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Robin L. Tanamachi and Pamela L. Heinselman

ABSTRACT

On 31 May 2013, a polarimetric WSR-88D located in Norman, Oklahoma (KOUN), was used to collect sectorized volumetric observations in a tornadic supercell. Because only a fraction of the full azimuthal volume was observed, rapid volume update times of ~1–2 min were achieved. In addition, the number of pulses used in each radial was larger than is conventional, increasing the statistical robustness of the calculated polarimetric variables. These rapid observations serve as a proxy for those of a future dual-polarized phased-array radar. Through comparison with contemporaneous observations from two nearby dual-polarized WSR-88Ds [Twin Lakes, Oklahoma (KTLX), and near University of Oklahoma Westheimer Airport in Norman (KCRI)], a number of instances in which the rapidly scanned KOUN radar detected or better resolved (in a temporal sense) features of severe convective storms are highlighted. In particular, the polarimetric signatures of merging updrafts, a rapidly descending giant hail core, an anticyclonic tornado, and a dissipating storm cell are examined. These observations provided insights into the rapid evolution of severe convective storms that could not be made (or would have been made with much lower confidence) with current, operational WSR-88D scanning strategies. Possible implications of these rapid updates for the warning decision process are discussed.

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Robin L. Tanamachi, Pamela L. Heinselman, and Louis J. Wicker

Abstract

On 24 May 2011, a tornadic supercell (the El Reno, Oklahoma, storm) produced tornadoes rated as category 3 and 5 events on the enhanced Fujita scale (EF3 and EF5, respectively) during a severe weather outbreak. The transition (“handoff”) between the two tornadoes occurred as the El Reno storm merged with a weaker, ancillary storm. To examine the impacts of the merger on the dynamics of these storms, a series of three-dimensional cloud-scale analyses are created by assimilating 1-min volumetric observations from the National Weather Radar Testbed’s phased array radar into a numerical cloud model using the local ensemble transform Kalman filter technique. The El Reno storm, its updrafts, and vortices in the analyzed fields are objectively identified, and the changes in these objects before, during, and after the merger are examined. It is found that the merger did not cause the tornado handoff, which preceded the updraft merger by about 5 min. Instead, the handoff likely resulted from midlevel mesocyclone occlusion, in which the midlevel mesocyclone split and a portion is shed rearward with respect to storm motion. During the merger process, the midlevel mesocyclone and updraft structure in the El Reno storm became relatively disorganized. New updraft pulses that formed above colliding outflow boundaries between the two storms tilted environmental vorticity from low levels to generate an additional midlevel vortex that later merged with the El Reno storm’s midlevel mesocyclone. Once the ~10-min merger process was complete, the El Reno storm and its mesocyclone rapidly reintensified, as access to buoyant inflow sector air was restored.

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Robin L. Tanamachi, Wayne F. Feltz, and Ming Xue

Abstract

On the morning of 12 June 2002, a series of upper boundary layer (UBL) rapid drying and moistening events (RDEs and RMEs, respectively) occurred at the “Homestead” site of the International H2O Project (IHOP_2002). Over a period of 10 h, atmospheric water vapor in the UBL decreased or increased within a matter of minutes four separate times. High-temporal-resolution data of the RDEs and RMEs collected by numerous instruments deployed for this intensive observation period are presented. The results of an Advanced Regional Prediction System (ARPS) simulation of the weather conditions around the time period reproduced one of the two RDE–RME pairs with reasonably accurate amplitude and timing. Both the observational data and ARPS numerical model output indicate that the second RDE–RME pair resulted from the interaction between a dry air mass descending from the Rocky Mountains and a cold pool–internal undular bore couplet propagating over the Homestead site from a mesoscale convective complex to the north. The RDEs and RMEs, which were rarely observed during IHOP_2002, are believed to be an indirect indicator of such bores.

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Robin L. Tanamachi, Daniel T. Dawson II, and Loran Carleton Parker
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Robin L. Tanamachi, Daniel T. Dawson II, and Loran Carleton Parker

Abstract

A summer course has been developed at Purdue University that leverages students’ intrinsic desire to observe tornadoes as a motivator for learning severe storms forecasting. Relative to previous “storm chasing” courses described in the literature, the Students of Purdue Observing Tornadic Thunderstorms for Research (SPOTTR) course is enhanced by active learning exercises, career exploration activities, and the inclusion of research-grade meteorological instrumentation in order to provide an authentic in-field experiential learning scenario. After teaching severe weather forecasting skills and deployment techniques for several meteorological instruments (such as a mobile radar, radiosondes, and disdrometers), the instructors then guide the students on a 1-week field trip to the Great Plains, where the group executes a miniature field campaign to collect high-quality meteorological observations in and near severe storms. On days with no targetable severe weather, the participants visit sites deemed beneficial to the students’ professional development. The final week of the course is spent performing retrospective case studies based on the observations collected, and distilling lessons learned. Surveys given to SPOTTR students show that students’ understanding of severe storms forecasting, technical skills, and career aspirations all improved as a result of having participated in the SPOTTR course, affirming the efficacy of the course design.

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Vincent T. Wood, Robin L. Tanamachi, and Luther W. White

Abstract

Previous studies have neglected to distinguish between a central pressure deficit due to a tornado itself and due to a parent mesocyclone in which the tornado is embedded. To obtain improved understanding of the influences of larger-scale vortex variability on smaller-scale tornado pressure deficits, a parametric tangential wind model supplemented with a cyclostrophic speed equation was used to explore the role that the variability plays in influencing radial pressure deficits by deducing radial pressure deficit distributions from radial profiles of hypothetically superpositioned, dual-maxima tangential velocities in the free atmosphere, where a dominant swirling flow was in approximate cyclostrophic balance. The cyclostrophic approximation was partitioned into two separate components, allowing one to scrutinize and determine which of the concentric vortices contributes most significantly to the tornado pressure minima. The model parametrically constructed a smaller-scale, stronger vortex (rapidly swirling flow) that was centered within a larger-scale, weaker vortex (slowly swirling flow) to represent a tornado centered within a supercell, low-level, parent mesocyclone above a tornado boundary layer. The radial pressure deficit fluctuations were varied by changing one of five key velocity-controlling parameters assigned to one vortex to represent a variety of vortex strengths. Based on eight experiments, the larger-scale, weaker (smaller scale, stronger) vortex contributed less (more) to the total pressure deficit than the smaller-scale, stronger (larger scale, weaker) vortex. The stronger vortex centered within the larger-scale, weaker vortex has a larger central pressure minimum than it does in the absence of the larger-scale vortex.

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Robin L. Tanamachi, Howard B. Bluestein, Stephen S. Moore, and Robert P. Madding

Abstract

During the spring seasons of 2003 and 2004, an infrared thermal camera was deployed in and around supercell thunderstorms in an attempt to retrieve the temperature at the cloud base of a mesocyclone prior to tornadogenesis. The motivation for this exercise was to obtain temperature information that might indicate the thermal structure, timing, and extent of the rear-flank downdraft (RFD) and possibly elucidate its relationship to tornadogenesis.

An atmospheric transmissivity study was conducted to account for the effects of atmospheric transmission on the measured temperatures, and to determine an ideal range of distances from which infrared images of a wall cloud or a tornado could be safely captured while still retrieving accurate cloud temperatures. This range was found to be 1.5–3 km.

Two case days are highlighted in which the infrared camera was deployed within 1.5–3 km of a tornado; the visible and infrared images are shown side by side for comparison. On the single occasion on which the tornadogenesis phase was captured, the infrared images show no strong horizontal temperature gradients. From the infrared images taken of tornadoes, it can be inferred that the infrared signal from the tornado consisted primarily of infrared emissions from lofted dust particles or cloud droplets, and that the infrared signal from the tornado condensation funnel was easily obscured by infrared emissions from lofted dust particles or intervening precipitation curtains.

The deployment of the infrared camera near supercell thunderstorms and the analysis of the resulting images proved challenging. It is concluded that the infrared camera is a useful tool for measuring cloud-base temperature gradients provided that distance and viewing angle constraints are met and that the cloud base is unobscured by rain or other intervening infrared emission sources. When these restrictions were met, the infrared camera successfully retrieved horizontal temperature gradients along the cloud base and vertical temperature gradients (close to the moist adiabatic lapse rate) along the tornado funnel.

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Robin L. Tanamachi, Howard B. Bluestein, Jana B. Houser, Stephen J. Frasier, and Kery M. Hardwick

Abstract

On 4 May 2007, a supercell produced an EF-5 tornado that severely damaged the town of Greensburg, Kansas. Volumetric data were collected in the “Greensburg storm” by the University of Massachusetts X-band, mobile, polarimetric Doppler radar (UMass X-Pol) for 70 min; 10 tornadoes were detected. This mobile Doppler radar dataset is one of only a few documenting an EF-5 tornado and the supercell’s transition from short-track, cyclic tornado production (mode 1) to long-track tornado production (mode 2). Using bootstrap confidence intervals, it is determined that the mode-2 tornadoes moved in the same direction as the supercell vault. In contrast, the mode-1 tornadoes moved to the left with respect to the vault.

From polarimetric data collected in this storm, the authors infer the presence of large, oblate drops (high Z DR, high ρ hv) in the forward flank and surrounding some of the tornadoes. The authors speculate that the weak-echo column (WEC) in the Greensburg tornado, which extended above 10 km AGL, was caused primarily by the centrifuging of hydrometeors at low levels and rapid upward transport of relatively scatterer-free air at upper levels. This WEC was collocated at low levels with a low-Z DR, low-ρ hv column, indicating lofted debris.

Dual-Doppler analyses, generated at ~10-min intervals using data from UMass X-Pol and the Dodge City, Kansas, Weather Surveillance Radar-1988 Doppler (WSR-88D), were used to locate updrafts and downdrafts near the hook echo. In the immediate vicinity of tornadoes, diminished Z DR values downstream of analyzed downdrafts may indicate the ingestion by tornadoes of relatively small drops, fallout of larger drops, or a combination of both.

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Robin L. Tanamachi, Howard B. Bluestein, Wen-Chau Lee, Michael Bell, and Andrew Pazmany

Abstract

On 15 May 1999, a storm intercept team from the University of Oklahoma collected high-resolution, W-band Doppler radar data in a tornado near Stockton, Kansas. Thirty-five sector scans were obtained over a period of approximately 10 min, capturing the tornado life cycle from just after tornadogenesis to the decay stage. A low-reflectivity “eye”—whose diameter fluctuated during the period of observation—was present in the reflectivity scans. A ground-based velocity track display (GBVTD) analysis of the W-band Doppler radar data of the Stockton tornado was conducted; results and interpretations are presented and discussed. It was found from the analysis that the axisymmetric component of the azimuthal wind profile of the tornado was suggestive of a Burgers–Rott vortex during the most intense phase of the life cycle of the tornado. The temporal evolution of the axisymmetric components of azimuthal and radial wind, as well as the wavenumber-1, -2, and -3 angular harmonics of the azimuthal wind, are also presented. A quasi-stationary wavenumber-2 feature of the azimuthal wind was analyzed from 25 of the 35 scans. It is shown, via simulated radar data collection in an idealized Burgers–Rott vortex, that this wavenumber-2 feature may be caused by the translational distortion of the vortex during the radar scans. From the GBVTD analysis, it can be seen that the maximum azimuthally averaged azimuthal wind speed increased while the radius of maximum wind (RMW) decreased slightly during the intensification phase of the Stockton tornado. In addition, the maximum azimuthally averaged azimuthal wind speed, the RMW, and the circulation about the vortex center all decreased simultaneously as the tornado decayed.

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Daniel T. Dawson II, Louis J. Wicker, Edward R. Mansell, and Robin L. Tanamachi

Abstract

The early tornadic phase of the Greensburg, Kansas, supercell on the evening of 4 May 2007 is simulated using a set of storm-scale (1-km horizontal grid spacing) 30-member ensemble Kalman filter (EnKF) data assimilation and forecast experiments. The Next Generation Weather Radar (NEXRAD) level-II radar data from the Dodge City, Kansas (KDDC), Weather Surveillance Radar-1988 Doppler (WSR-88D) are assimilated into the National Severe Storms Laboratory (NSSL) Collaborative Model for Multiscale Atmospheric Simulation (COMMAS). The initially horizontally homogeneous environments are initialized from one of three reconstructed soundings representative of the early tornadic phase of the storm, when a low-level jet (LLJ) was intensifying. To isolate the impact of the low-level wind profile, 0–3.5-km AGL wind profiles from Vance Air Force Base, Oklahoma (KVNX), WSR-88D velocity-azimuth display (VAD) analyses at 0130, 0200, and 0230 UTC are used. A sophisticated, double-moment bulk ice microphysics scheme is employed.

For each of the three soundings, ensemble forecast experiments are initiated from EnKF analyses at various times prior to and shortly after the genesis of the Greensburg tornado (0200 UTC). Probabilistic forecasts of the mesocyclone-scale circulation(s) are generated and compared to the observed Greensburg tornado track. Probabilistic measures of significant rotation and observation-space diagnostic statistics are also calculated. It is shown that, in general, the track of the Greensburg tornado is well predicted, and forecasts improve as forecast lead time decreases. Significant variability is also seen across the experiments using different VAD wind profiles. Implications of these results regarding the choice of initial mesoscale environment, as well as for the “Warn-on-Forecast” paradigm for probabilistic numerical prediction of severe thunderstorms and tornadoes, are discussed.

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