Center for Severe Weather Research

Boulder City, CO, United States

Center for Severe Weather Research

Boulder City, CO, United States
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Wurman J.,Center for Severe Weather Research | Kosiba K.,Center for Severe Weather Research | Robinson P.,Center for Severe Weather Research | Marshall T.,Haag Engineering
Bulletin of the American Meteorological Society | Year: 2014

R esearchers, recreational storm chasers, stormchasing tours, storm spotters, television reporters, and others have been pursuing tornadic storms for centuries to satisfy a variety of goals. As early as 1755, Benjamin Franklin describes a tornado chase on horseback (van Doren 1938). Until recently, within the storm-chasing and research community, there have been no known fatalities or significant injuries directly caused by tornadoes. © 2014 American Meteorological Society.


Wakimoto R.M.,U.S. National Center for Atmospheric Research | Stauffer P.,U.S. National Center for Atmospheric Research | Lee W.-C.,U.S. National Center for Atmospheric Research | Atkins N.T.,Lyndon State College | Wurman J.,Center for Severe Weather Research
Monthly Weather Review | Year: 2012

A ground-based velocity track display (GBVTD) analysis of the LaGrange, Wyoming, tornado on 5 June 2009 during the Second Verification of the Origins of Rotation in Tornadoes Experiment (VORTEX2) is photogrammetrically combined with a series of pictures of the funnel cloud. This analysis reveals the relationship between the vertical velocity, radial and tangential velocities, perturbation pressure, vertical vorticity, and angular momentum with the visual features of the tornado. An intense axial downdraft was evident and was supported by a downward-directed perturbation pressure gradient. The radial inflow at low levels was weak and difficult to retrieve owing to a combination of centrifuging of hydrometeors/debris in the intense circulation and the inability of the radar beam to fully resolve the flow. The tornado was weakening during the analysis period, which was supported by angular momentum being advected out of the tornado. The availability of a dual-Doppler wind synthesis for this tornadic event provided a unique opportunity to assess the assumptions in the GBVTD methodology. The analysis suggests that the simplified GBVTD equations that have been applied in past studies of tornadoes are not appropriate in the present case. The most accurate retrieval of the radial velocities requires that a higher-order term that is typically neglected be retained. A quantitative assessment of the impact of centrifuging of hydrometeors on the synthesized wind field was attempted. The results suggest that the radial and vertical velocity profile near and within the tornado core can be significantly altered for tornadoes (EF2) that are accompanied by a small radius of maximum wind and relatively weaker low-level inflow. ©2012 American Meteorological Society.


Wakimoto R.M.,U.S. National Center for Atmospheric Research | Atkins N.T.,Lyndon State College | Wurman J.,Center for Severe Weather Research
Monthly Weather Review | Year: 2011

This study presents a single-Doppler radar analysis combined with cloud photography of the LaGrange, Wyoming, tornado on 5 June 2009 in an attempt to relate the radar-observed hook echo, weak-echo hole (WEH), and rotational couplet to the visual characteristics of the tornado. The tornado was rated EF2. The circulation at low levels went through two intensification periods based on azimuthal shear measurements. The first intensification was followed by the appearance of a brief funnel cloud. The second intensification was coincident with the appearance of a second funnel cloud that remained in contact with the ground until the tornado dissipated. A deep WEH rapidly formed within the hook echo after damaging wind was identified at the ground and before the appearance of a funnel cloud. The echo pattern through the hook echo on 5 June undergoes a dramatic evolution. Initially, the minimum radar reflectivities are near the surface (15 dBZ) and theWEH does not suggest a tapered structure near the ground. Subsequently, higher reflectivities appear at low levels when the funnel cloud makes contact with the ground. During one analysis time, the increase of the echo within the WEH at low levels results in a couplet of high/low radar reflectivity in the vertical. This increase in echo at low levels is believed to be associated with lofted debris although none was visibly apparent until the last analysis time. The WEH was nominally wider than the visible funnel cloud. The dataset provides the first detailed analysis of the double-ring structure within a hook echo that has been reported in several studies. The inner high-reflectivity region is believed to be a result of lofted debris. At higher-elevation angles, a small secondary WEH formed within the first WEH when debris was lofted and centrifuged. A feature noted in past studies showing high-resolution vertical cross sections of single-Doppler velocity normal to the radar beam is an intense rotational couplet of negative and positive values in the lowest few hundred meters. This couplet was also evident in the analysis of the LaGrange tornado. The couplet was asymmetric with stronger negative velocities owing to the motion of the tornado toward the radar. The damaging wind observed by radar extended well beyond the condensation funnel in the lowest few hundred meters. However, another couplet indicating strong rotation was also noted aloft in a number of volume scans. The decrease in rotational velocities between the low-and upper-level couplets may be related to air being forced radially outward from the tornado center at a location above the intense inflow. © 2011 American Meteorological Society.


Marquis J.,Pennsylvania State University | Richardson Y.,Pennsylvania State University | Markowski P.,Pennsylvania State University | Dowell D.,U.S. National Center for Atmospheric Research | Wurman J.,Center for Severe Weather Research
Monthly Weather Review | Year: 2012

Dual-Doppler wind synthesis and ensemble Kalman filter analyses produced by assimilating Doppler-on-Wheels velocity data collected in four tornadic supercells are examined in order to further understand the maintenance of tornadoes. Although tornado-scale features are not resolved in these analyses, larger-scale processes involved with tornado maintenance are well represented. The longest-lived tornado is maintained underneath the midlevel updraft within a zone of low-level horizontal convergence along a rear-flank gust front for a considerable time, and dissipates when horizontally displaced from the midlevel updraft. The shortest-lived tornado resides in a similar zone of low-level convergence briefly, but dissipates underneath the location of the midlevel updraft when the updraft becomes tilted and low-level convergence is displaced several kilometers from the tornado. This suggests that a location beneath the midlevel updraft is not always a sufficient condition for tornado maintenance, particularly in the presence of strongly surging outflow. Tornadoes in two other storms persist within a band of low-level convergence in the outflow air (a possible secondary rear-flank gust front), suggesting that tornado maintenance can occur away from the main boundary separating the outflow air and the ambient environment. In at least one case, tilting of horizontal vorticity occurs near the tornado along the secondary gust front, as evidenced by three-dimensional vortex line arching. This observation suggests that a relatively cold secondary rear-flank downdraft may assist with tornado maintenance through the baroclinic generation and tilting of horizontal vorticity, despite the fact that parcels composing them would be more negatively buoyant than the preceding outflow air. © 2012 American Meteorological Society.


Wurman J.,Center for Severe Weather Research | Kosiba K.,Center for Severe Weather Research
Weather and Forecasting | Year: 2013

Avariety of vortex configurations observed at finescale with DopplerOn 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 multiplevortex 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. © 2013 American Meteorological Society.


Kosiba K.,Center for Severe Weather Research | Wurman J.,Center for Severe Weather Research | Richardson Y.,Pennsylvania State University | Markowski P.,Pennsylvania State University | And 2 more authors.
Monthly Weather Review | Year: 2013

The 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, mobilemesonet, 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 tornado forms. Tornadogenesis does not proceed steadily. Strengthening, weakening, and renewed intensification of the tornado are documented in photographic, reflectivity,Doppler velocity, and dual-Doppler fields and are associated with, and shortly follow, changes in the secondary rear-flank downdraft, convergence, location of the vortex relative to the updraft/downdraft couplet, tilting and stretching near and in the developing tornado, and the evolution of total circulation. © 2013 American Meteorological Society.


Wurman J.,Center for Severe Weather Research | Kosiba K.,Center for Severe Weather Research | Robinson P.,Center for Severe Weather Research
Bulletin of the American Meteorological Society | Year: 2013

Observations inside a tornado, integrated with fine-resolution rapid-scan Doppler on Wheels data, and real-time observations of damage, reveal for the first time the three-dimensional structure of a tornado near the ground and help evaluate the enhanced Fujita scale. © 2013 American Meteorological Societyis.


Toth M.,Purdue University | Trapp R.J.,Purdue University | Wurman J.,Center for Severe Weather Research | Kosiba K.A.,Center for Severe Weather Research
Weather and Forecasting | Year: 2013

In the United States, visual observations of tornadoes and/or the existence of tornado damage currently provide the sole evidence of tornadogenesis in association with a mesocyclone or other radar-detected stormscale vortex. The severity of the tornado damage is currently the only means of estimating the intensity of tornadoes, radar detected or otherwise. The limitations of the damage-based record of tornado occurrence and intensity are well known and motivated this research. Weather Surveillance Radar-1988 Doppler (WSR- 88D) measurements of the translating tornadic flow were compared with (semi-) coordinated measurements obtained near the surface with mobile radar. On the basis of a small yet fairly broad sample of tornadoes, high linear correlation was found between the vortex intensity (rotation plus translation) quantified using WSR- 88D data and that quantified using Doppler on Wheels data. The possible effects of Doppler radar sampling on these results were explored through experiments with a simple vortex model. These experiments argued that the likelihood is high that a tornado would be sampled in a favorable way during at least one radar scan. Hence, the suggestion from this work is that WSR-88Ds (or similar operational radars) can potentially be used in isolation to estimate low-level tornado intensity. The proposed estimation is by way of a linear regression model, and application of this model is relevant only once a tornado is already confirmed. © 2013 American Meteorological Society.


Kosiba K.A.,Center for Severe Weather Research | Wurman J.,Center for Severe Weather Research
Weather and Forecasting | Year: 2013

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 17m 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 3.5m 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 ̃5m AGL, about 15% higher than at 10m AGL and 25% higher than at ̃40m 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-7ms-1) is at or below 10-14m 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. © 2013 American Meteorological Society.


Kosiba K.,Center for Severe Weather Research | Wurman J.,Center for Severe Weather Research
Journal of the Atmospheric Sciences | Year: 2010

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. © 2010 American Meteorological Society.

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