity data reveal that a dominant feature in the Union City, Okla., tornadic thunderstorm is a core mesocyclonic circulation, 2–6 km in diameter, extending to at least 9 km above ground. There is an apparent flow through the precipitation echo at low levels and divergence at high levels. Considerable similarity appears between mid-level flow structure around the mesocyclone core and that observed around a solid rotating cyclinder embedded in classical potential flow. As tornado time approaches, core circulation tangential velocities increase while diameter decreases. Simultaneously, the collapse of storm top and extensive echo overhang suggest updraft weakening.

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Rodger A. Brown, Leslie R. Lemon, and Donald W. Burgess

Abstract

Doppler radar measurements in the Union City, Okla., tornadic storm of 24 May 1973 led to discovery of a unique tornadic vortex signature (TVS) in the field of mean Doppler velocity data. The distinct character of this signature and its association with the tornado are verified using a model that simulates Doppler velocity measurements through a tornado. Temporal and spatial variations of the TVS reveal previously unknown tornado characteristics. The TVS originates at storm mid-levels within a parent mesocyclone, descends to the ground with the tornado (extending vertically at least 10 km), and finally dissipates at all heights when the tornado dissipates. NSSL Doppler radar data from 1973 through 1976 reveal 10 signatures; eight were associated with tornadoes or funnel clouds, while no reports are available for the other two. Since the TVS first appears aloft tens of minutes before tornado touchdown, the signature has decided potential for real-time warning.

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Robert P. Davies-Jones, Donald W. Burgess, Leslie R. Lemon, and Daniel Purcell

Abstract

Changes in tornado structure were accompanied by corresponding changes in damage intensity and debris configuration. Initially, damage was light over a 200 m wide path but the vortical nature of the winch was clearly evident. During the mature stage, damage was severe and still showed signs of circulation Intriguing and mysterious microscale patterns were observed in wheat fields. In the shrinking and decaying stages, heavy damage occurred over a 100 m wide path and there was evidence of strong radial inflow in the lowest meter above the surface. Generally, debris was thrown ahead of the vortex, with heavy objects coming to rest on the right forward side. Signs of circulation were no longer apparent in the debris configuration. Flow relative to the moving vortex appeared asymmetrical with strongest winds on the right side of the funnel.

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Rodger A. Brown, Donald W. Burgess, John K. Carter, Leslie R. Lemon, and Dale Sirmans

Some results of the first 10 cm dual-Doppler radar measurements in a tornadic storm are presented. A mesoscale cyclonic circulation confirms proposed single Doppler vortex signature and indicates that the curved reflectivity hook echo is around the periphery of the circulation. The interpolated tornado position is within the mesocyclone where high-variance Doppler velocity spectra suggest strong velocity gradients.

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Charles A. Doswell III, Leslie R. Lemon, and Robert A. Maddox

Several key events in the history of modern meteorology are reviewed and analyzed in light of the current state of forecasting. A common thread in much of the material reviewed is the need for greater interaction between research meteorologists and forecasters. Greatly hindering this desirable goal is the inadequate training system for forecast meteorologists. Some possible means for altering the structures within which forecaster training takes place are examined. Responses and commitment to improve the current situation are solicited.

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Leslie R. Lemon, Ralph J. Donaldson Jr., Donald W. Burgess, and Rodger A. Brown

Significant advances in single-Doppler radar application to severe storm study and identification have been made since 1965. Mesocyclones have been detected by Doppler radar and found to precede severe weather occurrence by several tens of minutes. A typical mesocyclone evolution leading to tornado development has also been documented. The tornado vortex itself has a revealing signature in Doppler radar data, the tornadic vortex signature (TVS). Statistics of both mesocyclone and TVS association with confirmed severe weather are presented in this paper. Doppler radar provides the potential for improving severe thunderstorm warnings. Experiments are underway to test the operational use of this new tool in storm warning and flight advisory services.

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