Joshua Wurman

Design and Deployment of a Portable, Pencil-Beam, Pulsed, 3-cm Doppler Radar

Abstract A portable, pencil-beam, pulsed, Doppler, 3-cm wavelength radar has been constructed to study a wide variety of meteorological phenomena including tornadoes, severe storms, and boundary layer processes. The new radar, the Doppler on Wheels (DOW), has full scanning capability, a real-time display and archiving, and is mounted on a truck for easy portability and full mobility. This portability allows the radar to be brought to within a kilometer of rare meteorological phenomena. At this range, the pencil beam of the radar is very narrow, permitting significantly higher-resolution measurements (at 3-km range, 64 m × 64 m × 75 m) than are usually possible with stationary or airborne systems. The radar employs a new high-powered, PC-based, digital intermediate frequency (IF) data acquisition scheme called the PIRAQ. The radar has successfully collected data in several tornadoes and tornadic storms and has been used to detect dust devils and other boundary layer structures. The sensitivity and mobility...

Finescale Radar Observations of the Dimmitt, Texas (2 June 1995), Tornado

Abstract The mature and dissipating stages of a strong tornado were observed from close range by the prototype Doppler On Wheels mobile radar. Volumetric observations repeated eight times over an 840-s period with resolution volumes at the center of the tornado as low as 61 m × 61 m × 75 m = 2.8 × 105 m3 revealed new details about three-dimensional tornado vortex structure and evolution. Observed structures included a conical debris envelope, a low-reflectivity eye, multiple windfield maxima, and multiple semiconcentric bands of reflectivity surrounding the eye. The three-dimensional structure of the debris and single-Doppler wind field were well characterized, as well as more rapid dissipation of the tornado aloft compared to near the ground. Volumetric measures of tornado strength are introduced. A downdraft exhibiting w ∼ −30 m s−1, indicative of a partial two-cell vortex, was observed only during the earliest radar scans when the tornado was near maximum intensity. Comparisons with simple conceptual m...

The 30 May 1998 Spencer, South Dakota, Storm. Part II: Comparison of Observed Damage and Radar-Derived Winds in the Tornadoes

Abstract A violent supercell tornado passed through the town of Spencer, South Dakota, on the evening of 30 May 1998 producing large gradients in damage severity. The tornado was rated at F4 intensity by damage survey teams. A Doppler On Wheels (DOW) mobile radar followed this tornado and observed the tornado at ranges between 1.7 and 8.0 km during various stages of the tornados life. The DOW was deployed less than 4.0 km from the town of Spencer between 0134 and 0145 UTC, and during this time period, the tornado passed through Spencer, and peak Doppler velocity measurements exceeded 100 m s−1. Data gathered from the DOW during this time period contained high spatial resolution sample volumes of approximately 34 m × 34 m × 37 m along with frequent volume updates every 45–50 s. The high-resolution Doppler velocity data gathered from low-level elevation scans, when sample volumes are between 20 and 40 m AGL, are compared to extensive ground and aerial damage surveys performed by the National Weather Servic...

Diagnosed Three-Dimensional Axisymmetric Structure of the Mulhall Tornado on 3 May 1999

On 3 May 1999, an unusually large tornado that caused F4-level damage and killed several people was intercepted by the Doppler on Wheels (DOW) mobile radar near Mulhall, Oklahoma, from a range of 4 to 9 km, resulting in high-resolution volumetric data every 55 s up to 1.5-km altitude over a period of 14 min. For the first time, the evolution and three-dimensional structure of a tornado were deduced using the ground-based velocity track display (GBVTD) technique. After the circulation center was determined, the tangential wind and radial wind were derived from the GBVTD technique at each radius and height. In addition, the axisymmetric vertical velocity, angular momentum, vorticity, and perturbation pressure were deduced from the tangential and radial wind fields. This study focuses on the axisymmetric aspects of this tornado. The primary circulation of the Mulhall tornado consisted of an 84 m s 1 peak axisymmetric tangential wind with the radius of maximum wind (RMW) ranging from 500 to 1000 m. The secondary circulation exhibited a two-cell structure characterized by a central downdraft surrounded by an annular updraft near the RMW. The calculated maximum pressure deficit from a 3-km radius to the tornado center at 50-m altitude was 80 hPa. The maximum vorticity during the first 8 min of observation was located inside the RMW away from the tornado center. This vorticity profile satisfied the necessary condition of barotropic instability. As the tornado weakened afterward, the vorticity monotonically increased toward the center. The computed swirl ratios were between 2 and 6, consistent with the observed multiple vortex radar signatures and the vorticity pattern. Swirl ratios were generally smaller during the weakening phase.

Centrifuging of Hydrometeors and Debris in Tornadoes: Radar-Reflectivity Patterns and Wind-Measurement Errors

High-resolution Doppler radar observations of tornadoes reveal a distinctive tornado-scale signature with the following properties: a reflectivity minimum aloft inside the tornado core (described previously as an “eye”), a high-reflectivity tube aloft that is slightly wider than the tornado core, and a tapering of this high-reflectivity tube near the ground. The results of simple one-dimensional and two-dimensional models demonstrate how these characteristics develop. Important processes in the models include centrifugal ejection of hydrometeors and/or debris by the rotating flow and recycling of some objects by the nearsurface inflow and updraft. Doppler radars sample the motion of objects within the tornado rather than the actual airflow. Since objects move at different speeds and along different trajectories than the air, error is introduced into kinematic analyses of tornadoes based on radar observations. In a steady, axisymmetric tornado, objects move outward relative to the air and move more slowly than the air in the tangential direction; in addition, the vertical air-relative speed of an object is less than it is in still air. The differences between air motion and object motion are greater for objects with greater characteristic fall speeds (i.e., larger, denser objects) and can have magnitudes of tens of meters per second. Estimates of these differences for specified object and tornado characteristics can be obtained from an approximation of the one-dimensional model. Doppler On Wheels observations of the 30 May 1998 Spencer, South Dakota, tornado demonstrate how the apparent tornado structure can change when the radar-scatterer type changes. When the Spencer tornado entered the town and started lofting debris, changes occurred in the Doppler velocity and reflectivity fields that are consistent with an increase in mean scatterer size.

The Multiple-Vortex Structure of a Tornado

Abstract The structure and behavior of multiple subtornadic-scale vortices in a tornado were examined and were compared with laboratory, conceptual, and numerical models. Unique radar observations of an exceptionally large and violent tornado obtained with a Doppler on Wheels mobile radar on 3 May 1999 in northern Oklahoma provided the opportunity, for the first time ever with quantitative radar measurements, to characterize the size, strength, motion, horizontal and vertical structure, and persistence of multiple vortices in a tornado. Doppler velocity, received power, and spectral-width data were used to study the vortices. The structures of the multiple subtornadic-scale vortices were similar to that of tornadic vortices in certain respects. They exhibited doughnut-shaped received power maxima and/or hooks surrounding comparatively clear central eyes. Doppler velocity differences across the vortices decreased with height. However, the vortices exhibited intense small-scale shears at their centers that ...

The Second Verification of the Origins of Rotation in Tornadoes Experiment: VORTEX2

The second Verification of the Origins of Rotation in Tornadoes Experiment (VORTEX2), which had its field phases in May and June of 2009 and 2010, was designed to explore i) the physical processes of tornadogenesis, maintenance, and demise; ii) the relationships among tornadoes, tornadic storms, and the larger-scale environment; iii) numerical weather prediction and forecasting of supercell thunderstorms and tornadoes; and iv) the wind field near the ground in tornadoes. VORTEX2 is by far the largest and most ambitious observational and modeling study of tornadoes and tornadic storms ever undertaken. It employed 13 mobile mesonet–instrumented vehicles, 11 ground-based mobile radars (several of which had dual-polarization capability and two of which were phased-array rapid scan), a mobile Doppler lidar, four mobile balloon sounding systems, 42 deployable in situ observational weather stations, an unmanned aerial system, video and photogrammetric teams, damage survey teams, deployable disdrometers, and othe...

Low-Level Winds in Tornadoes and Potential Catastrophic Tornado Impacts in Urban Areas

Using an axisymmetric model of tornado structure tightly constrained by high-resolution wind field measurements collected by Doppler on Wheels (DOW) mobile radars, the potential impacts of intense tornadoes crossing densely populated urban areas are evaluated. DOW radar measurements combined with in situ low-level wind measurements permit the quantification of low-level tornadic winds that would impact structures. Axisymmetric modeled wind fields from actual and hypothetical tornadoes are simulated to impact high-density residential and commercial districts of several major cities. U.S. census block data, satellite imagery, and other sources are used to characterize and count the number of structures impacted by intense winds, up to 132 m s−1, and estimate the level and cost of resulting damage. Census data are used to estimate residential occupancy and human casualties. Results indicate that a large and intense tornado crossing through residential portions of Chicago, Illinois, could result in tragic con...

Dual-doppler and single-doppler analysis of a tornadic storm undergoing mergers and repeated tornadogenesis

Abstract Dual-Doppler observations with unprecedented finescale spatial and temporal resolution are used to characterize the vector wind field in and near a tornado occurring near Kiefer, Oklahoma, on 26 May 1997. Analyses of the dual-Doppler vector wind fields document in detail, for the first time, several structures associated with the tornado: a proximate updraft region, a rear-flank downdraft wrapping around the tornado, a double gust front structure occluding near the tornado, and a region of enhanced vorticity separated from the tornado that may have been associated with cyclic tornadogenesis. The analyses are compared to conceptual and computer models of tornadic storms. A subsequent tornadogenesis was observed with radar every 18 s, providing a finescale temporal view of the genesis process. The genesis process was complex and the evolution of tornado intensity parameters was not monotonic in time. Low-level rotation contracted and intensified, then broadened, then contracted and intensified a se...

Tornado Maintenance Investigated with High-Resolution Dual-Doppler and EnKF Analysis

AbstractDual-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, particu...