Verification Of The Origins Of Rotation In Tornadoes Experiment Research Articles

Observed Bulk Hook Echo Drop Size Distribution Evolution in Supercell Tornadogenesis and Tornadogenesis Failure

AbstractThe time preceding supercell tornadogenesis and tornadogenesis “failure” has been studied extensively to identify differing attributes related to tornado production or lack thereof. Studies from the Verification of the Origins of Rotation in Tornadoes Experiment (VORTEX) found that air in the rear-flank downdraft (RFD) regions of non- and weakly tornadic supercells had different near-surface thermodynamic characteristics than that in strongly tornadic supercells. Subsequently, it was proposed that microphysical processes are likely to have an impact on the resulting thermodynamics of the near-surface RFD region. One way to view proxies to microphysical features, namely drop size distributions (DSDs), is through use of polarimetric radar data. Studies from the second VORTEX used data from dual-polarization radars to provide evidence of different DSDs in the hook echoes of tornadic and non-tornadic supercells. However, radar-based studies during these projects were limited to a small number of cases preventing result generalizations. This study compiles 68 tornadic and 62 non-tornadic supercells using Weather Surveillance Radar–1988 Doppler (WSR-88D) data to analyze changes in polarimetric radar variables leading up to, and at, tornadogenesis and tornadogenesis failure. Case types generally did not show notable hook echo differences in variables between sets, but did show spatial hook echo quadrant DSD differences. Consistent with past studies, differential radar reflectivity factor (ZDR) generally decreased leading up to tornadogenesis and tornadogenesis failure; in both sets, estimated total number concentration increased during the same times. Relationships between DSDs and the near-storm environment, and implications of results for nowcasting tornadogenesis, also are discussed.

Progress toward Characterization of the Atmospheric Boundary Layer over Northern Alabama Using Observations by a Vertically Pointing, S-Band Profiling Radar during VORTEX-Southeast

AbstractDuring spring 2016 and spring 2017, a vertically pointing, S-band Frequency Modulated Continuous Wave radar (UMass FMCW) was deployed in northern Alabama under the auspices of the Verification of the Origins of Rotation in Tornadoes Experiment (VORTEX)-Southeast. In total, ~14 weeks of data were collected, in conditions ranging from quiescent clear skies to severe thunderstorms. The principal objective of these deployments was to characterize the boundary layer evolution near the VORTEX-Southeast domain. In this paper, we describe intermediate results in service of this objective. Specifically, we describe updates to the UMass FMCW system, document its deployments for VORTEX-Southeast, and apply four automated algorithms: 1) a dealiasing algorithm to the Doppler velocities, 2) a fuzzy logic scatterer classification scheme to separate precipitation from nonprecipitation observations, 3) a brightband/melting-layer identification algorithm for stratiform precipitation, and 4) an extended Kalman filter–based convective boundary layer depth (mixing height) measurement algorithm for nonprecipitation observations. Results from the latter two applications are qualitatively verified against retrieved soundings from a collocated thermodynamic profiling system.

A Solo-Based Automated Quality Control Algorithm for Airborne Tail Doppler Radar Data

AbstractAn automated quality control preprocessing algorithm for removing nonweather radar echoes in airborne Doppler radar data has been developed. This algorithm can significantly reduce the time and experience level required for interactive radar data editing prior to dual-Doppler wind synthesis or data assimilation. The algorithm uses the editing functions in the Solo software package developed by the National Center for Atmospheric Research to remove noise, Earth-surface, sidelobe, second-trip, and other artifacts. The characteristics of these nonweather radar returns, the algorithm to identify and remove them, and the impacts of applying different threshold levels on wind retrievals are presented. Verification was performed by comparison with published Electra Doppler Radar (ELDORA) datasets that were interactively edited by different experienced radar meteorologists. Four cases consisting primarily of convective echoes from the Verification of the Origins of Rotation in Tornadoes Experiment (VORTEX), Bow Echo and Mesoscale Convective Vortex Experiment (BAMEX), Hurricane Rainband and Intensity Change Experiment (RAINEX), and The Observing System Research and Predictability Experiment (THORPEX) Pacific Asian Regional Campaign (T-PARC)/Tropical Cyclone Structure-2008 (TCS08) field experiments were used to test the algorithm using three threshold levels for data removal. The algorithm removes 80%, 90%, or 95% of the nonweather returns and retains 95%, 90%, or 85% of the weather returns on average at the low-, medium-, and high-threshold levels. Increasing the threshold level removes more nonweather echoes at the expense of also removing more weather echoes. The low threshold is recommended when weather retention is the highest priority, and the high threshold is recommended when nonweather removal is the highest priority. The medium threshold is a good compromise between these two priorities and is recommended for general use. Dual-Doppler wind retrievals using the automatically edited data compare well to retrievals from interactively edited data.

Interactions between a Supercell and a Quasi-Stationary Frontal Boundary

Abstract The case presented here is submitted as an example of a previously undocumented type of interaction between a supercell thunderstorm and a frontal boundary. During the afternoon of 8 June 1995, a supercell thunderstorm formed near a quasi-stationary frontal boundary and then moved northeast across Beaver County in the Oklahoma Panhandle. Its motion took it away from the boundary and deeper into the cool air. As the storm matured and strengthened, a portion of the boundary to the south of the supercell moved northward and briefly became entrained in the low-level circulation of the storm. This northward advance of the boundary was subsequently followed by a southward motion back to near its original location. High-density spatial and temporal observations from the Oklahoma Mesonet and the Verification of the Origins of Rotation in Tornadoes Experiment (VORTEX) Mobile Mesonetwork are presented to document the northward advance of the boundary into the supercell circulation.

Vortex Lines within Low-Level Mesocyclones Obtained from Pseudo-Dual-Doppler Radar Observations

Abstract Vortex lines passing through the low-level mesocyclone regions of six supercell thunderstorms (three nontornadic, three tornadic) are computed from pseudo-dual-Doppler airborne radar observations obtained during the Verification of the Origins of Rotation in Tornadoes Experiment (VORTEX). In every case, at least some of the vortex lines emanating from the low-level mesocyclones form arches, that is, they extend vertically from the cyclonic vorticity maximum, then turn horizontally (usually toward the south or southwest) and descend into a broad region of anticyclonic vertical vorticity. This region of anticyclonic vorticity is the same one that has been observed almost invariably to accompany the cyclonic vorticity maximum associated with the low-level mesocyclone; the vorticity couplet straddles the hook echo of the supercell thunderstorm. The arching of the vortex lines and the orientation of the vorticity vector along the vortex line arches, compared to the orientation of the ambient (barotropic) vorticity, are strongly suggestive of baroclinic vorticity generation within the hook echo and associated rear-flank downdraft region of the supercells, and subsequent lifting of the baroclinically altered/generated vortex lines by an updraft. Discussion on the generality of these findings, possible implications for tornadogenesis, and the similarity of the observed vortex lines to vortex lines in larger-scale convective systems are included as well.

A comparison of the midlevel kinematic characteristics of a pair of supercell thunderstorms observed by airborne Doppler radar

The midlevel kinematic characteristics of two supercell thunderstorms observed during the Verification of the Origins of Rotation in Tornadoes Experiment (VORTEX), one tornadic and the other nontornadic, are compared using airborne pseudo-dual-Doppler wind retrievals. The most significant difference between the two cases was that a midlevel rear-inflow jet was observed in the nontornadic supercell (12 May 1995), whereas such rear-to-front, storm-relative flow was absent at midlevels in the tornadic supercell (16 May 1995). The midlevel jet in the nontornadic supercell was located on the north flank of a prominent region of anticyclonic vertical vorticity. Some speculations are provided pertaining to the possible importance of the midlevel kinematic differences between the two storms.

Evolution of Low-Level Angular Momentum in the 2 June 1995 Dimmitt, Texas, Tornado Cyclone

Abstract The life cycle of the 2 June 1995 Dimmitt, Texas, tornado cyclone, observed during the Verification of the Origins of Rotation in Tornadoes Experiment (VORTEX), is described. The tornado cyclone here is defined as a significantly axisymmetric flow larger than the visible tornado and characterized by increasing angular momentum with increasing radius. Its life cycle included three phases with somewhat differing evolution of angular momentum, herein called intensifying, transition, and weakening. During the intensifying stage, the funnel and debris cloud gradually increased in size. The azimuthally averaged secondary circulation of the larger-scale tornado cyclone, as determined using high-resolution single-Doppler data obtained by a mobile radar, was primarily inward and upward, consistent with the presence of a wall cloud outside the tornado. The azimuthally averaged angular momentum increased monotonically away from the tornado, so inward advection allowed the angular momentum to increase slowly with time in part of the tornado cyclone. During the transition phase, downdrafts began to occur within the tornado cyclone. The transport of angular momentum by the secondary circulation nearly was offset by eddy flux convergence of angular momentum so that the azimuthally averaged angular momentum tendency was only weakly negative at most radii. The tornado was visually impressive during this stage, featuring a 400-m diameter debris cloud extending to cloud base, while the surrounding wall cloud shrank and eroded. During the weakening phase, the funnel and debris cloud gradually shrank, and the funnel went through a rope stage prior to disappearing. The weakening phase was characterized by extensive downdrafts at all radii outside the tornado, and large-scale near-ground outflow as observed by mobile mesonet systems in a portion of the tornado cyclone. The secondary circulation acted to transport smaller angular momentum downward from aloft, and outward along the ground. All terms of the angular momentum budget became negative throughout most of the low-level (0–800-m AGL) tornado cyclone during the weakening phase. Several hypotheses for this evolution are evaluated, including changes in water loading in the tornado cyclone, cooling of the near-ground air, and the distribution of tangential velocity with height with its concomitant influence on the nonhydrostatic vertical pressure gradient force.

A Preliminary Survey of Rear-Flank Descending Reflectivity Cores in Supercell Storms

Abstract This paper develops a definition of a supercell reflectivity feature called the descending reflectivity core (DRC). This is a reflectivity maximum pendant from the rear side of an echo overhang above a supercell weak-echo region. Examples of supercells with and without DRCs are presented from two days during the Verification of the Origins of Rotation in Tornadoes Experiment (VORTEX), as well as one day with tornadic high-precipitation supercell storms in central Kansas. It was found that in all cases, tornado formation was preceded by the descent of a DRC. However, the sample reported herein is much too small to allow conclusions regarding the overall frequency of DRC occurrence in supercells, or the frequency with which DRCs precede tornado formation. Although further research needs to be done to establish climatological frequencies, the apparent relationship observed between DRCs and impending tornado formation in several supercells is important enough to warrant publication of preliminary findings.

Surface In Situ Observations within the Outflow of Forward-Flank Downdrafts of Supercell Thunderstorms

Abstract In the long-standing conceptual model of a supercell thunderstorm, the forward-flank downdraft (FFD) and its associated negative buoyancy originate from precipitation loading and the latent chilling of air due to the melting and evaporation of precipitation. The horizontal buoyancy gradient within the outflow of the FFD has been identified as an important source of low-level, streamwise vorticity in three-dimensional numerical simulations of supercells. These simulations have demonstrated that the formation of low-level mesocyclones is critically dependent on the baroclinic generation of horizontal vorticity within the FFD outflow. Despite the implied dynamical importance of the FFD outflow in the evolution of supercell thunderstorms, only a very limited number of thermodynamic observations have been obtained within FFD outflow. The range of thermodynamic conditions within FFD outflow is not well known, nor is it known whether any systematic relationship exists between the thermodynamic characteristics of FFD outflow and the intensity of low-level mesocyclones and/or tornadogenesis. In this paper, in situ observations obtained at the ground by a mobile mesonet within FFD outflow are used to investigate whether any relationship exists between the thermodynamic characteristics of the outflow and low-level mesocyclogenesis and/or tornadogenesis. The data were obtained within both tornadic and nontornadic supercells (12 cases total) during the Verification of the Origins of Rotation in Tornadoes Experiment (VORTEX) from 1994 to 1995, and in smaller field campaigns during the 1997–99 period.

The 8 June 1995 McLean, Texas, Storm. Part II: Cyclic Tornado Formation, Maintenance, and Dissipation

Abstract On 8 June 1995, scientists participating in the Verification of the Origins of Rotation in Tornadoes Experiment (VORTEX) collected a unique dataset with the Electra Doppler Radar (ELDORA). The ELDORA observations document the sequential life cycles of storm-scale circulations associated with three large tornadoes in a supercell thunderstorm near McLean, Texas. A qualitative description of the evolution of the storm was provided in Part I of this paper. During the first stage of development of each storm-scale circulation, interaction of the updraft with the environmental low-level horizontal vorticity produced a vorticity column that increased in intensity with height. As the vortex matured, vorticity increased greatly at low levels (i.e., below 2 km AGL) and exceeded that aloft. Each tornadic vortex was located near the rear side of the updraft, where the surrounding low-level horizontal vorticity was modified locally, most likely by weak baroclinity within the storm. Tilting of low-level horizo...

The Suppression of Deep Moist Convection near the Southern Great Plains Dryline

Abstract Deep moist convection failed to initiate over the Texas Panhandle on 6 May 1995 despite expectations to the contrary by the forecasters for the Verification of the Origins of Rotation in Tornadoes Experiment (VORTEX). The National Centers for Environmental Prediction–National Center for Atmospheric Research (NCEP–NCAR) reanalysis is used to identify a midlevel migratory short-wave ridge propagating across the area during the time of maximum diurnal heating. Subsidence values of 1.0 × 10−3 hPa s−1 for a period exceeding several hours were associated with the ridge and led to substantial (∼3 K) warming in the midlevels and to the formation of a strong inversion around 600 hPa. A number of independent observations are examined to corroborate the hypothesis that the passage of the midlevel short-wave ridge at the time of maximum diurnal heating is ultimately responsible for the absence of deep moist convection. Twelve-hour forecasts of the 1995 NCEP operational model suite and the Pennsylvania State ...

Airborne Pseudo–Dual Doppler Analysis of a Dryline–Outflow Boundary Intersection

Abstract On 3 June 1995, as part of the Verification of the Origins of Rotation in Tornadoes Experiment (VORTEX), the Electra Doppler Radar (ELDORA) onboard the National Center for Atmospheric Research (NCAR) Electra aircraft made possible a high-resolution examination of clear-air motions in the Texas panhandle in and around the intersection of the dryline and a surface baroclinic boundary, a location commonly referred to as the “triple point.” The ELDORA observations, as well as conclusions drawn from analyses of these data, are presented and discussed. A transverse secondary circulation associated with the dryline is visualized through analyses of the ELDORA data. Typical values of rising and sinking air are found to be 2 m s−1 and 2–3 m s−1, respectively. These vertical velocities are approximately the same as those indicated by in situ data collected onboard the Electra. Because the maximum in rising motion is found at the western edge of the dewpoint gradient and because the low-level relative airfl...

The 8 June 1995 McLean, Texas, Storm. Part I: Observations of Cyclic Tornadogenesis

Abstract On 8 June 1995 scientists participating in the Verification of the Origins of Rotation in Tornadoes Experiment (VORTEX) collected airborne Doppler radar data in a storm that produced a family of tornadoes near McLean, Texas. The Electra Doppler Radar (ELDORA) scanned three significant tornadoes during their formative and mature stages; one of the tornadoes was of F4/F5 intensity. Evidence from pseudo-dual-Doppler analyses of the ELDORA data reveals a process of cyclic tornado formation qualitatively similar to that depicted in previous conceptual models. In particular, the rear-flank gust front appears to play a major role in determining the location of the next vortex in the series. When a tornado forms, a small region (3–5 km wide) of outflow surges ahead of the tornado, producing a local bulge in the gust front. A new vorticity maximum may form near the leading edge of the outflow. In contrast to what is suggested by earlier conceptual models, intersection of the rear-flank gust front with a w...

The Interaction of a Pacific Cold Front with Shallow Air Masses East of the Rocky Mountains

Abstract The study presented here describes the interactions that occurred between an advancing Pacific cold front and shallow Gulf of Mexico and Arctic air masses situated east of the Rocky Mountains during the Verification of the Origins of Rotation in Tornadoes Experiment (VORTEX) field campaign on 17–18 April over Oklahoma and adjacent states. These interactions were driven largely by the complex topography of the region. Four air masses of distinctly different origin (i.e., Pacific polar, high-altitude continental, Gulf of Mexico, and Arctic), and the boundaries that separated them (i.e., Pacific cold front, dryline, and Arctic front), were observed within the experimental domain. This event produced more than $1 million worth of damage in the experimental domain due to severe weather. A dense network of ground-based in situ and remote observing systems and two research aircraft equipped with in situ sensors and Doppler radars gathered data that allowed the authors to document the passage of a vigoro...

The Garden City, Kansas, Storm during VORTEX 95. Part II: The Wall Cloud and Tornado

Abstract On 16 May 1995, a supercell storm produced an F1 tornado near Garden City, Kansas, during VORTEX (Verification of the Origins of Rotation in Tornadoes Experiment). This event provided the first opportunity to synthesize data collected by a new airborne radar platform called ELDORA (Electra Doppler radar) developed by the National Center for Atmospheric Research. The evolution of the low- and midlevel mesocyclone was presented in a companion paper. In this paper, the sequence of events that triggered tornadogenesis within the mesocyclone circulation is shown. The occlusion downdraft, documented in Part I, appears to promote “vortex breakdown” resulting in the generation of multiple vorticity centers. It is one of these centers that intensifies into the Garden City tornado. The structural relationship between the observed wall cloud and the radar reflectivity associated with the developing hook echo and the kinematic wind field is also presented. Although the wall cloud appears to be uniform there ...

Doppler Radar Analysis of the Northfield, Texas, Tornado of 25 May 1994

Abstract A large tornado was observed near Northfield, Texas, on 25 May 1994 during the Verification of the Origins of Rotation in Tornadoes Experiment (VORTEX). An analysis of the tornado and its parent storm is discussed. Doppler wind velocity spectra of the tornado and its parent circulation, which were computed from data collected by a low-power, portable, FM-CW (frequency-modulated continuous-wave), 3-cm-wavelength Doppler radar, are presented at increments in the range of 78 m. The FM-CW radar data from the tornado are the first ever collected of high enough quality to analyze. The CW spectra computed from data collected by the portable radar, a pseudo-dual-Doppler analysis of airborne Doppler radar data collected by a National Oceanic and Atmospheric Administration P-3 aircraft, photogrammetric analysis of a video of the tornado, and a ground-based damage survey are discussed in the context of the FM-CW spectra. This study is unique in that both ground-based and airborne Doppler radar systems probe...

A Mobile Mesonet for Finescale Meteorological Observations

Abstract A mobile weather observing system (mobile mesonet) was designed to augment existing meteorological networks in the study of severe local storms and other mesoscale weather phenomena in conjunction with the Verification of the Origins of Rotation in Tornadoes Experiment (VORTEX). Fifteen mobile mesonet units were built, each consisting of meteorological instruments mounted on standard automobiles. for high temporal and spatial resolution observations. While the most accurate measurements are possible from stationary mobile mesonet vehicles, accurate observations also are possible from moving vehicles. The mobile mesonet instruments measure pressure (600–1100 mb), temperature (−33° to 48°C), relative humidity (0%–100%), and wind direction and speed (0°–360° and 0–60 m s−1). Onboard each vehicle, a Global Positioning System (GPS) receiver and a flux-gate compass obtain universal time, vehicle location (latitude, longitude, altitude), and vehicle heading and speed. A standard laptop computer stores d...

ELDORA Observations during VORTEX 95

Airborne radar platforms have played an increasingly important role in advancing our understanding of storms that are either too remote or occur too infrequently for ground-based radars. Recognizing this potential, the National Center for Atmospheric Research and the Centre de Recherche en Physique de l'Environnment Terrestre et Planetaire have designed and developed ELDORA (Electra Doppler Radar), a tail Doppler radar with the following unique capabilities: 1) increased accuracy and sensitivity by averaging more independent samples in the radar pulse volume, 2) higher spatial resolution in the along-track direction by using a faster rotation rate of the antenna, and 3) a large unambiguous velocity measuring interval by means of a dual pulse repetition frequency. Although the first field deployment of ELDORA occurred in 1993, it was not until VORTEX (Verification of the Origins of Rotation in Tornadoes Experiment) that the system was operating up to its full capabilities. Examples of the radar's ability to detect clear-air phenomena are presented along with high-resolution images near severe local storms and tornadoes.

Observations on the Origins of Rotation: The Newcastle Tornado during VORTEX 94

Abstract Observations of a strong (F3) tornado near Newcastle, Texas, on 29 May 1994 during the Verification of the Origins of Rotation in Tornadoes Experiment (VORTEX) are presented. The visual characteristics and intensity of the tornado were revealed by a photogrammmetric analysis of pictures taken by chase teams and by a detailed damage survey. The tornado developed from a low-level shear feature along the flanking line of a supercell. Vortex stretching of this feature to tornadic intensity occurred under the influence of an intense updraft from a rapidly growing storm along the flanking line. No apparent midlevel mesocyclone accompanied this tornado. In contrast, the supercell was characterized by a well-defined mid- and low-level mesocyclone; however, no tornado was observed by ground chase teams. The implications of these findings on the current understanding of tornadogenesis are discussed.