Bruce D. Lee

The Numerical Simulation of Non-Supercell Tornadogenesis. Part I: Initiation and Evolution of Pretornadic Misocyclone Circulations along a Dry Outflow Boundary

Abstract High-resolution three-dimensional simulations are used to study misocyclone initiation and development along the leading edge of an outflow boundary. Model conditions were designed such that this development could be simulated independent of moist processes. The outflow boundary is allowed to propagate into a region of southerly low-level flow which results in a vertical vortex sheet along the outflow’s leading edge. Lobe and cleft instabilities follow and provide perturbations for the subsequent development of horizontal shearing instabilities. These growing instabilities are the inaugural circulations of leading edge misocyclones with wavelengths ranging from 1.6 to 3.2 km. The structure of these modeled misocyclones compares favorably to observed pretornadic misocyclones along outflow boundaries in northeast Colorado. Vortex sheet dynamics are observed to exert substantial control over the structure of the evolving outflow leading edge. The vertical vortex sheet passes through discrete evoluti...

Thermodynamic Analysis of Supercell Rear-Flank Downdrafts from Project ANSWERS

Abstract Data collected during Project Analysis of the Near-Surface Wind and Environment along the Rear-flank of Supercells (ANSWERS) provided an opportunity to test recently published associations between rear-flank downdraft (RFD) thermodynamic characteristics and supercell tornadic activity on a set of 10 events from the northern plains. On average, RFDs associated with tornadic supercells had surface equivalent potential temperature and virtual potential temperature values only slightly lower than storm inflow values. RFDs associated with nontornadic supercells had mean group equivalent potential temperature and virtual potential temperature values that were colder relative to storm inflow values than their respective tornadic counterparts. Additionally, the analysis revealed that RFDs associated with tornadic supercells had higher CAPE and lower convective inhibition than the RFDs of nontornadic supercells, on average. The results of this study provide further support for the general concept that a t...

Near-Ground Pressure and Wind Measurements in Tornadoes*

Since the spring of 2002, tornadoes were sampled on nine occasions using Hardened In-Situ Tornado Pressure Recorder probes, video probes, and mobile mesonet instrumentation. This study describes pressure and, in some cases, velocity data obtained from these intercepts. In seven of these events, the intercepted tornadoes were within the radar-indicated or visually identified location of the supercell low-level mesocyclone. In the remaining two cases, the intercepted tornadoes occurred outside of this region and were located along either the rear-flank downdraft gust front or an internal rear-flank downdraft surge boundary. The pressure traces, sometimes augmented with videography, suggest that vortex structures ranged from single-cell to two-cell, quite similar to the swirl-ratio-dependent continuum of vortex structures shown in laboratory and numerical simulations. Although near-ground tornado observations are quite rare, the number of contemporary tornado measurements now available permits a comparative range of observed pressure deficits for a wide variety of tornado sizes and intensities to be presented.

The Numerical Simulation of Nonsupercell Tornadogenesis. Part II: Evolution of a Family of Tornadoes along a Weak Outflow Boundary

Abstract Nonsupercell tornadogenesis along a weak outflow boundary has been simulated using a three-dimensional moist convective cloud model. Thermodynamic conditions similar to those observed for nonsupercell tornado (NST) events of the High Plains were utilized in the model initialization. As the ensemble system of storm, outflow boundary, and leading edge vortices evolve, six distinct life cycle stages for the development and decay of NSTs are documented that span a period of about 35 min. Consistent with the results of Part I of this numerical study, vortex sheet dynamics exert considerable control over the outflow leading edge. The progression of pretornadic life cycle stages serves to concentrate vertical vorticity effectively along the outflow boundary in discrete misocyclone circulations aligned in a 3-km wavelength pattern. The organization of larger-scale misocyclones and ultimate intensification to initial tornadic intensity occurs coincident with the rapid development of deep convection overhe...

A Numerical Case Study of Convection Initiation along Colliding Convergence Boundaries in Northeast Colorado

Abstract A numerical cloud model has been used to simulate convective storm development on 17 July 1987 in northeast Colorado. The study involves the simulation of convergence along atmospheric boundaries and the subsequent development of convection. The model was initialized using observed conditions for this case day from the Convection Initiation and Downburst Experiment (CINDE). A two-dimensional version of the Clark NCAR nested grid model is employed. Results indicate that convection in boundary line collision cases can be successfully simulated by using actual observed atmospheric data. Gradual deepening of the moisture layer in the convergence zone was shown to destabilize the local atmosphere leading to the initiation of deep convection on this day. The modeled storm approximated the depth and intensity of the observed storms and displayed many of the features of the actual event. Sensitivity studies revealed that the timing and intensity of convection along boundaries is greatly affected by alter...

The Bowdle, South Dakota, Cyclic Tornadic Supercell of 22 May 2010: Surface Analysis of Rear-Flank Downdraft Evolution and Multiple Internal Surges

AbstractMobile mesonet sampling in the hook echo/rear-flank downdraft (RFD) region of a tornadic supercell near Bowdle, South Dakota, provided the opportunity to examine RFD thermodynamic and kinematic attributes and evolution. Focused analysis of the fifth low-level mesocyclone cycle that produced two significant tornadoes including a violent tornado, revealed four RFD internal surge (RFDIS) events. RFDISs appeared to influence tornado development, intensity, and demise by altering the thermodynamic and kinematic character of the RFD region bounding the pretornadic and tornadic circulations. Significant tornadoes developed and matured when the RFD, modulated by internal surges, was kinematically strong, only weakly negatively buoyant, and very potentially buoyant. In contrast, the demise of the Bowdle tornado was concurrent with a much cooler RFDIS that replaced more buoyant and far more potentially buoyant RFD air near the tornado. This surge also likely contributed to a displacement of the tornado from...

Analysis of Tornado-Induced Tree Fall Using Aerial Photography from the Joplin, Missouri, and Tuscaloosa–Birmingham, Alabama, Tornadoes of 2011

AbstractIn this study, aerial imagery of tornado damage is used to digitize the falling direction of trees (i.e., tree fall) along the 22 May 2011 Joplin, Missouri, and 27 April 2011 Tuscaloosa–Birmingham, Alabama, tornado tracks. Normalized mean patterns of observed tree fall from each tornado’s peak-intensity period are subjectively compared with results from analytical vortex simulations of idealized tornado-induced tree fall to characterize mean properties of the near-surface flow as depicted by the model. A computationally efficient method of simulating tree fall is applied that uses a Gumbel distribution of critical tree-falling wind speeds on the basis of the enhanced Fujita scale. Results from these simulations suggest that both tornadoes had strong radial near-surface winds. A few distinct tree-fall patterns are identified at various locations along the Tuscaloosa–Birmingham tornado track. Concentrated bands of intense tree fall, collocated with and aligned parallel to the axis of underlying vall...

The 19 April 1996 Illinois Tornado Outbreak. Part II: Cell Mergers and Associated Tornado Incidence

Abstract In the 19 April 1996 Illinois tornado outbreak, cell mergers played a very important role in the convective evolution. With a large number of cells forming within a short time period, the early stages of cell organization were marked by cell merger interactions and cell attrition that led to a pattern of isolated tornadic supercells as described in Part I of this study. Twenty-six mergers were documented and analyzed. Storm-rotation-induced differential cell propagation accounted for 58% of these 26 cell mergers while differing cell speeds prompted 27% of the mergers. Cell merger characterizations were utilized to describe the cell reflectivity coalescence morphology including aspects of new cell development, development along the periphery of an existing cell, or an upward pulse in the cell intensity of a dominant cell. In cases where the merging cells were of similar intensity, a rapidly developing cellular pulse “bridging” the two echoes was often observed. When the relationship between short-...

Surface Analysis near and within the Tipton, Kansas, Tornado on 29 May 2008

Abstract Data collected by a mesonet within the near-tornado environment and in the Tipton tornado on 29 May 2008 provided a rare opportunity to analyze rear-flank downdraft (RFD) outflow properties closely bounding a tornado and to characterize parcel thermodynamics being ingested into a tornado from the rear-flank downdraft. Parcels moving into the tornado on its right flank had very small negative buoyancy and considerable potential buoyancy. Measurements within and very near the tornado showed similar buoyancy characteristics to the storm inflow. Analyzed surface divergence and videographic evidence indicated that the RFD outflow just to the right and wrapping in front of the tornado was supported by parcels moving out of a narrow downdraft bordering the right flank of the tornado. Surface flow field analysis showed that parcels moved out of the downdraft-associated divergence region and into the right side of, as well as in front of, the tornado. An internal RFD surge boundary was positioned roughly ...

The 19 April 1996 Illinois Tornado Outbreak. Part I: Cell Evolution and Supercell Isolation

Abstract In this study of the 19 April 1996 Illinois tornado outbreak, 109 cells were tracked using radar data to understand the transition of the cell configuration from a considerable number of initial cells to a small subset of supercells after several hours of evolution. Of these 109 cells, 85 developed along three synoptic boundaries (dryline, warm front, and dryline–warm front occlusion) between 1940 and 2230 UTC. A large majority of these 85 cells formed in a 1-h period between 2040 and 2140 UTC. With a considerable number of cells initiating within a short time period, the early stages of cell organization were marked by cell merger interactions and cell attrition that led to a pattern of isolated tornadic supercells. Cell-type initiation analysis revealed that storms that would become supercells were initiated, on average, 17 min before nonsupercell storms. Cyclonic supercells, with mean storm life spans of 214 min, had much longer lives than nonsupercell storms. Anticyclonic supercells resulting...