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Christopher C. Weiss
,
Howard B. Bluestein
, and
Andrew L. Pazmany

Abstract

The dryline has long been associated with the development of severe thunderstorms in the southern plains during the spring and early summer months. The propagation and structure of the dryline are closely tied to surface processes that are neither well understood nor well resolved with current observational capabilities. As a result, there are often large errors in forecasts of dryline position and structure.

Improvements in radar technology have allowed for better observations of the dryline in recent years. Here, very finescale radar observations taken with the University of Massachusetts—Amherst (UMass) mobile W-band radar during an International H2O Project (IHOP) double-dryline event on 22 May 2002 in the Oklahoma panhandle are presented. The observations are placed in the context of the dryline secondary circulation, which describes flow in a plane normal to the dryline. The narrow, half-power beamwidth of the antenna on the W band (0.18°) permitted the measurements of channels of upward (8–9 m s−1 over a horizontal distance of 50–100 m) and downward vertical velocity, greater in absolute magnitude than that previously reported in dryline field studies.

A ground-based variational pseudo-multiple-Doppler processing technique is introduced, which is used to decompose time series of RHI velocity data into horizontal and vertical wind components. The technique is applied to a retrograding dryline from 22 May 2002. Finescale structure of the retreating dryline interface is presented.

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Howard B. Bluestein
,
Christopher C. Weiss
, and
Andrew L. Pazmany

Abstract

This two-part paper details an analysis of high-resolution wind and reflectivity data collected by a mobile, W-band Doppler radar: The dataset captures the near-surface life history of a tornado in a supercell in north-central Nebraska on 5 June 1999. The formation of the tornado vortex near the ground is described from a sequence of sector scans ranging from 30-s intervals prior to tornadogenesis to 10–15-s intervals during much of the lifetime of the tornado.

Cyclonic vortices of 100–200 m width were found along a bow-shaped line of enhanced radar reflectivity, at what appears to have been the leading edge of a rear-flank gust front. At the time of tornadogenesis, one of these vortices was located just ahead of the nose of the bow-shaped radar echo and a jet, which were embedded within a larger-scale cyclone. At other times, small-scale cyclonic vortices coexisted with the tornado along an arc-shaped line extending to its north and northeast but did not appear to interact with the tornado. The evolution of all vortices and their associated reflectivity signatures was on a timescale shorter than 30 s, indicating that during tornadogenesis the flow pattern was highly unsteady. Mechanisms by which a smaller-scale vortex or vortices and a bow-shaped echo may have played a role in tornadogenesis are suggested. The structure of the tornado vortex near the ground, as a function of time, is discussed in Part II.

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Howard B. Bluestein
,
Christopher C. Weiss
, and
Andrew L. Pazmany

Abstract

A mobile, W-band Doppler radar scanned, at close range, portions of a tornado near Happy, Texas, on 5 May 2002. Simultaneous boresighted video images were also recorded, which facilitated correlating the radar-observed features of the tornado with its visual features. Range–height indicators (RHIs) of radar reflectivity and Doppler velocity were collected that detail, with high spatial resolution, aspects of the vertical structure of the tornado near the ground.

Most of the RHIs showed a column of a weak-echo hole from about 60 m above the ground up to the top of the domain at 800–1000 m above the ground; the hole was roughly 40% broader about 100 m above the ground as it was above, resulting in a characteristic pear-shaped vertical cross section of reflectivity. In this tornado, the condensation funnel was much narrower than the width of the weak-echo hole; the visible debris cloud near the ground was approximately just as wide as the hole above 150 m. The mean depth of the debris cloud was around 200 m. The vertical structure of the Doppler-velocity field exhibited a narrow band of high wind speeds about 200–400 m above the ground, consistent with airflow inward toward and cyclonically about the tornado. Possible reasons for the observed structure of the tornado are offered.

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Howard B. Bluestein
,
Christopher C. Weiss
, and
Andrew L. Pazmany

Abstract

Analyses of a dust-devil dataset collected in northwest Texas are presented. The data were collected just above the ground at close range with a mobile, W-band (3-mm wavelength) Doppler radar having an azimuthal (radial) resolution of 3–5 m (30 m) at the range of the dust devils. Most dust devils appeared as quasi-circular rings of relatively high radar reflectivity. Four dust-devil vortices were probed, three of which were cyclonic and one anticyclonic. Documentation was obtained of a pair of adjacent cyclonic vortices rotating cyclonically around each other.

Approximate radial profiles of azimuthal and radial wind components and of radar reflectivity are detailed and discussed. The diameters of the core of the dust devils ranged from 30 to 130 m; the latter diameters are much wider than that of typical dust devils in a homogeneous environment. The widest vortex was cyclonic and exhibited evidence of a two-cell structure (i.e., sinking motion near the center and rising motion just outside the radius of maximum wind), a broad, calm eye, and an annulus of maximum vorticity just inside the radius of maximum wind. As the vortex widened, it developed an asymmetry, and some evidence was found that two waves propagated cyclonically around it. The narrowest dust devil had the structure of a Rankine combined vortex, that is, a central core of constant vorticity surrounded by potential flow. Owing to very strong radial shear of the azimuthal wind, the vorticity in the dust-devil cores ranged from 0.5 to 1 s−1, which is as high as the vorticity in some tornadoes. However, the maximum ground-relative wind speeds in each dust devil were only 6.5–13.5 m s−1. The location of the highest radar reflectivity was located at or within the radius of maximum wind. In the widest dust devil, the vorticity estimated from the Doppler shear associated with its vortex signature was much less than the smaller-scale vorticity ring estimated from the azimuthal wind profile. It is therefore suggested that the vorticity estimated from the Doppler shear in tornadoes may be underestimated significantly when the tornado vortex exhibits a two-cell structure and that Doppler shear alone may not be a good indicator of vortex intensity.

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Howard B. Bluestein
,
Wen-Chau Lee
,
Michael Bell
,
Christopher C. Weiss
, and
Andrew L. Pazmany

Abstract

This is Part II of a paper detailing an analysis of high-resolution wind and reflectivity data collected by a mobile, W-band Doppler radar; the analysis depicts the near-surface life history of a tornado in a supercell in north-central Nebraska on 5 June 1999. The structure of the tornado vortex near the ground is described from a sequence of sector scans at 10–15-s intervals during much of the lifetime of the tornado. The formation of the tornado vortex near the ground is described in .

The wind and reflectivity features in the tornado evolved on timescales of 10 s or less. A time history of the azimuthally averaged azimuthal and radial wind profiles and the asymmetric components of the azimuthal and radial wind fields in the tornado were estimated by applying the ground-based velocity track display (GBVTD) technique to the Doppler wind data. If the magnitude of the asymmetric part of the radial wind component were indeed much less than that of the azimuthal wind component (a necessary requirement for application of the GBVTD technique), then the azimuthal wind field was dominated by quasi-stationary wavenumber-2 disturbances for most of the lifetime of the tornado. The radius of maximum wind (RMW) contracted as the tornado intensified and increased as the tornado dissipated. Shorter-timescale oscillations in azimuthal wind speed and RMW were found that could be manifestations of inertial oscillations. Evidence was also found that the tornado vortex was two-celled when it was most intense. During the “shrinking stage,” the vortex remained relatively wide and intense, even though the condensation funnel had narrowed substantially.

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Matthew R. Kramar
,
Howard B. Bluestein
,
Andrew L. Pazmany
, and
John D. Tuttle

Abstract

During spring 2001 in the Southern Plains, a recurring, hitherto undocumented reflectivity signature that the authors have called the “Owl Horn” signature (because the radar reflectivity pattern resembles the profile of the Great Horned Owl) was observed on a mobile, X-band radar display. The reflectivity signature was always located at the rear side of a developing supercell, spanned the entire rear side of the storm, and was always seen on low-level plan position indicator (PPI) scans. It lasted on the order of only 5–10 min and was not an artifact of the radar.

A study of the Owl Horn signature was undertaken using the Tracking Radar Echoes by Correlation technique (TREC) to estimate the wind field. TREC has previously been applied to clear-air and hurricane environments, and to the internal motions of severe storms, but not to their evolution. The characteristics of the signature are presented, and then, through the application of TREC to the radar reflectivity data (Doppler wind data were not available in 2001) collected during May and June 2001, the horizontal wind field was estimated around and in the Owl Horn signature.

Instances of the Owl Horn in numerical model storm simulations were investigated. The numerical simulations were used to identify conditions under which the signature occurs, the process by which it is created is discussed, and its dependence upon the environmental wind shear is examined. Results indicate that the hodograph shape and magnitude influence the production of the Owl Horn signature. Supercell-magnitude shear is required, and some curvature—particularly low-level curvature—is essential to the production of the feature. The Owl Horn signature is formed when horizontal vorticity is tilted into the vertical by expanding outflow through a positive feedback mechanism with the outflow.

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Robin L. Tanamachi
,
Howard B. Bluestein
,
Wen-Chau Lee
,
Michael Bell
, and
Andrew Pazmany

Abstract

On 15 May 1999, a storm intercept team from the University of Oklahoma collected high-resolution, W-band Doppler radar data in a tornado near Stockton, Kansas. Thirty-five sector scans were obtained over a period of approximately 10 min, capturing the tornado life cycle from just after tornadogenesis to the decay stage. A low-reflectivity “eye”—whose diameter fluctuated during the period of observation—was present in the reflectivity scans. A ground-based velocity track display (GBVTD) analysis of the W-band Doppler radar data of the Stockton tornado was conducted; results and interpretations are presented and discussed. It was found from the analysis that the axisymmetric component of the azimuthal wind profile of the tornado was suggestive of a Burgers–Rott vortex during the most intense phase of the life cycle of the tornado. The temporal evolution of the axisymmetric components of azimuthal and radial wind, as well as the wavenumber-1, -2, and -3 angular harmonics of the azimuthal wind, are also presented. A quasi-stationary wavenumber-2 feature of the azimuthal wind was analyzed from 25 of the 35 scans. It is shown, via simulated radar data collection in an idealized Burgers–Rott vortex, that this wavenumber-2 feature may be caused by the translational distortion of the vortex during the radar scans. From the GBVTD analysis, it can be seen that the maximum azimuthally averaged azimuthal wind speed increased while the radius of maximum wind (RMW) decreased slightly during the intensification phase of the Stockton tornado. In addition, the maximum azimuthally averaged azimuthal wind speed, the RMW, and the circulation about the vortex center all decreased simultaneously as the tornado decayed.

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Howard B. Bluestein
,
Michael M. French
,
Robin L. Tanamachi
,
Stephen Frasier
,
Kery Hardwick
,
Francesc Junyent
, and
Andrew L. Pazmany

Abstract

A mobile, dual-polarization, X-band, Doppler radar scanned tornadoes at close range in supercells on 12 and 29 May 2004 in Kansas and Oklahoma, respectively. In the former tornadoes, a visible circular debris ring detected as circular regions of low values of differential reflectivity and the cross-correlation coefficient was distinguished from surrounding spiral bands of precipitation of higher values of differential reflectivity and the cross-correlation coefficient. A curved band of debris was indicated on one side of the tornado in another. In a tornado and/or mesocyclone on 29 May 2004, which was hidden from the view of the storm-intercept team by precipitation, the vortex and its associated “weak-echo hole” were at times relatively wide; however, a debris ring was not evident in either the differential reflectivity field or in the cross-correlation coefficient field, most likely because the radar beam scanned too high above the ground. In this case, differential attenuation made identification of debris using differential reflectivity difficult and it was necessary to use the cross-correlation coefficient to determine that there was no debris cloud. The latter tornado’s parent storm was a high-precipitation (HP) supercell, which also spawned an anticyclonic tornado approximately 10 km away from the cyclonic tornado, along the rear-flank gust front. No debris cloud was detected in this tornado either, also because the radar beam was probably too high.

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Howard B. Bluestein
,
Christopher C. Weiss
,
Michael M. French
,
Eric M. Holthaus
,
Robin L. Tanamachi
,
Stephen Frasier
, and
Andrew L. Pazmany

Abstract

The University of Massachusetts W- and X-band, mobile, Doppler radars scanned several tornadoes at close range in south-central Kansas on 12 May 2004. The detailed vertical structure of the Doppler wind and radar reflectivity fields of one of the tornadoes is described with the aid of boresighted video. The inside wall of a weak-echo hole inside the tornado was terminated at the bottom as a bowl-shaped boundary within several tens of meters of the ground. Doppler signatures of horizontal vortices were noted along one edge in the lowest 500 m of the tornado. The vertical structure of Doppler velocity displayed significant variations on the 100-m scale. Near the center of the tornado, a quasi-horizontal, radial bulge of the weak-echo hole at ∼500–600 m AGL dropped to about 400 m above the ground and was evident as a weak-echo band to the south of the tornado. It is suggested that this feature represents echo-weak material transported radially outward by a vertical circulation. Significant vertical variations of Doppler velocity were found in the surface friction layer both inside and outside the tornado core. The shape of a weak-echo notch that was associated with a hook echo wrapped around the tornado is described. Highest Doppler velocities were located outside the condensation funnel. The structure of the other tornadoes is also described, but with much less detail. Some of the analyses are compared with numerical simulations of tornado-like vortices done elsewhere.

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Howard B. Bluestein
,
Stephen G. Gaddy
,
David C. Dowell
,
Andrew L. Pazmany
,
John C. Galloway
,
Robert E. McIntosh
, and
Herbert Stein

Abstract

Counterrotating 500-m-scale vortices in the boundary layer are documented in the right-moving member of a splitting supercell thunderstorm in northeastern Oklahoma on 17 May 1995 during the Verification of the Origins of Rotation in Tornadoes Experiment. A description is given of these vortices based upon data collected at close range by a mobile, 3-mm wavelength (95 GHz), pulsed Doppler radar. The vortices are related to a storm-scale, pseudo-dual-Doppler analysis of airborne data collected by the Electra Doppler radar (ELDORA) using the fore–aft scanning technique and to a boresighted video of the cloud features with which the vortices were associated. The behavior of the storm is also documented from an analysis of WSR-88D Doppler radar data.

The counterrotating vortices, which were associated with nearly mirror image hook echoes in reflectivity, were separated by 1 km. The cyclonic member was associated with a cyclonically swirling cloud base. The vortices were located along the edge of a rear-flank downdraft gust front, southeast of a kink in the gust front boundary, a location previously found to be a secondary region for tornado formation. The kink was coincident with a notch in the radar echo reflectivity. A gust front located north of the kink, along the edge of the forward-flank downdraft, was characterized mainly by convergence and density current–like flow, while the rear-flank downdraft boundary was characterized mainly by cyclonic vorticity.

Previously documented vortices along gust fronts have had the same sense of rotation as the others in the group and are thought to have been associated with shearing instabilities. The symmetry of the two vortices suggests that they may have been formed through the tilting of ambient horizontal vorticity. Although the vortices did not develop into tornadoes, it is speculated that similar vortices could be the seeds from which some tornadoes form.

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