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Boulder City, CO, United States

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. Source

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. Source

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. Source

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. Source

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. Source

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