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Karen Kosiba and Joshua Wurman
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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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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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Karen Kosiba and Joshua Wurman

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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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Karen A. Kosiba and Joshua Wurman

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The finescale three-dimensional structure and evolution of the near-surface boundary layer of a tornado (TBL) is mapped for the first time. The multibeam Rapid-Scan Doppler on Wheels (RSDOW) collected data at several vertical levels, as low as 4, 6, 10, 12, 14, and 17 m above ground level (AGL), contemporaneously at 7-s intervals for several minutes in a tornado near Russell, Kansas, on 25 May 2012. Additionally, a mobile mesonet anemometer measured winds at 3.5 m AGL in the core flow region. The radar, anemometer, and ground-based velocity-track display (GBVTD) analyses reveal the peak wind intensity is very near the surface at ~5 m AGL, about 15% higher than at 10 m AGL and 25% higher than at ~40 m AGL. GBVTD analyses resolve a downdraft within the radius of maximum winds (RMW), which decreased in magnitude when varying estimates for debris centrifuging are included. Much of the inflow (from −1 to −7 m s−1) is at or below 10–14 m AGL, much shallower than reported previously. Surface outflow precedes tornado dissipation. Comparisons between large-eddy simulation (LES) predictions of the corner flow swirl ratio Sc and observed tornado intensity changes are consistent.

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Karen A. Kosiba and Joshua Wurman

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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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Joshua Wurman, Karen Kosiba, and Paul Robinson

Direct observations of the winds inside a tornado were obtained with an instrumented armored vehicle, the Tornado Intercept Vehicle (TIV), and integrated with finescale mobile Doppler radar (Doppler on Wheels) data revealing, for the first time, the structure of the near-ground three-dimensional wind field in and around the core region of a strong tornado, and permitting comparison with conceptual models. Inward and upward spiraling near-surface flow, upward motion near the surface, and an axial downdraft aloft are documented, as well as a periodic oscillation in tornado intensity. Simultaneous video documentation of damage occurring during the tornado is related to the direct wind observations, permitting the first comparisons of the time history of damage to the time history of directly measured winds and a limited evaluation of the underlying assumptions and quantitative relationships in the enhanced Fujita (EF) scale.

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Karen Kosiba, Joshua Wurman, Forrest J. Masters, and Paul Robinson

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Data collected from a Doppler on Wheels (DOW) mobile radar deployed in Port Arthur, Texas, near the point of landfall of Hurricane Rita (2005) and from two Florida Coastal Monitoring Program 10-m weather stations (FCMP-WSs) are used to characterize wind field variability, including hurricane boundary layer (HBL) streaks/rolls, during the hurricane's passage. DOW data, validated against nearby weather station data, are combined with surface roughness fields derived from land-use mapping to produce fine spatial scale, two-dimensional maps of the 10 m above ground level (AGL) open-terrain exposure and exposure-influenced winds over Port Arthur. The DOW collected ~3000 low-elevation radar sweeps at 12-s intervals for >10 h during the passage of the hurricane. This study focuses on the 2–3-h period when the western eyewall passed over Port Arthur. Finescale HBL wind streaks are observed to have length scales of O(300 m), smaller than previously identified in other HBL studies. The HBL streaks are tracked as they pass over an FCMP-WS located in flat, open terrain and another FCMP-WS located near a subdivision. DOW data collected over the FCMP-WS are reduced to anemometer height, using roughness lengths calculated from DOW and FCMP-WS data. Variations in the radar-observed winds directly over the FCMP-WS are very well correlated, both in their timing and magnitude, with wind gusts observed by the weather stations, revealing directly for the first time the surface manifestation of these wind streaks that are observed frequently by radar >100 m AGL. This allows for the generation of spatially filled maps of small-scale wind fluctuations over Port Arthur during the hurricane eyewall's passage using DOW-measured winds.

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Joshua Wurman, Karen Kosiba, Paul Robinson, and Tim Marshall

A large and violent tornado/multiple-vortex mesocyclone (MVMC) tracked east and northeastward near El Reno, Oklahoma, on 31 May 2013, causing eight fatalities, including storm chasers/researchers attempting to deploy in situ instrumentation. Subvortices moved within and near the MVMC, some in trochoidal-like patterns, with ground-relative translational velocities ranging from 0 to 79 m s−1, the fastest ever documented. Doppler on Wheels (DOW) measurements in one of these subvortices exceeded 115 m s−1 at 114 m AGL. With assumptions concerning radar-unobserved components of the velocity, peak wind speeds of 130–150 m s−1 are implied, comparable to the strongest ever measured. Only enhanced Fujita scale 3 (EF-3) damage was documented, likely because of a paucity of well-built structures and the most intense winds being confined to small, rapidly moving subvortices, resulting in only subsecond gusts. The region enclosing the maximum winds of the tornado/MVMC extended ~2 km. DOW-measured winds > 50 m s−1 (> 30 m s−1) extended far beyond the radius of maximum winds (RMW) extending >5 km (7 km), comparable to the widest ever documented. A strong multiple-vortex anticyclonic tornado with dual-polarization debris signatures is documented.

A subvortex tracking eastward within the larger tornado/MVMC intensified, moved north, and then moved northwestward, becoming briefly nearly stationary near/over a research team's vehicle, transporting it ~600 m generally eastward, killing the team. An experienced media team's vehicle was destroyed inside the tornado/MVMC, resulting in injuries. The circumstances leading to these incidents are analyzed using DOW data. The anomalous—and likely unpredictable in real time—path of the interior subvortex likely contributed to these deaths and injuries. The risks associated with chasing and scientific missions near and particularly inside large and complex MVMC/tornado vortices are discussed.

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James Marquis, Yvette Richardson, Paul Markowski, David Dowell, Joshua Wurman, Karen Kosiba, Paul Robinson, and Glen Romine

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High-resolution Doppler radar velocities and in situ surface observations collected in a tornadic supercell on 5 June 2009 during the second Verification of the Origins of Rotation in Tornadoes Experiment (VORTEX2) are assimilated into a simulated convective storm using an ensemble Kalman filter (EnKF). A series of EnKF experiments using a 1-km horizontal model grid spacing demonstrates the sensitivity of the cold pool and kinematic structure of the storm to the assimilation of these observations and to different model microphysics parameterizations. An experiment is performed using a finer grid spacing (500 m) and the most optimal data assimilation and model configurations from the sensitivity tests to produce a realistically evolving storm. Analyses from this experiment are verified against dual-Doppler and in situ observations and are evaluated for their potential to confidently evaluate mesocyclone-scale processes in the storm using trajectory analysis and calculations of Lagrangian vorticity budgets. In Part II of this study, these analyses will be further evaluated to learn the roles that mesocyclone-scale processes play in tornado formation, maintenance, and decay. The coldness of the simulated low-level outflow is generally insensitive to the choice of certain microphysical parameterizations, likely owing to the vast quantity of kinematic and in situ thermodynamic observations assimilated. The three-dimensional EnKF wind fields and parcel trajectories resemble those retrieved from dual-Doppler observations within the storm, suggesting that realistic four-dimensional mesocyclone-scale processes are captured. However, potential errors are found in trajectories and Lagrangian three-dimensional vorticity budget calculations performed within the mesocyclone that may be due to the coarse (2 min) temporal resolution of the analyses. Therefore, caution must be exercised when interpreting trajectories in this area of the storm.

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