Karen Kosiba

Genesis of the Goshen County, Wyoming, Tornado on 5 June 2009 during VORTEX2

AbstractThe genesis of a strong and long-lived tornado observed during the second Verification of the Origins of Rotation in Tornadoes Experiment (VORTEX2) in Goshen County, Wyoming, on 5 June 2009 is studied. Mobile radar, mobile mesonet, rawinsonde, and photographic data are used to produce an integrated analysis of the evolution of the wind, precipitation, and thermodynamic fields in the parent supercell to deduce the processes that resulted in tornadogenesis. Several minutes prior to tornadogenesis, the rear-flank downdraft intensifies, and a secondary rear-flank downdraft forms and cyclonically wraps around the developing tornado. Kinematic and thermodynamic analyses suggest that horizontal vorticity created in the forward flank and hook echo is tilted and then stretched near the developing tornado. Tilting and stretching are enhanced in the developing low-level circulation as the secondary rear-flank downdraft develops, intensifies, and wraps around the circulation center. Shortly thereafter, the to...

In Situ, Doppler Radar, and Video Observations of the Interior Structure of a Tornado and the Wind–Damage Relationship

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.

The Three-Dimensional Axisymmetric Wind Field Structure of the Spencer, South Dakota, 1998 Tornado

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.

The Pretornadic Phase of the Goshen County, Wyoming, Supercell of 5 June 2009 Intercepted by VORTEX2. Part II: Intensification of Low-Level Rotation

AbstractThe dynamical processes responsible for the intensification of low-level rotation prior to tornadogenesis are investigated in the Goshen County, Wyoming, supercell of 5 June 2009 intercepted by the second Verification of the Origins of Rotation in Tornadoes Experiment (VORTEX2). The circulation of material circuits that converge upon the low-level mesocyclone is principally acquired along the southern periphery of the forward-flank precipitation region, which is a corridor characterized by a horizontal buoyancy gradient; thus, much of the circulation appears to have been baroclinically generated. The descending reflectivity core (DRC) documented in Part I of this paper has an important modulating influence on the circulation of the material circuits. A circuit that converges upon the low-level mesocyclone center prior to the DRC’s arrival at low levels (approximately the arrival of the 55-dBZ reflectivity isosurface in this case) loses some of its previously acquired circulation during the final f...

The Role of Multiple-Vortex Tornado Structure in Causing Storm Researcher Fatalities

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 (> 3...

The Pretornadic Phase of the Goshen County, Wyoming, Supercell of 5 June 2009 Intercepted by VORTEX2. Part I: Evolution of Kinematic and Surface Thermodynamic Fields

AbstractThe authors analyze the pretornadic phase (2100–2148 UTC; tornadogenesis began at 2152 UTC) of the Goshen County, Wyoming, supercell of 5 June 2009 intercepted by the second Verification of the Origins of Rotation in Tornadoes Experiment (VORTEX2). The analysis relies on radar data from the Weather Surveillance Radar-1988 Doppler (WSR-88D) in Cheyenne, Wyoming (KCYS), and a pair of Doppler-on-Wheels (DOW) radars, mobile mesonet observations, and mobile sounding observations.The storm resembles supercells that have been observed in the past. For example, it develops a couplet of counter-rotating vortices that straddle the hook echo within the rear-flank outflow and are joined by arching vortex lines, with the cyclonic vortex becoming increasingly dominant in the time leading up to tornadogenesis. The outflow in the hook echo region, where sampled, has relatively small virtual potential temperature θυ deficits during this stage of evolution. A few kilometers upstream (north) of the location of maxim...

Finescale Single- and Dual-Doppler Analysis of Tornado Intensification, Maintenance, and Dissipation in the Orleans, Nebraska, Supercell

Abstract Finescale single- and dual-Doppler observations are used to diagnose the three-dimensional structure of the wind field surrounding a tornado that occurred near the town of Orleans, Nebraska, on 22 May 2004. The evolution of the vorticity and divergence fields and other structures near the tornado are documented in the lowest kilometer. Changes in tornado intensity are compared to the position of the tornado relative to primary and secondary gust fronts. Circulation on scales of a few kilometers surrounding the tornado remains relatively constant during the analysis period, which spans the intensifying and mature periods of the tornado’s life cycle. Stretching of vertical vorticity and tilting of horizontal vorticity are diagnosed, but the latter is near or below the threshold of detectability in this analysis during the observation period in the analyzed domain. Low-level circulation within 500 m of the tornado increased several minutes before vortex-relative and ground-relative near-surface wind...

Finescale Radar Observations of Tornado and Mesocyclone Structures

AbstractA 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...

The 2015 Plains Elevated Convection at Night Field Project

AbstractThe central Great Plains region in North America has a nocturnal maximum in warm-season precipitation. Much of this precipitation comes from organized mesoscale convective systems (MCSs). This nocturnal maximum is counterintuitive in the sense that convective activity over the Great Plains is out of phase with the local generation of CAPE by solar heating of the surface. The lower troposphere in this nocturnal environment is typically characterized by a low-level jet (LLJ) just above a stable boundary layer (SBL), and convective available potential energy (CAPE) values that peak above the SBL, resulting in convection that may be elevated, with source air decoupled from the surface. Nocturnal MCS-induced cold pools often trigger undular bores and solitary waves within the SBL. A full understanding of the nocturnal precipitation maximum remains elusive, although it appears that bore-induced lifting and the LLJ may be instrumental to convection initiation and the maintenance of MCSs at night.To gain ...

The Three-Dimensional Structure and Evolution of a Tornado Boundary Layer

AbstractThe 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 prec...