Formal Publications:

Conference Papers:

Other Research:




Near-Ground Pressure and Wind Measurements in Tornadoes

Christopher D. Karstens, Timothy M. Samaras, Bruce D. Lee, William A. Gallus, Jr., and Catherine A. Finley

ABSTRACT

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.

Journal Article (AMS Link)




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

Bruce D. Lee, Catherine A. Finley, and Christopher D. Karstens

ABSTRACT

Mobile 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 pre-tornadic 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 the storm updraft. Development of the first tornado and rapid intensification of the Bowdle tornado occurred when a RFDIS boundary convergence zone interacted with the pre-tornadic and tornadic circulations, respectively. In the latter case, a strong vertical vortex sheet along a RFDIS boundary appeared to be a near-surface cyclonic vorticity source for the tornado. A downdraft closely bounding the right flank of the developing first tornado and intensifying Bowdle tornado provided some of the inflow to these circulations. For the Bowdle tornado, parcels were also streaming toward the tornado from its immediate east and northeast. A cyclonic-anticyclonic vortex couplet was observed during a portion of each significant tornado cycle.

Journal Article (AMS Link)




Analysis of Tornado-Induced Tree-Fall Using Aerial Photography from the Joplin, MO, and Tuscaloosa-Birmingham, AL, Tornadoes of 2011

Christopher D. Karstens, William A. Gallus, Jr., Bruce D. Lee, and Catherine A. Finley

ABSTRACT

In 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, MO, and 27 April 2011 Tuscaloosa-Birmingham, AL, tornado tracks. Normalized mean patterns of observed tree-fall from each tornado’s peak intensity period are subjectively compared to 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 based on the Enhanced Fujita scale. Results from these simulations suggest 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 valley channels, extend well beyond the primary damage path. These damage patterns are hypothesized to be the result of flow acceleration caused by channeling within valleys. Another distinct pattern of tree-fall, likely not linked to the underlying topography, may have been associated with a rear-flank downdraft (RFD) internal surge during the tornado’s intensification stage. Here, the wind field was strong enough to produce tornado-strength damage well beyond the visible funnel cloud. This made it difficult to distinguish between tornado- and RFD-related damage, and thus illustrates an ambiguity in ascertaining tornado damage path width in some locations.

Journal Article (AMS Link)




Understanding Terrain Impacts on Tornado Flow through Tree-fall Analysis of the Joplin and Tuscaloosa-Birmingham Tornadoes of 2011 and through Numerical and Laboratory Vortex Simulations

Christopher D. Karstens, William A. Gallus, Jr., Partha P. Sarkar, Bruce D. Lee, and Catherine A. Finley

ABSTRACT

Aerial imagery of tornado damage taken of the 22 May 2011 Joplin, MO and 27 April 2011 Tuscaloosa-Birmingham, AL tornado tracks was used to digitize the falling direction of trees (i.e., tree-fall) along each damage path. The digitized tree-fall was used to compute a normalized mean cross-section of observed tree-fall within the various life-cycle stages from each tornado. These mean patterns of tree-fall were subjectively compared to results from analytical vortex simulations of idealized tornado-induced tree-fall in an attempt to characterize mean properties of the near-surface flow as depicted by the model. A computationally efficient method of simulating tree-fall was also developed that uses a random distribution of critical tree-falling wind speeds based on the Enhanced Fujita scale. Results from these simulations suggest both tornadoes had a highly radial configuration of the near-surface wind field.

A few distinct tree-fall patterns were 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 valley channels, extended well beyond the primary damage path. These damage patterns are hypothesized to be the result of speed-up caused by channeling of the near-surface inflow within valleys. Another distinct pattern of tree-fall, likely not linked to the underlying topography, may have been associated with a rear-flank downdraft (RFD) internal surge during the tornado's intensification stage. Here the wind field was strong enough to produce tornado-strength damage well beyond the visible funnel cloud. This made it difficult to distinguish between tornado- and RFD-related damage, and thus illustrates an ambiguity in ascertaining tornado damage path width in some locations.

Laboratory experiments were also performed, using Iowa State University's Tornado and Microburst Simulator, to better understand topographically-induced effects on a translating tornado-like vortex. Simulations performed with idealized 2-D models of a ridge and an escarpment reveal that the vortex track deviates in a sinusoidal manner. The evolution of vortex structure is consistent with behaviors related to vortex stretching and compression through the conservation of potential vorticity in an inviscid, homogeneous fluid. These results are compared to observations of tornado tracks from the 27 April 2011 tornado outbreak that show similar patterns of track deflection when crossing a significant rise or fall in elevation. Additionally, a 3-D section of the Tuscaloosa-Birmingham, AL tornado track was constructed to investigate the hypothesized flow-channeling effect. Preliminary tests imply channeling could only occur if the tornado had primarily radial near-surface flow that would already be aligned with the valley.

Presentation (AMS Link)




Supplemental Damage Indicators Discovered in Recent Strong Tornadoes

Christopher D. Karstens, William A. Gallus, Jr., Partha P. Sarkar, and Timothy P. Marshall

ABSTRACT

As part of an effort to improve the understanding of near-surface winds in tornadoes, surveys of tornado damage were conducted following a few major tornado events that have occurred since 2010. In addition to assessing a wide range of standard EF-scale damage indicators (DIs), a few non-standard or supplemental DIs were identified, including:
  • Parking stops (Joplin, MO)
  • Manhole covers (Joplin, MO)
  • Storm grates (Joplin, MO)
  • Transmission line tower (Bowdle, SD)
  • Grain Bins (Mapleton, IA)
  • Antique-style light poles (Joplin, MO)
Documentation of each bulleted DI will be provided. Additionally, the track-relative locations of each DI were identified and compared to the EF-ratings of nearby standard EF-scale DIs, where possible. An estimation of minimum wind speed necessary to produce the observed degree of damage (DOD) was made, where possible, by using the weight and cross-sectional area of the DI as well as the estimated pressure coefficient as induced by the atmospheric pressure drop inside the tornado (using laboratory vortex measurements). For example, preliminary analysis of lofted parking stops offers support for the assigned EF-5 rating in the Joplin tornado. Additionally, the transmission line tower's observed DOD (collapsed and removed from original location) in the Bowdle, SD tornado exceeded the highest DOD provided in the EF-scale (collapsed tower). Recommendations for how to implement these supplemental DIs in the EF-scale will be provided.

Poster Available Upon Request




Damage Survey of the Tuscaloosa-Birmingham Tornado on 27 April 2011

Jim Stefkovich, Timothy P. Marshall, John DeBlock, James G. LaDue, and Christopher D. Karstens

ABSTRACT

On April 27, 2011 a large and destructive tornado traveled through Tuscaloosa, AL and continued northeastward through the northwest suburbs of Birmingham. The tornado killed 64 people and injured more than 1500. Overall, the tornado traveled 80.7 miles (130 km) and was rated high end EF-4 on the Enhanced Fujita Scale. This paper will present findings from our damage surveys and discuss the use of the EF-scale. It was found that many houses were swept clean from their foundations. However, these foundations offered little resistance against transferred wind loads as they were typically constructed with unreinforced, concrete masonry units (CMUs). Thus, there was no safe place for residents to seek appropriate shelter against such a violent tornado.

Manuscript (AMS Link)
Presentation (AMS Link)




Observation and Laboratory Experimentation of Tornadoes Translating Over Complex Topography

Christopher D. Karstens, William A. Gallus, Jr., and Partha P. Sarkar

ABSTRACT

Although most tornadoes documented in the U.S. occur in areas with nearly flat terrain, tornadoes do occur in regions where the terrain is quite undulating. Yet, the understanding of the effects of topography on tornado structure and intensity remains extremely limited. Many of the tornadoes occurring on 27 April 2011 tracked through regions with complex topography, particularly in Alabama and northwest Georgia on northward through parts of the Appalachians. Shortly after the event, aerial orthophotos were taken by NOAA's National Geodetic Survey along significant stretches of the damage paths. These photos offer an unprecedented opportunity to study tornado damage, especially in areas with significant tree canopy and/or rough terrain that would make a ground survey nearly impossible. In this study, the orthophoto imagery is combined with ground survey information, conducted by the lead author in early May, to geospatially relate variations in the patterns and intensity of tree damage to the underlying terrain using a Digital Elevation Model (DEM). Preliminary analysis has revealed some interesting behavior. For example, trees in low-lying areas or valleys show convergent fall patterns and intense damage is noted. Trees along ridge lines, however, are often damaged to a lesser extent compared to adjacent areas. Composites of examples such as these will be presented.

In addition, laboratory experiments were performed, using Iowa State University's Tornado and Microburst Simulator, to better understand terrain-induced effects on tornadoes. The simulator was configured to produce a low swirl- and high swirl-ratio tornado-like vortex that translates over idealized two-dimensional models of an escarpment and a hill. Preliminary results suggest that when translating to lower elevations, the vortex transitions toward a lower swirl-ratio configuration with a slightly narrower and more intense core. Measurements of surface pressure, three-dimensional winds, and derived swirl-ratios from these experiments are compared to control vortex simulations with a flat lower boundary, and to studies of straight-line boundary layer wind tunnel tests with similar topographic models. Information gained from the experiments is also compared to the observations of tree-fall patterns and varying damage intensity in regions where the modeled and observed terrain features are comparable.

Poster Available Upon Request




Technologies for Improving Operational Forecasting and Severe Storms Research

Christopher D. Karstens and Daryl Herzmann

ABSTRACT

This presentation discusses several projects undertaken in the last few years serving many different purposes within the operational forecasting and research communities. A short description of each project is given below. In addition to providing an overview of each topic, plans for future work and potential future projects are discussed.

1. National Bufkit Data Distribution/Archive and Meteogram Visualization Tool

Beginning 23 January 2009, generation of publicly accessible NAM, GFS, and RUC Bufkit profiles began for nearly 900 sites in the Contiguous U.S., and an archive of these data began on 30 December 2010. A web-based meteogram visualization tool was also created to display temperature, apparent temperature, dew point, wind speed and direction, precipitation, and accumulated snowfall meteograms from the aforementioned data sources. In addition, the meteogram plots include NAM MOS, GFS MOS, and the nearest ASOS observations, provided via the Iowa Environmental Mesonet (IEM), and National Weather Service (NWS) forecasts via the National Digital Forecast Database (NDFD). Discussion of post-processing techniques, implementation of empirically-based algorithms, and community usage and incorporation of constructive feedback is provided.

2. Interactive Radar/Warning Workshop

Starting in 2010, the annual radar/warning workshop held at the Central Iowa NWA Severe Storms and Doppler Radar conference transitioned from a presentation-style format to an interactive, competition format intended to be engaging and provide a constructive learning environment. The diverse group of attendees is now split into small teams, consisting of 5-10 people, tasked with issuing severe weather warnings from laptop computers running Gibson-Ridge radar software and a custom-designed warning generation tool. In addition to the laptop computers (and several accommodating power strips), core infrastructure includes a wireless controller, two wireless access points, and a central data server for distributing level-II radar files. The intent is to create a simulated real-time severe weather scenario. Discussion of component integration, workshop setup, case study and radar-site transposing, the warning generation tool, and scoring techniques is provided.

3. Data Acquisition and Real-time Display/Tracking Software for Project TWISTEX

The Tactical Weather Instrumented Sampling in/near Tornadoes EXperiment (TWISTEX) is a field project that collects surface observations near severe storms and tornadoes using mobile mesonet instrumentation mounted on four vehicles. The mesonet racks consist of a variety of instruments that use various protocols for acquisition (e.g., TCP, RS-232, USB, NMEA). Additionally, communication among team members is essential to ensure real-time coordination and to ensure proper vehicle spacing in adverse driving conditions. To fulfill these needs, a custom-designed program was written which includes the following components:

  • Mesonet and vehicle checklist
  • Mobile Chat Client
  • Data acquisition from multiple instruments simultaneously and creation of data logs
  • Real-time display of acquired data and automated monitoring
  • Generation and distribution of GIS placefiles for display in Gibson-Ridge radar software in real-time
  • GIS coordination tools (e.g., navigational waypoints, instrumentation waypoints, etc.)
  • Integration with SpotterNetwork.org for severe weather reporting.

An overview of this program and its usage during the past two severe storms seasons is discussed.

Poster (PDF)




Analysis of near-surface wind flow in close proximity to tornadoes

Christopher D. Karstens, Timothy M. Samaras, William A. Gallus, Jr., Catherine A. Finley, and Bruce D. Lee

ABSTRACT

Results from numerical tornado simulations indicate that variations in near-surface inflow have a profound impact on vortex structure and intensity, providing motivation to collect near-surface wind measurements in/near tornadoes. Since 2008, the Tactical Weather Instrumented Sampling in/near Tornadoes EXperiment (TWISTEX) has collected wind measurements in close proximity to at least five tornadoes using mobile mesonet stations, and in one of these events, an in situ instrument was deployed to make direct or near-direct wind measurements within the tornado core. These tornadoes were well documented, presenting an opportunity to analyze the near-surface wind flow near each tornado.

In each case, a detailed analysis of the tornado track, visual vortex structure, and vortex intensity will be presented. The detailed track records will be used to transform near-surface velocity measurements onto a tornado-relative grid, from which estimations of decomposed velocities (radial and tangential) will be made. Where possible, the temporal evolution of these decomposed velocities will be related to the visual vortex structure and intensity, which may provide some insight into the life cycle of these tornadoes.

Extended Abstract (PDF)




Mobile mesonet observations of the rear-flank downdraft evolution associated with a violent tornado near Bowdle, SD on 22 May 2010

Catherine A. Finley, Bruce D. Lee, Matthew Grzych, Christopher D. Karstens, and Timothy M. Samaras

ABSTRACT

There are many unresolved issues regarding tornadogenesis in supercells. One question that has received attention in the research community is the role of the rear-flank downdraft (RFD) and rear-flank downdraft gust front (RFDGF) in the tornadogenesis and maintenance process. TWISTEX (Tactical Weather-Instrumented Sampling in/near Tornadoes EXperiment) is an experiment designed to collect and analyze near-surface data in the RFD/RFDGF regions and in tornadoes.

On 22 May 2010, the TWISTEX mobile mesonet intercepted a tornadic supercell south of Mobridge, South Dakota. Teams followed the storm northeast as it cycled, collecting high-resolution data within multiple RFDs and associated RFDGFs over a 2 hour period. Data collected on this day includes an approximately 30 minute long RFD sampling period in close proximity to an EF-4 tornado which passed near Bowdle, SD.

An analysis of the high-resolution kinematic and thermodynamic data will be presented, and the implications for tornadogenesis and maintenance will be discussed, including:

  • The thermodynamic and kinematic characterization and time evolution of the RFD associated with the EF-4 Bowdle tornado, including internal RFD gradients and multiple kinematic surges within the RFD. Teams also sampled the RFD in the post-genesis but rapidly strengthening phase of this tornado.
  • A comparison of the thermodynamic and kinematic characteristics of various RFD surges throughout the first several hours of the storm's life. Other RFD samples from this storm included an initial non-tornadic RFD, and the RFDs associated with two other tornadic cycles both immediately prior to, and following the Bowdle tornado.

Extended Abstract (PDF)




Surface observations of the rear-flank downdraft evolution associated with the Aurora, NE tornado of 17 June 2009

Bruce D. Lee, Catherine A. Finley, Christopher D. Karstens, and Timothy M. Samaras

ABSTRACT

On 17 June 2009 a supercell with “classic” structure produced numerous tornadoes in the Grand Island and Aurora, NE area. The rear-flank downdraft (RFD) outflow and tornado proximity environment were sampled for extensive periods by a mobile mesonet deployed as part of the Tactical Weather-Instrumented Sampling in/near Tornadoes EXperiment (TWISTEX). Of particular interest, the mesonet collected a long-duration dataset within the RFD outflow and along/near the RFD gust front preceding and concurrent with the most prominent tornado episode from this supercell. Given generally slow storm motion, part of the mobile mesonet maintained roughly similar storm-relative positions through most of the tornado lifecycle (that lasted approximately 20 min), allowing the opportunity to examine RFD outflow evolution. There were at least two RFD internal surges with this event that visually appeared to coincide with tornadogenesis and tornado intensification, respectively. The mesonet sampled the first surge and perhaps part of the second surge. Data collected on the edge of the tornado from a Hardened In-Situ Tornado Pressure Recorder probe (which also records temperature and relative humidity) may allow a thermodynamic comparison between parcels being ingested into the tornado and parcels residing in the bounding RFD outflow. Detailed analysis of this event will be presented.

Extended Abstract (PDF)




Wind measurements within a tornado core

Timothy M. Samaras, Christopher D. Karstens, Bruce D. Lee, and Catherine A. Finley

ABSTRACT

A new multi-measurement In-situ instrument has been designed and fielded to collect wind speed data a three different heights up to a maximum height of 2 meters. This instrument is also capable of collecting accurate temperature and humidity measurements in the presence of high wind and debris by using a similar radiation shield and aspiration fan system originally described by Straka, et. al., (1996) for mobile mesonet operations.

The new instrument was successfully deployed on May 22, 2010 in the path of a developing EF4 tornado west of Bowdle, South Dakota where several wind velocity measurements were taken concurrently at differing heights from .7 meters to 2 meters. The result of these measurements will be presented and compared to previous attempted measurements in the past.




TWISTEX 2008: In Situ and Mobile Mesonet Observations of Tornadoes

Christopher D. Karstens and T. M. Samaras, A. Laubach, B. D. Lee, C. A. Finley, W. A. Gallus Jr., F. L. Haan, Jr.

ABSTRACT

In May and June of 2008, the Tactical Weather Instrumented Sampling in/near Tornadoes EXperiment (TWISTEX) project acquired several datasets in and near tornadic circulations, as well as within associated supercell rear-flank downdraft (RFD) surges. These environments were sampled using Hardened In-situ Tornado Pressure Recorder (HITPR) probes, photogrammetric probes, and mobile mesonet instrumentation.

Preliminary analysis of the datasets show at least three tornadic circulations well-removed from intense low-level mesocyclones passed across one or more mobile mesonet vehicles. The first, on May 10th, occurred near Broken Bow, OK where a linear transect of a developing tornadic circulation was measured. The second, on May 23rd near Quinter, KS, appeared to be a satellite tornado associated with an intense tornadic mesocyclone. The third, on May 29th, occurred near Tipton, KS where a small anti-cyclonic circulation was measured. In addition, a separate tornado near Tipton, KS passed directly across two HITPR probes and one photogrammetric probe deployed just ahead of it.

A complete summary of these datasets will be presented and related to analysis of video and damage. In addition, these datasets will be compared to turbulent vortex measurements obtained from the Iowa State University WiST tornado simulator as well as to prior results from laboratory and numerical simulations.

Extended Abstract (PDF)

Poster (PDF)




Simulations of Near-Ground Hurricane Winds Influenced by Built Structures

Christopher D. Karstens and W. A. Gallus Jr.

ABSTRACT

In this study efforts are made to improve understanding, and potentially forecasting, of near-ground hurricane winds as they interact with built structures and environments. Major landfalling hurricanes from the 2004 and 2005 seasons are simulated using a 4-km grid spacing version of the Weather Research and Forecasting (WRF) model initialized using GFS analyses. Preliminary results suggest the model's depiction of hurricane intensity is underpredicted when initialized with GFS output. A simulation of Hurricane Rita using this configuration made landfall with a central minimum pressure of 952 mb, while Hurricane Rita's measured central minimum pressure was 937 mb at landfall. As a result, wind speeds generated by WRF are less than the observed. In an attempt to improve upon these simulations, WRF Three-Dimensional Variational Data Assimilation (3DVAR) will be used to assimilate observations taken near the model initialization time.

Upon landfall, vertical wind profiles will be extracted from WRF model output near coastal regions that are encompassed by the region of highest forecasted wind speeds. These profiles will be used to initialize a three-dimensional wind tunnel domain in Fluent, a computational fluid dynamics (CFD) solver for numerically simulated complex flows. Contained within the wind tunnel are several structure configurations of interest, including a single building, parallel buildings, a suburban array, and a city environment. The wind tunnel and built structures can be created using the Fluent preprocessor, Gambit, while Fluent is used to resolve the microscale flow patterns. To simulate the effect of time which would result in changing wind directions, the structures will be rotated within the tunnel domain. Distributions of wind velocities at various elevations within close proximity of the structures will be presented.

Extended Abstract (PDF)

Poster (PDF)




Boundary-layer Decoupling Affects on Tornadoes

Christopher D. Karstens

Class Project: Mt 507

ABSTRACT

The North American low-level jet is known to have substantial impacts on the climatology of central and eastern regions of the United States. However, the development of the low-level jet may also have an impact on the low-level shear, which could rapidly alter an environment toward becoming more favorable for producing tornadoes. This study investigates this mechanism's contribution to enhancing low-level shear by analyzing the historical tornado records and through a case study analysis.

Extended Abstract (PDF)

Presentation (PDF)




A Comparison of Mid-level Frontogenesis to Radar-Indicated Heavy Snowbands

Christopher D. Karstens

Mentor: William A. Gallus Jr.

ABSTRACT

Frontogenesis and conditional symmetric instability have been researched extensively, becoming widely used as a means of predicting small scale heavy snow events. The secondary ageostrophic circulation induced by frontogenesis has been shown to produce narrow bands of precipitation, sustained for periods of time by the continual release of moist symmetric instability. This study analyzes six levels of frontogenesis to determine if a specific level is consistently in close proximity to the radar-indicated snowband detected at the same time. The closest level for three cases was examined in greater detail to emphasize a relationship between frontogenesis and moist symmetric instability. Results show frontogenesis in the 700mb to 750mb layer was close to the snowband location within reasonable accuracy, but is shown to vary. In addition, two conceptual models of frontogenesis in relation to saturated equivalent potential vorticity are verified.

Senior Thesis (PDF)

Presentation (PDF)


Updated 29 June 2012