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Joshua Wurman

Abstract

The structure and behavior of multiple subtornadic-scale vortices in a tornado were examined and were compared with laboratory, conceptual, and numerical models. Unique radar observations of an exceptionally large and violent tornado obtained with a Doppler on Wheels mobile radar on 3 May 1999 in northern Oklahoma provided the opportunity, for the first time ever with quantitative radar measurements, to characterize the size, strength, motion, horizontal and vertical structure, and persistence of multiple vortices in a tornado. Doppler velocity, received power, and spectral-width data were used to study the vortices. The structures of the multiple subtornadic-scale vortices were similar to that of tornadic vortices in certain respects. They exhibited doughnut-shaped received power maxima and/or hooks surrounding comparatively clear central eyes. Doppler velocity differences across the vortices decreased with height. However, the vortices exhibited intense small-scale shears at their centers that could not be explained by the inability to resolve core flow regions adequately. Even though the distances between wind speed maxima were typically about 250 m, approximately one-half of the total shear in most vortices was concentrated across 50 m or less. This was in contrast to the approximately solid-body rotation exhibited in the core flow region of the parent tornado. It is hypothesized that either the very rapid motion of the vortices or small-scale transient updrafts caused this phenomenon. The shear across the vortices, about 100 m s–1, was about one-half of the total shear across the tornado, about 170 m s–1. The amplitude of the vortices was consistent with some, but not all, numerical and laboratory predictions. The central shear regions of the vortices exhibited estimated vertical vorticities of 4–8 s–1, the highest ever observed in tornadic flows. Wind speed changes of 50 m s–2, corresponding to 5 times the acceleration of gravity, would have been experienced by stationary observers impacted by the multiple vortices. The vortices appeared to translate around the tornado at a fraction of the peak azimuthally averaged tangential velocity of the parent tornado, consistent with some theoretical and computational predictions. It was not possible to rule out, however, that, in the absence of any upstream propagation, the vortices merely translated at the peak azimuthally averaged tangential velocity of the parent tornado at the radius of the vortices as predicted in other studies. Individual vortices were trackable for at least 40 s, revolving at least 180° around the parent tornado. The multiple vortices were most prominent during the weakening phase of the tornado, as peak azimuthally averaged tangential winds dropped from over 80 to less than 70 m s–1, and just after the radius of the peak flow region had contracted somewhat, possibly indicating changes in the swirl ratio.

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Joshua Wurman

A bistatic dual-Doppler weather radar network consisting of only one transmitter and a nontransmitting, nonscanning, low-cost bistatic receiver was deployed in the Boulder, Colorado, area during 1993.

The Boulder network took data in a variety of weather situations, including low-reflectivity stratiform snowfall, several convective cells, and a hailstorm. Dual-Doppler vector wind fields were retrieved and compared to those from a traditional, two-transmitter dual-Doppler network. The favorable results from these comparisons indicate that the bistatic dual-Doppler technique is viable and practical.

Bistatic multiple-Doppler networks have significant scientific and economic advantages accruing from the use of only single sources of illumination. Individual spatial volumes are viewed simultaneously from multiple look angles, minimizing storm evolution–induced errors. The passive receivers in a bistatic network do not require expensive transmitters, moving antenna hardware, or operators. Thus, they require only a small percentage of the investment needed to field traditional transmitting radars.

Bistatic systems can be deployed affordably to provide three-dimensional fields of full-vector winds, including directly measured vertical precipitation particle velocities for numerous applications in meteorological research, aviation, forecasting, media, and education.

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Karen Kosiba and Joshua Wurman
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Joshua Wurman and Karen Kosiba

Abstract

A variety of vortex configurations observed at finescale with Doppler On Wheels (DOW) radars in and near the hook echoes of supercell thunderstorms are described. These include marginal/weak tornadoes, often with no documented condensation funnels, debris rings, or low-reflectivity eyes; multiple-vortex mesocyclones; multiple simultaneous tornadoes; satellite tornadoes; cyclonic–anticyclonic tornado pairs; multiple vortices within other multiple vortices; tornadoes with quasi-concentric multiple wind field maxima; lines of vortices outside tornadoes; and horizontal vortices. The kinematic structures of these different phenomena are documented and compared. The process of multiple vortex circulations evolving from and into tornadoes is documented. DOW observations suggest that there is no clear spatial-scale separation between multiple-vortex tornadoes and larger multiple-vortex circulations.

These different vortex configurations motivate a refined definition of what constitutes a tornado, excluding many multiple, weak, embedded, and tornado-associated vortices.

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Joshua Wurman and Karen Kosiba

Abstract

Strong hurricanes cause severe, but highly variable, wind damage to homes and community infrastructure. It has been speculated, but not previously shown, that damage variability is caused by tornadoes or other small-scale phenomena. Here, the authors present the first mapping and tracking of persistent tornado-scale vortices (TSVs) in the eyewall and the first documentation of the likely role of eyewall mesovortices (MVs) and TSVs in enhancing surface winds and damage. Unprecedented finescale observations in the eyewall of Hurricane Harvey (2017) were obtained by a Doppler on Wheels (DOW) radar deployed inside the eye. These observations reveal several persistent eyewall MVs revolving about the eye, as well as superimposed subkilometer-scale TSVs. Wind field perturbations associated with TSVs and MVs are less than those typical in supercell tornadoes, but since they are embedded in strong background eyewall flow, they are likely responsible for the enhancement of surface wind gusts and significant damage, including destroyed buildings and lofted vehicles. Potential climate change may result in more frequent intense and/or rapidly intensifying hurricanes; thus, understanding and forecasting the causes of hurricane wind damage is a high priority.

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Shinsuke Satoh and Joshua Wurman

Abstract

The accuracy of wind vectors derived from bistatic dual-Doppler synthesis was investigated. The investigation was based on mathematical examinations and observation data analyses in both convective and stratiform cases. Accurate wind vectors were calculated within a range of 40° < β < 150°, where β was the bistatic scatter angle. The accuracy is mainly governed by β as the first principle, while sidelobe contamination and the sensitivity of the low-gain bistatic antenna are practically the main obstacles in retrieving accurate wind fields in convective and stratiform echoes, respectively. The sidelobe contamination around strong convective echoes with large gradients of reflectivity can be eliminated by comparing the measured bistatic reflectivity and a clean bistatic radar reflectivity, which is derived from the measured transmitting radar reflectivity and a bistatic radar equation. The radar equation includes the bistatic resolution volume and a bistatic antenna pattern estimated by the long-time observation of stratiform echoes. The equation also leads to the distribution of minimum detectable reflectivity of the bistatic receiver. The multiple bistatic radar observation in stratiform echoes produced three kinds of horizontal wind vectors, which were almost coincident with each other, in the overdetermined region. The composite wind fields are calculated by a simple variational method.

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Joshua Wurman and Swarndeep Gill

Abstract

The mature and dissipating stages of a strong tornado were observed from close range by the prototype Doppler On Wheels mobile radar. Volumetric observations repeated eight times over an 840-s period with resolution volumes at the center of the tornado as low as 61 m × 61 m × 75 m = 2.8 × 105 m3 revealed new details about three-dimensional tornado vortex structure and evolution. Observed structures included a conical debris envelope, a low-reflectivity eye, multiple windfield maxima, and multiple semiconcentric bands of reflectivity surrounding the eye. The three-dimensional structure of the debris and single-Doppler wind field were well characterized, as well as more rapid dissipation of the tornado aloft compared to near the ground. Volumetric measures of tornado strength are introduced. A downdraft exhibiting w ∼ −30 m s−1, indicative of a partial two-cell vortex, was observed only during the earliest radar scans when the tornado was near maximum intensity. Comparisons with simple conceptual models of vortices are presented and asymmetries are described. Possible reasons for the lack of radar-observed surface convergence are discussed. Comparisons between observed winds and damage are presented and a potential Fujita scale is introduced.

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Karen Kosiba and Joshua Wurman

Abstract

The three-dimensional axisymmetric wind field structure of the violent Spencer, South Dakota, 1998 tornado was analyzed using the ground-based velocity track display (GBVTD) method. Data from a Doppler on Wheels mobile radar, collected at very close range to the tornado, were used to conduct the GBVTD calculations at a very fine (16 m) resolution. The analysis revealed a two-cell vortex with a very strong axial downdraft throughout the observation period, radial inflow jets preceding intensification and a decrease in inflow preceding weakening, swirl ratio values consistent with observed multiple vortex structure, and other features of the vortex.

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Wen-Chau Lee and Joshua Wurman

Abstract

On 3 May 1999, an unusually large tornado that caused F4-level damage and killed several people was intercepted by the Doppler on Wheels (DOW) mobile radar near Mulhall, Oklahoma, from a range of 4 to 9 km, resulting in high-resolution volumetric data every 55 s up to 1.5-km altitude over a period of 14 min. For the first time, the evolution and three-dimensional structure of a tornado were deduced using the ground-based velocity track display (GBVTD) technique. After the circulation center was determined, the tangential wind and radial wind were derived from the GBVTD technique at each radius and height. In addition, the axisymmetric vertical velocity, angular momentum, vorticity, and perturbation pressure were deduced from the tangential and radial wind fields. This study focuses on the axisymmetric aspects of this tornado.

The primary circulation of the Mulhall tornado consisted of an 84 m s−1 peak axisymmetric tangential wind with the radius of maximum wind (RMW) ranging from 500 to 1000 m. The secondary circulation exhibited a two-cell structure characterized by a central downdraft surrounded by an annular updraft near the RMW. The calculated maximum pressure deficit from a 3-km radius to the tornado center at 50-m altitude was −80 hPa. The maximum vorticity during the first 8 min of observation was located inside the RMW away from the tornado center. This vorticity profile satisfied the necessary condition of barotropic instability. As the tornado weakened afterward, the vorticity monotonically increased toward the center. The computed swirl ratios were between 2 and 6, consistent with the observed multiple vortex radar signatures and the vorticity pattern. Swirl ratios were generally smaller during the weakening phase.

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Karen A. Kosiba and Joshua Wurman

Abstract

Two Doppler on Wheels (DOW) mobile radars collected fine-spatial-scale dual-Doppler data in the right-front quadrant and eye of Hurricane Frances (2004) as it made landfall near Stuart, Florida. A 5.7-km dual-Doppler baseline established a dual-Doppler domain south and east of Fort Pierce, Florida, encompassing a 5.5 km × 5.5 km horizontal area, with a grid spacing of 20 m, allowing for the resolution of subkilometer-scale horizontal structures and associated kinematics. Three-dimensional vector wind analyses of the boundary layer revealed the presence of linear coherent structures with a characteristic wavelength of 400–500 m near the surface that increased in size and became more cellular in shape with increasing height. Average horizontal perturbation winds were proportional to average total horizontal winds. Within the eye of the hurricane, the features lost linear coherency despite a high mean wind speed, possibly due to changes in stability. A slight decrease in the characteristic wavelength of boundary layer structures was documented as the winds cross the barrier islands east of Fort Pierce. Vertical flux of horizontal momentum caused by individual vortical structures was substantially higher than values employed in turbulence parameterization schemes, but the domain-wide average flux was substantially lower than that in individual structures, likely due to the transient nature of the most intense portions of the structures. Analysis of the turbulent kinetic energy (TKE) yielded values comparable to those reported in previous observational studies over the open ocean. However, there was substantial variability in TKE within the dual-Doppler domain, emphasizing the challenge in obtaining representative samples using non-3D measurements such as dropsondes.

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