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Shawn S. Murdzek
,
Paul M. Markowski
,
Yvette P. Richardson
, and
Robin L. Tanamachi

Abstract

A supercell produced a nearly tornadic vortex during an intercept by the Second Verification of the Origins of Rotation in Tornadoes Experiment on 26 May 2010. Using observations from two mobile radars performing dual-Doppler scans, a five-probe mobile mesonet, and a proximity sounding, factors that prevented this vortex from strengthening into a significant tornado are examined. Mobile mesonet observations indicate that portions of the supercell outflow possessed excessive negative buoyancy, likely owing in part to low boundary layer relative humidity, as indicated by a high environmental lifted condensation level. Comparisons to a tornadic supercell suggest that the Prospect Valley storm had enough far-field circulation to produce a significant tornado, but was unable to converge this circulation to a sufficiently small radius. Trajectories suggest that the weak convergence might be due to the low-level mesocyclone ingesting parcels with considerable crosswise vorticity from the near-storm environment, which has been found to contribute to less steady and weaker low-level updrafts in supercell simulations. Yet another factor that likely contributed to the weak low-level circulation was the inability of parcels rich in streamwise vorticity from the forward-flank precipitation region to reach the low-level mesocyclone, likely owing to an unfavorable pressure gradient force field. In light of these results, we suggest that future research should continue focusing on the role of internal, storm-scale processes in tornadogenesis, especially in marginal environments.

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Michael Colbert
,
David J. Stensrud
,
Paul M. Markowski
, and
Yvette P. Richardson

Abstract

In support of the Next Generation Global Prediction System (NGGPS) project, processes leading to convection initiation in the North American Mesoscale Forecast System, version 3 (NAMv3) are explored. Two severe weather outbreaks—occurring over the southeastern United States on 28 April 2014 and the central Great Plains on 6 May 2015—are forecast retrospectively using the NAMv3 CONUS (4 km) and Fire Weather (1.33 km) nests, each with 5-min output. Points of convection initiation are identified, and patterns leading to convection initiation in the model forecasts are determined. Results indicate that in the 30 min preceding convection initiation at a grid point, upward motion at low levels of the atmosphere enables a parcel to rise to its level of free convection, above which it is accelerated by the buoyancy force. A moist absolutely unstable layer (MAUL) typically is produced at the top of the updraft. However, when strong updrafts are collocated with large vertical gradients of potential temperature and moisture, noisy vertical profiles of temperature, moisture, and hydrometeor concentration develop beneath the rising MAUL. The noisy profiles found in this study are qualitatively similar to those that resulted in NAMv3 failures during simulations of Hurricane Joaquin in 2015. The CM1 cloud model is used to reproduce these noisy profiles, and results indicate that the noise can be mitigated by including explicit vertical diffusion in the model. Left unchecked, the noisy profiles are shown to impact convective storm features such as cold pools, precipitation, updraft helicity intensity and tracks, and the initiation of spurious convection.

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Alicia M. Klees
,
Yvette P. Richardson
,
Paul M. Markowski
,
Christopher Weiss
,
Joshua M. Wurman
, and
Karen K. Kosiba

Abstract

On 10 June 2010, the second Verification of the Origins of Rotation in Tornadoes Experiment (VORTEX2) armada collected a rare set of observations of a nontornadic and a tornadic supercell evolving in close proximity to each other. The storms and their environments were analyzed using single- and dual-Doppler radar, mobile mesonet, deployable surface mesonet, and mobile sounding data, with the goal of understanding why one supercell produced no tornadoes while the other produced at least two. Outflow temperature deficits were similar for the two storms, both within the normal range for weakly tornadic supercells but somewhat cold relative to significantly tornadic supercells. The storms formed in a complex environment, with slightly higher storm-relative helicity near the tornadic supercell. The environment evolved significantly in time, with large thermodynamic changes and increases in storm-relative helicity, leading to conditions much more favorable for tornadogenesis. After a few hours, a new storm developed between the supercells, likely leading to the demise of the nontornadic supercell before it was able to experience the enhanced environmental conditions. Two tornadoes developed within the single mesocyclone of the other supercell. After the dissipation of the second tornado, rapid rearward motion of low- to midlevel circulations may have inhibited further tornado production in this storm.

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Patrick S. Skinner
,
Christopher C. Weiss
,
Michael M. French
,
Howard B. Bluestein
,
Paul M. Markowski
, and
Yvette P. Richardson

Abstract

Observations collected in the second Verification of the Origins of Rotation in Tornadoes Experiment during a 15-min period of a supercell occurring on 18 May 2010 near Dumas, Texas, are presented. The primary data collection platforms include two Ka-band mobile Doppler radars, which collected a near-surface, short-baseline dual-Doppler dataset within the rear-flank outflow of the Dumas supercell; an X-band, phased-array mobile Doppler radar, which collected volumetric single-Doppler data with high temporal resolution; and in situ thermodynamic and wind observations of a six-probe mobile mesonet.

Rapid evolution of the Dumas supercell was observed, including the development and decay of a low-level mesocyclone and four internal rear-flank downdraft (RFD) momentum surges. Intensification and upward growth of the low-level mesocyclone were observed during periods when the midlevel mesocyclone was minimally displaced from the low-level circulation, suggesting an upward-directed perturbation pressure gradient force aided in the intensification of low-level rotation. The final three internal RFD momentum surges evolved in a manner consistent with the expected behavior of a dynamically forced occlusion downdraft, developing at the periphery of the low-level mesocyclone during periods when values of low-level cyclonic azimuthal wind shear exceeded values higher aloft. Failure of the low-level mesocyclone to acquire significant vertical depth suggests that dynamic forcing above internal RFD momentum surge gust fronts was insufficient to lift the negatively buoyant air parcels comprising the RFD surges to significant heights. As a result, vertical acceleration and the stretching of vertical vorticity in surge parcels were limited, which likely contributed to tornadogenesis failure.

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Matthew R. Kumjian
,
Kevin A. Bowley
,
Paul M. Markowski
,
Kelly Lombardo
,
Zachary J. Lebo
, and
Pavlos Kollias
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Christopher C. Weiss
,
David C. Dowell
,
John L. Schroeder
,
Patrick S. Skinner
,
Anthony E. Reinhart
,
Paul M. Markowski
, and
Yvette P. Richardson

Abstract

Observations obtained during the second Verification of the Origin of Rotation in Tornadoes Experiment (VORTEX2) are analyzed for three supercell intercepts. These intercepts used a fleet of deployable “StickNet” probes, complemented by mobile radars and a mobile mesonet, to map state quantities over the expanse of target storms.

Two of the deployments occurred for different stages of a supercell storm near and east of Dumas, Texas, on 18 May 2010. A comparison of the thermodynamic and kinematic characteristics of the storm provides a possible explanation for why one phase was weakly tornadic and the other nontornadic. The weakly tornadic phase features a stronger horizontal virtual temperature gradient antiparallel to the forward-flank reflectivity gradient and perpendicular to the near-surface flow direction, suggesting that air parcels could acquire more significant baroclinic vorticity as they approach the low-level mesocyclone.

The strongly tornadic 10 May 2010 case near Seminole, Oklahoma, features comparatively small virtual and equivalent potential temperature deficits, suggesting the strength of baroclinic zones may be less useful than the buoyancy near the mesocyclone for assessing tornado potential. The distribution of positive pressure perturbations and backed ground-relative winds within the forward flank are consistent with the notion of a “starburst” pattern of diverging winds associated with the forward-flank downdraft.

Narrow (~1 km wide) zones of intense baroclinic vorticity generation of O(~10−4) s−2 are shown to exist within precipitation on the forward and left sides of the mesocyclone in the Dumas intercepts, not dissimilar from such zones identified in recent high-resolution numerical studies.

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Paul M. Markowski
,
Nathan T. Lis
,
David D. Turner
,
Temple R. Lee
, and
Michael S. Buban

Abstract

Observations of near-surface vertical wind profiles and vertical momentum fluxes obtained from a Doppler lidar and instrumented towers deployed during VORTEX-SE in the spring of 2017 are analyzed. In particular, departures from the predictions of Monin–Obukhov similarity theory (MOST) are documented on thunderstorm days, both in the warm air masses ahead of storms and within the cool outflow of storms, where MOST assumptions (e.g., horizontal homogeneity and a steady state) are least credible. In these regions, it is found that the nondimensional vertical wind shear near the surface commonly exceeds predictions by MOST. The departures from MOST have implications for the specification of the lower boundary condition in numerical simulations of convective storms. Documenting departures from MOST is a necessary first-step toward improving the lower boundary condition and parameterization of near-surface turbulence (“wall models”) in storm simulations.

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Howard B. Bluestein
,
Robert M. Rauber
,
Donald W. Burgess
,
Bruce Albrecht
,
Scott M. Ellis
,
Yvette P. Richardson
,
David P. Jorgensen
,
Stephen J. Frasier
,
Phillip Chilson
,
Robert D. Palmer
,
Sandra E. Yuter
,
Wen-Chau Lee
,
David C. Dowell
,
Paul L. Smith
,
Paul M. Markowski
,
Katja Friedrich
, and
Tammy M. Weckwerth

To assist the National Science Foundation in meeting the needs of the community of scientists by providing them with the instrumentation and platforms necessary to conduct their research successfully, a meeting was held in late November 2012 with the purpose of defining the problems of the next generation that will require radar technologies and determining the suite of radars best suited to help solve these problems. This paper summarizes the outcome of the meeting: (i) Radars currently in use in the atmospheric sciences and in related research are reviewed. (ii) New and emerging radar technologies are described. (iii) Future needs and opportunities for radar support of high-priority research are discussed. The current radar technologies considered critical to answering the key and emerging scientific questions are examined. The emerging radar technologies that will be most helpful in answering the key scientific questions are identified. Finally, gaps in existing radar observing technologies are listed.

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