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This study builds upon recent rapid-scan radar observations of mesocyclonic tornadogenesis in supercells by investigating the formation of seven tornadoes (four from a single cyclic supercell), most of which include samples at heights < 100 m above radar level. The spatiotemporal evolution of the tornadic vortex signatures (TVSs), maximum velocity differentials across the vortex couplet, and pseudovorticity are analyzed. In general, the tornadoes formed following a non-descending pattern of evolution, although one case was descending over time scales O( Significance Statement It is generally well understood that tornadoes form over short time scales [i.e., O(∼60) s]. Despite this fact, detailed scientific measurements of tornado evolution during and just prior to genesis remains limited, particularly very near the ground and on time and space scales sufficient to observe tornado processes. Multiple recent studies have supported a non-descending evolution of rotation in supercell tornadoes, but the small number of analyzed cases is still insufficient for generalization. This study investigates seven new cases of tornadogenesis using high spatiotemporal resolution radar data that include near-ground level observations to examine the evolution of rotation with time and height. For the time scales observable by the radar platform [i.e., O(∼30) s], genesis occurred predominately following a non-descending manner in five out of the seven tornadoes studied, while the vertical evolution of two tornadoes were sensitive to the criterion used to define a “tornadic” vortex signature.

Mobile, polarimetric radar data were collected on a series of tornadoes that occurred near Dodge City, Kansas. A poststorm survey revealed a series of tornadic debris swaths in several dirt fields and high-resolution pictures of the tornado documented the visual characteristics of the tornado and the lofted debris cloud. The main rotational couplet associated with the tornado was identified in the single-Doppler velocities; however, no secondary rotational couplets were resolved in the low-level data performed during two consecutive volume scans. Numerical simulations have suggested that cycloidal damage swaths can result when debris is deposited as the low-level inflow turns upward in the corner region of the updraft annulus of the tornado core. This mechanism can dominate even when suction vortices are present in the simulations and can produce these swaths in the absence of these smaller-scale vortices. It is hypothesized that the observed cycloidal damage swaths were a result of the low-level inflow in the corner region of the tornado and not by the existence of suction vortices. Polarimetric data were combined with photographs of the tornado in order to document the lofted debris cloud and its relationship with the funnel. This analysis provided an opportunity to investigate whether recent findings describing the cross-correlation coefficient ρhv and differential reflectivity ZDR signatures of the lofted debris cloud could be replicated. Regions of low ρhv at the periphery of the funnel cloud suggesting high debris loading and a column of negative ZDR centered on the tornado believed to be produced by common debris alignment were noted. Significance Statement It is well known that some tornadoes produce smaller-scale vortices that rotate around the central axis of the main circulation. In addition, numerous aerial photographs have documented cycloidal debris marks within tornado damage tracks that traverse open fields. The prevailing theory shown in numerous textbooks is that these marks are produced by these vortices. The current study suggests that this widely accepted model for producing these marks may be incorrect. It is suggested that these cycloidal marks are produced by the main tornado circulation and not by the smaller-scale vortices in this case.

On 24 May 2016, a supercell that produced 13 tornadoes near Dodge City, Kansas, was documented by a rapid-scanning, X-band, polarimetric, Doppler radar (RaXPol). The anomalous nature of this storm, particularly the significant deviations in storm motion from the mean flow and number of tornadoes produced, is examined and discussed. RaXPol observed nine tornadoes with peak radar-derived intensities (ΔVmax) and durations ranging from weak (~60 m s−1) and short lived (150 m s−1) and long lived (>25 min). This case builds on previous studies of tornado debris signature (TDS) evolution with continuous near-surface sampling of multiple strong tornadoes. The TDS sizes increased as the tornadoes intensified but lacked direct correspondence to tornado intensity otherwise. The most significant growth of the TDS in both cases was linked to two substantial rear-flank-downdraft surges and subsequent debris ejections, resulting in growth of the TDSs to more than 3 times their original sizes. The TDS was also observed to continue its growth as the tornadoes decayed and lofted debris fell back to the surface. The TDS size and polarimetric composition were also found to correspond closely to the underlying surface cover, which resulted in reductions in ZDR in wheat fields and growth of the TDS in terraced dirt fields as a result of ground scouring. TDS growth with respect to tornado vortex tilt is also discussed.

The maximum upward vertical velocity at the leading edge of a density current is commonly −1. Studies of the vertical velocity, however, are relatively few, in part owing to the dearth of high-spatiotemporal-resolution observations. During the Plains Elevated Convection At Night (PECAN) field project, a mobile Doppler lidar measured a maximum vertical velocity of 13 m s−1 at the leading edge of a density current created by a mesoscale convective system during the night of 15 July 2015. Two other vertically pointing instruments recorded 8 m s−1 vertical velocities at the leading edge of the density current on the same night. This study describes the structure of the density current and attempts to estimate the maximum vertical velocity at their leading edges using the following properties: the density current depth, the slope of its head, and its perturbation potential temperature. The method is then be applied to estimate the maximum vertical velocity at the leading edge of density currents using idealized numerical simulations conducted in neutral and stable atmospheres with resting base states and in neutral and stable atmospheres with vertical wind shear. After testing this method on idealized simulations, this method is then used to estimate the vertical velocity at the leading edge of density currents documented in several previous studies. It was found that the maximum vertical velocity can be estimated to within 10%–15% of the observed or simulated maximum vertical velocity and indirectly accounts for parameters including environmental wind shear and static stability.

This study presents an investigation into relationships among topographic elevation, surface land cover, and tornado intensity using rapid scan, mobile Doppler radar observations of four tornadoes from the U.S. Central Plains. High spatiotemporal resolution observations of tornadic vortex signatures from the radar’s lowest elevation angle data (in most cases ranging from ~100 to 350 m above ground level) are coupled with digital elevation model (DEM) and 2011 National Land Cover Database (NLCD) data using a geographic information system (GIS). The relationships between 1) tornado intensity and topographic elevation or surface roughness and 2) changes in tornado intensity and changes in topographic elevation or surface roughness are investigated qualitatively, and statistical relationships are quantified and analyzed using a bootstrap permutation method for individual case studies and all cases collectively. Results suggest that there are statistically significant relationships for individual cases, but the relationships defy generalization and are different on a case-by-case basis, which may imply that they are coincidental, indicating a null correlation.

Tornadic supercells moved across parts of Oklahoma on the afternoon and evening of 9 May 2016. One such supercell, while producing a long-lived tornado, was observed by nearby WSR-88D radars to contain a strong anticyclonic velocity couplet on the lowest elevation angle. This couplet was located in a very atypical position relative to the ongoing cyclonic tornado and to the supercell’s updraft. A storm survey team identified damage near where this couplet occurred, and, in the absence of evidence refuting otherwise, the damage was thought to have been produced by an anticyclonic tornado. However, such a tornado was not seen in near-ground, high-resolution radar data from a much closer, rapid-scan, mobile radar. Rather, an elongated velocity couplet was observed only at higher elevation angles at altitudes similar to those at which the WSR-88D radars observed the strong couplet. This paper examines observations from two WSR-88D radars and a mobile radar from which it is argued that the anticyclonic couplet (and a similar one ~10 min later) were actually quasi-horizontal vortices centered ~1–1.5 km AGL. The benefits of having data from a radar much closer to the convective storm being sampled (e.g., better spatial resolution and near-ground data coverage) and providing more rapid volume updates are readily apparent. An analysis of these additional radar data provides strong, but not irrefutable, evidence that the anticyclonic tornado that may be inferred from WSR-88D data did not exist; consequently, upon discussions with the National Weather Service, it was not included in Storm Data.

Rapid-scan polarimetric data analysis of the dissipation of a likely violent supercell tornado that struck near Sulphur, Oklahoma, on 9 May 2016 is presented. The Rapid X-band Polarimetric Radar was used to obtain data of the tornado at the end of its mature phase and during its entire dissipation phase. The analysis is presented in two parts: dissipation characteristics of the tornadic vortex signature (TVS) associated with the tornado and storm-scale polarimetric features that may be related to processes contributing to tornado dissipation. The TVS exhibited near-surface radial velocities exceeding 100 m s−1 multiple times at the end of its mature phase, and then underwent a two-phased dissipation. Initially, decreases in near-surface intensity occurred rapidly over a ~5-min period followed by a slower decline in intensity that lasted an additional ~12 min. The dissipation of the TVS in time and height in the lowest 2 km above radar level and oscillatory storm-relative motion of the TVS also are discussed. Using polarimetric data, a well-defined low reflectivity ribbon is investigated for its vertical development, evolution, and relationship to the large tornadic debris signature (TDS) collocated with the TVS. The progression of the TDS during dissipation also is discussed with a focus on the presence of several bands of reduced copolar correlation coefficient that extend away from the main TDS and the eventual erosion of the TDS as the tornado dissipated. Finally, TVS and polarimetric data are combined to argue for the importance of a possible internal rear-flank downdraft momentum surge in contributing to the initial rapid dissipation of the tornado.

A detailed damage survey is combined with high-resolution mobile, rapid-scanning X-band polarimetric radar data collected on the Shawnee, Oklahoma, tornado of 19 May 2013. The focus of this study is the radar data collected during a period when the tornado was producing damage rated EF3. Vertical profiles of mobile radar data, centered on the tornado, revealed that the radar reflectivity was approximately uniform with height and increased in magnitude as more debris was lofted. There was a large decrease in both the cross-correlation coefficient (ρhv) and differential radar reflectivity (ZDR) immediately after the tornado exited the damaged area rated EF3. Low ρhv and ZDR occurred near the surface where debris loading was the greatest. The 10th percentile of ρhv decreased markedly after large amounts of debris were lofted after the tornado leveled a number of structures. Subsequently, ρhv quickly recovered to higher values. This recovery suggests that the largest debris had been centrifuged or fallen out whereas light debris remained or continued to be lofted. Range–height profiles of the dual-Doppler analyses that were azimuthally averaged around the tornado revealed a zone of maximum radial convergence at a smaller radius relative to the leading edge of lofted debris. Low-level inflow into the tornado encountering a positive bias in the tornado-relative radial velocities could explain the existence of the zone. The vertical structure of the convergence zone was shown for the first time.

This is a study of a tornadic supercell in Kansas on 14 May 2018 in which data of relatively high spatiotemporal resolution from a mobile, polarimetric, X-band, Doppler radar were integrated with GOES-16 geosynchronous satellite imagery, and with fixed-site, surveillance, S-band polarimetric Doppler radar data. The data-collection period spanned the early life of the storm from when it was just a series of ordinary cells, with relatively low cloud tops, through its evolution into a supercell with much higher cloud tops, continuing through the formation and dissipation of a brief tornado, and ending after the supercell came to a stop and reversed direction, produced another tornado, and collided with a quasi-linear convective system. The main goal of this study was to examine the relationship between the overshooting tops and radar observed features prior to and during tornadogenesis. The highest radar echo top was displaced about 10 km, mainly to the north or northeast of the main updraft and cloud top, from the supercell phase through the first tornado phase of the supercell phase, after which the updraft and the cloud top became more closely located and then jumped ahead; this behavior is consistent with what would be expected during cyclic mesocyclogenesis. The change in direction of the supercell later on occurred while the nocturnal low-level jet was intensifying. No relationship was apparent between changes in the highest cloud-top height and tornadogenesis, but changes in cloud-top heights (rapid increases and rapid decreases) were related to two phases in multicell evolution and to supercell formation.

This study documents the formation and part of the early evolution of a large, violent tornado near El Reno, Oklahoma, based on data from a mobile, polarimetric, rapid scan, X-band, Doppler radar. The main circulation associated with the tornado formed near the ground initially, ~90 s prior to the development of the vertically coherent vortex, which built upward through a vertical column of at least 3.5 km in less than 20 s, the update time of the Doppler radar data. Strong but broad rotation from 500 m to 1.5 km AGL also preceded the formation of the tornado at the surface by several minutes. A precipitation-loaded downdraft was observed in the right-forward flank of the storm, which could have enhanced evaporative cooling and allowed for a faster rate of baroclinic generation of low-level horizontal vorticity, while descending reflectivity cores in the right-rear quadrant might have enhanced low-level convergence to the rear of or along the leading edge of the rear-flank gust front. The intensification of the tornado occurred in spurts, not steadily, perhaps owing to surges in momentum at the surface associated with the precipitation-laden downdrafts. The tornado was highly tilted even when it was intensifying, calling into question the importance of a vertical juxtaposition of the mesocyclone aloft and the tornado at the surface. In this case study, while the development of a weak-echo hole was evidence of rotation, the absence of one did not mean that there was not a strong vortex, owing to the lofting of debris.

Mid-latitude jet streaks are known to produce conditions broadly supportive of tornado outbreaks, including forcing for large-scale ascent, increased wind shear, and decreased static stability. Although many processes may initiate a jet streak, we focus here on the development of jet maxima by interactions between the polar jet and tropopause polar vortices (TPVs). Originating from the Arctic, TPVs are long-lived circulations on the tropopause, which can be advected into the mid-latitudes. We hypothesize that when these vortices interact with the jet, they may contribute supplemental forcing for ascent and shear to tornado outbreaks, assuming other environmental conditions supportive of tornado development exist. Using a case set of significant tornado outbreak days from three states—Oklahoma, Illinois, and Alabama—we show that a vortex-jet streak structure is present (within 1250 km) in around two-thirds of tornado outbreaks. These vortices are commonly Arctic in origin (i.e., are TPVs) and are advected through a consistent path of entry into the mid-latitudes in the week before the outbreak, moving across the Northern Pacific and into the Gulf of Alaska before turning equatorward along the North American coast. These vortices are shown to be more intense and longer-lived than average. We further demonstrate that statistically significant patterns of wind shear, quasi-geostrophic forcing for ascent, and low static stability are present over the outbreak regions on the synoptic scale. In addition, we find that TPVs associated with tornadic events occur most often in the spring and are associated with greater low-level moisture when compared to non-tornadic TPV cases.

This is a brief summary of the names, characteristics, and dynamics and thermodynamics of subsynoptic-scale and smaller weather systems that can produce damaging surface winds in midlatitudes and wherever the damaging winds occur within the systems. Those systems associated with convective storms include tornadoes, gust fronts, and microbursts; those not associated with convective storms include downslope wind storms and, to a lesser extent, bores, katabatic winds, sting jets, dryline bulges, and the diurnal oscillation of the (nocturnal) low-level jet. Fundamental physical processes discussed include extreme positive or negative buoyancy, production of horizontal vorticity baroclinically, production of vertical vorticity through tilting and stretching, gravity-wave generation over orography and downstream propagation, and turbulent vertical mixing.

Rapid-scan radar observations of a supercell that produced near-record size hail in Oklahoma are examined. Data from the National Weather Radar Testbed Phased Array Radar (PAR) in Norman, Oklahoma, are used to study the overall character and evolution of the storm. Data from the nearby polarimetric KOUN WSR-88D and rapid-scanning X-band polarimetric (RaXPol) mobile radar are used to study the evolution of low- to midaltitude dual-polarization parameters above two locations where giant hailstones up to 16 cm in diameter were observed. The PAR observation of the supercell’s maximum storm-top divergent outflow is similar to the strongest previously documented value. The storm’s mesocyclone rotational velocity at midaltitudes reached a maximum that is more than double the median value for similar observations from other storms producing giant hail. For the two storm-relative areas where giant hail was observed, noteworthy findings include 1) the giant hail occurred outside the main precipitation core, in areas with low-altitude reflectivities of 40–50 dBZ; 2) the giant hail was associated with dual-polarization signatures consistent with past observations of large hail at 10-cm wavelength, namely, low ZDR, low ρHV, and low KDP; 3) the giant hail fell along both the northeast and southwest edges of the primary updraft at ranges of 6–10 km from the updraft center; and 4) with the exception of one isolated report, the giant hail fell to the northeast and northwest of the large tornado and the parent mesocyclone.

The number of case studies in the literature of nocturnal convection has increased during the past decade, especially those that utilize high-spatiotemporal-resolution datasets from field experiments such as the International H2O Project (IHOP_2002) and Plains Elevated Convection at Night (PECAN). However, there are few case studies of events for convection initiation without a nearby surface boundary. These events account for approximately 25% of all nocturnal convection initiation (CI) events. Unique characteristics of these events include a peak initiation time later at night, a preferred initiation location in northern Kansas and southern Nebraska, and a preferred north–south orientation to linear convective systems. In this study, four case studies of convection that is initiated without a nearby surface boundary are detailed to reveal a number of possible initiation mechanisms, including quasigeostrophic-aided ascent, elevated ascent associated with convergent layers (of unknown causes), the low-level jet, and gravity waves. The case studies chosen illustrate the wide variety of synoptic-scale conditions under which these events can occur.

A recent study found that surface hodographs over the Great Plains of the United States turn in a counterclockwise direction with time. This observed turning is opposite of the clockwise turning observed (and expected, based on theory) at higher altitudes. Using a mesoscale forecast model, the same study shows that it has the same hodograph behavior as found in the observations. The study further shows that the reason for this anomalous counterclockwise turning is the decoupling of the surface layer from the boundary layer after sunset and its recoupling after sunrise. The present paper presents a simple model for this behavior by extending a recent analytical model for the diurnal oscillation to include the surface-layer effect. In addition, selected solution features are analyzed in terms of several of the nondimensional input parameters.

A three-dimensional data assimilation (3DVar) least squares–type single-Doppler velocity retrieval (SDVR) algorithm is utilized to retrieve the wind field of a tornadic supercell using data collected by a mobile, phased-array, Doppler radar [Mobile Weather Radar (MWR) 05XP] with very high temporal resolution (6 s). It is found that the cyclonic circulation in the hook-echo region can be successfully recovered by the SDVR algorithm. The quality of the SDVR analyses is evaluated by dual-Doppler syntheses using data collected by two mobile Doppler radars [Doppler on Wheels 6 and 7 (DOW6 and DOW7, respectively)]. A comparison between the SDVR analyses and dual-Doppler syntheses confirms the conclusion reached by an earlier theoretical analysis that because of the temporally discrete nature of the radar data, the wind speed retrieved by single-Doppler radar is always underestimated, and this underestimate occurs more significantly for the azimuthal (crossbeam) wind component than for the radial (along beam) component. However, the underestimate can be mitigated by increasing the radar data temporal resolution. When the radar data are collected at a sufficiently high rate, the azimuthal wind component may be overestimated. Even with data from a rapid scan, phased-array, Doppler radar, our study indicates that it is still necessary to calculate the SDVR in an optimal moving frame of reference. Finally, the SDVR algorithm’s robustness is demonstrated. Even with a temporal resolution (2 min) much lower than that of the phased-array radar, the cyclonic flow structure in the hook-echo region can still be retrieved through SDVR using data observed by DOW6 or DOW7, although a difference in the retrieved fields does exist. A further analysis indicates that this difference is caused by the location of the radars.

This study documents the formation and evolution of secondary vortices associated within a large, violent tornado in Oklahoma based on data from a close-range, mobile, polarimetric, rapid-scan, X-band Doppler radar. Secondary vortices were tracked relative to the parent circulation using data collected every 2 s. It was found that most long-lived vortices (those that could be tracked for ≥15 s) formed within the radius of maximum wind (RMW), mainly in the left-rear quadrant (with respect to parent tornado motion), passing around the center of the parent tornado and dissipating closer to the center in the right-forward and left-forward quadrants. Some secondary vortices persisted for at least 1 min. When a Burgers–Rott vortex is fit to the Doppler radar data, and the vortex is assumed to be axisymmetric, the secondary vortices propagated slowly against the mean azimuthal flow; if the vortex is not assumed to be axisymmetric as a result of a strong rear-flank gust front on one side of it, then the secondary vortices moved along approximately with the wind.

Vertical shear in the boundary layer affects the mode of convective storms that can exist if they are triggered. In western portions of the southern Great Plains of the United States, vertical shear, in the absence of any transient features, changes diurnally in a systematic way, thus leading to a preferred time of day for the more intense modes of convection when the shear, particularly at low levels, is greatest. In this study, yearly and seasonally averaged wind observations for each time of day are used to document the diurnal variations in wind at the surface and in the boundary layer, with synoptic and mesoscale features effectively filtered out. Data from surface mesonets in Oklahoma and Texas, Doppler wind profilers, instrumented tower data, and seasonally averaged wind data for each time of day from convection-allowing numerical model forecasts are used. It is shown through analysis of observations and model data that the perturbation wind above anemometer level turns in a clockwise manner with time, in a manner consistent with prior studies, yet the perturbation wind at anemometer level turns in an anomalous, counterclockwise manner with time. Evidence is presented based on diagnosis of the model forecasts that the dynamics during the early evening boundary layer transition are, in large part, responsible for the behavior of the hodographs at that time: as vertical mixing in the boundary layer diminishes, the drag on the wind at anemometer level persists, leading to rapid deceleration of the meridional component of the wind. This deceleration acts to turn the wind to the left rather than to the right, as would be expected from the Coriolis force alone.

During the second Verification of the Origins of Rotation in Tornadoes Experiment (VORTEX2) field campaign, mobile radars observed a previously undocumented feature: the low-reflectivity ribbon (LRR). The LRR was characterized by reduced reflectivity ZH and differential reflectivity ZDR through a narrow region extending from the intersection of the hook and forward-flank regions of supercells. This study synthesizes kinematic and polarimetric radar observations with in situ measurements taken by the “StickNet” observing network. StickNet data have been used to establish that the LRR is associated with a localized minimum in pseudoequivalent potential temperature Pronounced drops in are observed by nine separate probes in three different supercell thunderstorms. Both single- and dual-Doppler analyses are used to examine the two- and three-dimensional structures of the winds within the LRR, revealing that the LRR is associated with cyclonic vertical vorticity aloft. Polarimetric radar observations are used to study the hydrometeor characteristics and the processes that cause those hydrometers to be present. Special consideration is given to the analysis of the vertical distribution of traditional and polarimetric variables, as well as the evolution of the kinematic fields retrieved by dual-Doppler analysis. The combination of thermodynamic, kinematic, and inferred microphysical observations supports a hypothesis that the LRR comprises sparse, large hail.

The objectives of this study are to determine the finescale characteristics of the wind and temperature fields associated with a prefrontal wind-shift line and to contrast them with those associated with a strong cold front. Data from a mobile, polarimetric, X-band, Doppler radar and from a surveillance S-band radar, temperature profiles retrieved from a thermodynamic sounder, and surface observations from the Oklahoma Mesonet are used to analyze a prefrontal wind-shift line in Oklahoma on 11 November 2013. Data from the same mobile radar and the Oklahoma Mesonet are used to identify the finescale characteristics of the wind field associated with a strong surface cold front in Oklahoma on 9 April 2013. It is shown that the prefrontal wind-shift line has a kinematic and thermodynamic structure similar to that of an intrusion (elevated density current), while the cold front has a kinematic structure similar to that of a classic density current. Other characteristics of the prefrontal wind-shift line and front are also discussed. Evidence of waves generated at the leading edge of the prefrontal wind-shift line is presented.

A high-resolution numerical model and polarimetric forward operator allow one to examine simulated convective storms from the perspective of observable polarimetric radar quantities, enabling a better comparison of modeled and observed deep moist convection. Part I of this two-part study described the model and forward operator used for all simulations and examined the structure and evolution of rings of reduced copolar cross-correlation coefficient (i.e., ρhv rings). The microphysical structure of upward extensions of enhanced differential reflectivity (ZDR columns and ZDR rings) and enhanced specific differential phase (KDP columns) near and within the updrafts of convective storms serve as the focus of this paper. In general, simulated ZDR columns are located immediately west of the midlevel updraft maximum and are associated with rainwater lofted above the 0°C level and wet hail/graupel, whereas ZDR rings are associated with wet hail located near and immediately east of the midlevel updraft maximum. The deepest areas of ZDR > 1 dB aloft are associated with supercells in the highest shear environments and those that have the most intense updrafts; the upper extent of the ZDR signatures is found to be positively correlated with the amount and mean-mass diameter of large hail aloft likely as a by-product of the shared correlations with updraft intensity and wind shear. Large quantities of rain compose the KDP columns, with the size and intensity of the updrafts directly proportional to the size and depth of the KDP columns.

A nocturnal maximum in rainfall and thunderstorm activity over the central Great Plains has been widely documented, but the mechanisms for the development of thunderstorms over that region at night are still not well understood. Elevated convection above a surface frontal boundary is one explanation, but this study shows that many thunderstorms form at night without the presence of an elevated frontal inversion or nearby surface boundary. This study documents convection initiation (CI) events at night over the central Great Plains from 1996 to 2015 during the months of April–July. Storm characteristics such as storm type, linear system orientation, initiation time and location, and others were documented. Once all of the cases were documented, surface data were examined to locate any nearby surface boundaries. The event’s initiation location relative to these boundaries (if a boundary existed) was documented. Two main initiation locations relative to a surface boundary were identified: on a surface boundary and on the cold side of a surface boundary; CI events also occur without any nearby surface boundary. There are many differences among the different nocturnal CI modes. For example, there appear to be two main peaks of initiation time at night: one early at night and one later at night. The later peak is likely due to the events that form without a nearby surface boundary. Finally, a case study of three nocturnal CI events that occurred during the Plains Elevated Convection At Night (PECAN) field project when there was no nearby surface boundary is discussed.

This paper describes the novel hybrid front-end beamforming architecture of a C-band mobile Polarimetric Atmospheric Imaging Radar (PAIR) system for weather applications. PAIR, a state-of-the-art radar on a mobile platform, will be shared with the scientific and radar communities to further research frontiers using its unprecedented high-temporal resolution and scanning flexibility. The system under development achieves dual polarization through novel polarimetric phased- array antenna design; improved detection capability through integrated solutions provided by solid state technology; faster update time through digital beamforming (DBF) in elevation; and a robust structure for fast deployment in severe weather. The concept and research applications and the development progress of PAIR will be reported in this paper.

High-resolution data of the tornadic debris signature (TDS) and weak-echo reflectivity band (WRB) associated with a large, violent tornado on 24 May 2011 in central Oklahoma are examined using a rapid-scan, X-band, polarimetric, mobile Doppler radar. Various characteristics of these features and their evolution are examined over time intervals of 20 s or less. The formation of the TDS, debris fallout, and inhomogeneities in the TDS structure, are analyzed from volumetric and single-elevation observations. Constant-radius vertical cross sections of Doppler velocity, reflectivity, and copolar cross-correlation coefficient are compared at various times during the tornado’s life cycle; from them it is found that the weak echo column (WEC) is considerably narrower than the TDS and the WEC is confined to the strong gradient of Doppler velocities in the tornado’s core. The TDS of the mature tornado extends radially outward, bound approximately by the 40 m s−1 radial isodop.Rapid-scan, near-surface data were collected for a period of 6 min, during which 2-s single-elevation PPI updates at 1° were available at heights below 100 m above radar level. During this period, a WRB associated with a visually observed horizontal vortex developed east of the tornado, along the leading edge of the secondary rear-flank gust front, as the tornado was rapidly intensifying. A relationship was noted between reduced radar-observed reflectivity and increased radar-observed radial convergence/divergence in the vicinity of the horizontal vortex as it strengthened. This feature is qualitatively analyzed and hypotheses explaining its generation and structure are discussed.

The United States experiences the most tornadoes of any country in the world. Given the catastrophic impact of tornadoes, concern has arisen regarding the variation in climatology of U.S. tornadoes under the changing climate. A recent study claimed that the temporal variability of tornado occurrence over the continental U.S. has increased since the 1970s. However, that study ignored the highly regionalized climatology of U.S. tornadoes. To address this issue, we examined the long-term trend of tornado temporal variability in each continental U.S. state. Based on the 64 year tornado records (1950–2013), we found that the trends in tornado temporal variability varied across the U.S., with only one third of the continental area or three out of 10 contiguous states (mostly from the Great Plains and Southeast, but where the frequency of occurrence of tornadoes is greater) displaying a significantly increasing trend. The other two-thirds area, where 60% of the U.S. tornadoes were reported (but the frequency of occurrence of tornadoes is less), however, showed a decreasing or a near-zero trend in tornado temporal variability. Furthermore, unlike the temporal variability alone, the combined spatial-temporal variability of U.S. tornado occurrence has remained nearly constant since 1950. Such detailed information on the climatological variability of U.S. tornadoes refines the claim of previous study and can be helpful for local mitigation efforts toward future tornado risks.

Past numerical simulation studies found that debris loading from sand-sized particles may substantially affect tornado dynamics, causing reductions in near-surface wind speeds up to 50%. To further examine debris loading effects, simulations are performed using a large-eddy simulation model with a two-way drag force coupling between air and sand. Simulations encompass a large range of surface debris fluxes that cause negligible to substantial impact on tornado dynamics for a high-swirl tornado vortex simulation. Simulations are considered for a specific case with a single vortex flow type (swirl ratio, intensity, and translation velocity) and a fixed set of debris and aerodynamic parameters. Thus, it is stressed that these findings apply to the specific flow and debris parameters herein and would likely vary for different flows or debris parameters. For this specific case, initial surface debris fluxes are varied over a factor of 16 384, and debris cloud mass varies by only 42% of this range because a negative feedback reduces near-surface horizontal velocities. Debris loading effects on the axisymmetric mean flow are evident when maximum debris loading exceeds 0.1 kg kg−1, but instantaneous maximum wind speed and TKE exhibit small changes at smaller debris loadings (greater than 0.01 kg kg−1). Initially, wind speeds are reduced in a shallow, near-surface layer, but the magnitude and depth of these changes increases with higher debris loading. At high debris loading, near-surface horizontal wind speeds are reduced by 30%–60% in the lowest 10 m AGL. In moderate and high debris loading scenarios, the number and intensity of subvortices also decrease close to the surface.

A detailed damage survey of the El Reno, Oklahoma, tornado of 31 May 2013 combined with rapid-scanning data recorded from two mobile radars is presented. One of the radars was equipped with polarimetric capability. The relationship between several suction vortices visually identified in pictures with the high-resolution Doppler velocity data and swath marks in fields is discussed. The suction vortices were associated with small shear features in Doppler velocity and a partial ringlike feature of high spectral width. For the first time, a suction vortex that created a swath mark in a field was visually identified in photographs and high-definition video while the rotational couplet was tracked by radar. A dual-Doppler wind synthesis of the tornadic circulation at low levels near the location of several storm chaser fatalities resolved ground-relative wind speeds in excess of 90 m s−1, greater than the minimum speed for EF5 damage. The vertical vorticity analysis revealed a rapid transition from a single tornadic vortex centered on the weak-echo hole (WEH) to suction vortices surrounding the WEH and collocated with the ring of enhanced radar reflectivities. Several bands/zones of enhanced convergence were resolved in the wind syntheses. One of the bands was associated with an internal or secondary rear-flank gust front. An inner band of convergence appeared to be a result of the positive bias in tornado-relative radial velocity owing to centrifuging of large lofted debris swirling within the tornado. An outer band of convergence formed at the northern edge of a region of strong inflow that was lofting small debris and dust into the storm.

Supercells dominated by mesocyclones, which tend to propagate to the right of the tropospheric pressure-weighted mean wind, on rare occasions produce anticyclonic tornadoes at the trailing end of the rear-flank gust front. More frequently, mesoanticyclones are found at this location, most of which do not spawn any tornadoes. In this paper, four cases are discussed in which the formation of anticyclonic tornadoes was documented in the plains by mobile or fixed-site Doppler radars. These brief case studies include the analysis of Doppler radar data for tornadoes at the following dates and locations: 1) 24 April 2006, near El Reno, Oklahoma; 2) 23 May 2008, near Ellis, Kansas; 3) 18 March 2012, near Willow, Oklahoma; and 4) 31 May 2013, near El Reno, Oklahoma. Three of these tornadoes were also documented photographically. In all of these cases, a strong mesocyclone (i.e., vortex signature characterized by azimuthal shear in excess of ~5 × 10−3 s−1 or a 20 m s−1 change in Doppler velocity over 5 km) or tornado was observed ~10 km away from the anticyclonic tornado. In three of these cases, the evolution of the tornadic vortex signature in time and height is described. Other features common to all cases are noted and possible mechanisms for anticyclonic tornadogenesis are identified. In addition, a set of estimated environmental parameters for these and other similar cases are discussed.

Polarimetric measurements recorded by a mobile X-band radar are combined with photographs of the Dodge City, Kansas, tornado to quantitatively document the evolving debris cloud. An inner annulus or tube of high radar reflectivity encircled the tornado at low levels. A column of low cross-correlation coefficient ρhv was centered on the funnel cloud during the early stage of the tornado’s life cycle. In addition, two areas of low ρhv were located near the inner annulus of high radar reflectivity and were hypothesized to be regions of high debris loading that have been reproduced in simulations of lofted debris. Another column of low ρhv was a result of strong wind speeds that were progressively lofting small debris and dust as inflow rotated around and within the weak echo notch of the hook echo. A column of negative differential reflectivity ZDR was also centered on the tornado and was hypothesized to result from common debris alignment. The polarimetric structure undergoes a dramatic transition when the debris cloud was prominent and enveloped most of the funnel cloud. The weak echo column (WEC) began to fill at lower levels as large amounts of debris were lofted into the circulation. The axis of minimum ρhv shifted to a radius just beyond the funnel cloud. A column of positive ZDR was collocated with the funnel surrounded by negative ZDR. The negative ZDR and low ρhv within the debris cloud were likely the result of some common debris alignment from wheat stems. The positive ZDR within the funnel signified the presence of a few hydrometeors.

Several data assimilation and forecast experiments are undertaken to determine the impact of special observations taken during the second Verification of the Origins of Rotation in Tornadoes Experiment (VORTEX2) on forecasts of the 5 June 2009 Goshen County, Wyoming, supercell. The data used in these experiments are those from the Mobile Weather Radar, 2005 X-band, Phased Array (MWR-05XP); two mobile mesonets (MM); and several mobile sounding units. Data sources are divided into “routine,” including those from operational Weather Surveillance Radar-1988 Dopplers (WSR-88Ds) and the Automated Surface Observing System (ASOS) network, and “special” observations from the VORTEX2 project. VORTEX2 data sources are denied individually from a total of six ensemble square root filter (EnSRF) data assimilation and forecasting experiments. The EnSRF data assimilation uses 40 ensemble members on a 1-km grid nested inside a 3-km grid. Each experiment assimilates data every 5 min for 1 h, followed by a 1-h forecast. All experiments are able to reproduce the basic evolution of the supercell, though the impact of the VORTEX2 observations was mixed. The VORTEX2 sounding data decreased the mesocyclone intensity in the latter stages of the forecast, consistent with observations. The MWR-05XP data increased the forecast vorticity above approximately 1 km AGL in all experiments and had little impact on forecast vorticity below 1 km AGL. The MM data had negative impacts on the intensity of the low-level mesocyclone, by decreasing the vertical vorticity and indirectly by decreasing the buoyancy of the inflow.

Unique observations of the interaction and likely merger of two cyclonic tornadoes are documented. One of the tornadoes involved in the interaction was the enhanced Fujita scale (EF5) El Reno–Piedmont, Oklahoma, tornado from 24 May 2011 and the other was a previously undocumented tornado. Data from three S-band radars: Twin Lakes, Oklahoma (KTLX); Norman, Oklahoma (KOUN); and the multifunction phased-array radar (MPAR), are used to detail the formation of the second tornado, which occurred to the northwest of the original tornado in an area of strong radial convergence. Radar data and isosurfaces of azimuthal shear provide evidence that both tornadoes formed within an elongated area of mesocyclone-scale cyclonic rotation. The path taken by the primary tornado and the formation location of the second tornado are different from previous observations of simultaneous cyclonic tornadoes, which have been most often observed in the cyclic tornadogenesis process. The merger of the two tornadoes occurred during the sampling period of a mobile phased-array radar—the Mobile Weather Radar, 2005 X-Band, Phased Array (MWR-05XP). MWR-05XP electronic scanning in elevation allowed for the merger process to be examined up to 4 km above radar level every 11 s. The tornadic vortex signatures (TVSs) associated with the tornadoes traveled around each other in a counterclockwise direction then merged in a helical manner up through storm midlevels. Upon merging, both the estimated intensity and size of the TVS associated with the resulting tornado increased dramatically. Similarities between the merger observed in this case and in previous cases also are discussed.

On 24 May 2011, a mobile, rapid-scan, X-band, polarimetric, Doppler radar (RaXPol) collected data on a supercell as it produced two tornadoes near El Reno, Oklahoma. The first tornado, rated an EF-3, was documented from intensification to decay, and the genesis and intensification of a second tornado that was rated an EF-5 was subsequently also documented. The objective of this study is to examine the spatiotemporal evolution of the rotation associated with the tornadoes (i) as the first tornado weakened to subtornadic intensity and (ii) as the second tornado formed and intensified. It is found that weakening did not occur monotonically. The transition from tornadic to subtornadic intensity over the depth of the radar volume (~4 km) occurred in less than 30 s, but this behavior is contingent upon the threshold for Doppler shear used to define the tornado. Similarly, the onset of a tornadic-strength Doppler velocity couplet occurred within a 30-s period over all elevations. Additionally, the evolution of storm-scale features associated with tornado dissipation and tornadogenesis is detailed. These features evolved considerably over relatively short time intervals (1–4 min). It is shown that during the transition period between the two tornadoes, two mesocyclones were present, but neither the tornadoes nor the mesocyclones evolved in a manner entirely consistent with any published conceptual model of supercell cycling, although certain aspects were similar to classic conceptual models. The mesocyclone and the tornado evolved differently from each other, in a manner that resembles a hybrid between the occluding and nonoccluding cyclic mesocyclogenesis models presented by Adlerman and Droegemeier.

On 31 May 2013 a broad, intense, cyclonic tornado and a narrower, weaker companion anticyclonic tornado formed in a supercell in central Oklahoma. This paper discusses the synoptic- and mesoscale environment in which the parent storm formed, based on data from the operational network of surface stations, rawinsondes, and WSR-88D radars, and from the Oklahoma Mesonet, a Doppler radar wind profiler, Rapid Refresh (RAP) analyses, and photographs. It also documents the overall behavior of the tornadoes and their relationships to features in their parent supercell based on data from a nearby, rapid-scan, polarimetric, mobile Doppler radar. The supercell formed near the intersection of a cold front and a dryline in an environment of moderately strong vertical shear and high CAPE, at the southern end of a line of multicell convective storms. The tornado damage path was as wide as 4.2 km according to the NWS damage assessment and ground-relative Doppler velocities of at least 135 m s−1 were found at the theoretical beam height of

This paper documents features that led to major forecast errors on the 12–24-h time scale in the nature and location of severe weather in the southern plains on 30 May 2012. Evidence is presented that the forecast errors were the result of 1) dry air that originated in a region of dissipating, elevated convective storms, and which was advected in a narrow tongue into western Oklahoma, inhibiting convective initiation; 2) the development of a cyclone along the dryline in western Texas, to the east of which several supercells formed; 3) the upscale development of the supercells into a mesoscale convective system (MCS) at nightfall; and 4) the dissipation of an MCS that had formed along a cold front in southwestern Kansas and was propagating into northwestern Oklahoma, as it encountered dry, subsiding air underneath the stratiform precipitation region of the rear portion of the MCS farther south. There was a meridionally oriented swath of high winds in clear air, in between the two MCSs. This swath of high winds may have been associated with a bore triggered at night by the MCSs approaching from the north, as the MCS collapsed, producing a gust front that propagated through stable, low-level air. This case study illustrates how the predictability of severe weather in a region can be extremely sensitive to the details of where nearby convective storms form and how they evolve. It also highlights the likely importance of the accurate representation of cloud microphysics and dynamics in numerical forecast models on predictability.

In the past four decades much has been discovered about tornado formation and structure from observations, laboratory models, and numerical-simulation experiments. Observations include nearby movies and photographs of tornadoes, fixed-site, airborne, and ground-based mobile Doppler radar remote measurements, and in situ measurements using instrumented probes. Laboratory models are vortex chambers and numerical-simulations are based on the governing fluid dynamical equations. However, questions remain: How and why do tornadoes form? and How does the wind field associated with them vary in space and time? Recent studies of tornadoes based on observations, particularly by radar, are detailed. The major aspects of numerically simulating a tornado and its formation are reviewed, and the dynamics of tornado formation and structure based on both observations and laboratory and numerical-simulation experiments are described. Finally, future avenues of research and suggested instrument development for furthering our knowledge are discussed.

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.

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.

A mobile, phased-array Doppler radar, the Mobile Weather Radar, 2005 X-band, Phased Array (MWR-05XP), has been used since 2007 to obtain data in supercells and tornadoes. Rapidly updating, volumetric data of tornadic vortex signatures (TVSs) associated with four tornadoes are used to investigate the time–height evolution of TVS intensity, position, and dissipation up through storm midlevels. Both TVS intensity and position were highly variable in time and height even during tornado mature phases. In one case, a TVS associated with a tornado dissipated aloft and a second TVS formed shortly thereafter while there was one continuous TVS near the ground. In a second case, the TVS associated with a long-lived, violent tornado merged with a second TVS (likely a second cyclonic tornado) causing the original TVS to strengthen. TVS dissipation occurred first at a height of ~1.5 km AGL and then at progressively higher levels in two cases; TVS dissipation occurred last in the lowest 1 km in three cases examined. Possible explanations are provided for the unsteady nature of TVS intensity and a conceptual model is presented for the initial dissipation of TVSs at ~1.5 km AGL.

Adding a mix of X- or C-band radars to the current Weather Surveillance Radar-1988 Doppler (WSR-88D) network could address several limitations of the network, including improvements to spatial gaps in low-level coverage and temporal sampling of volume scans. These limitations can result in missing critical information in highly dynamic events, such as tornadoes and severe straight-line wind episodes. To evaluate the potential value of a mixed-band radar network for severe weather operations, a case study is examined using data from X- and S-band radars. On 13 May 2009, a thunderstorm complex associated with a cold front moved southward into southwest Oklahoma. A tornado rapidly developed from an embedded supercell within the complex. The life cycle of the tornado and subsequent wind event was sampled by the experimental Collaborative Adaptive Sensing of the Atmosphere (CASA) radar testbed of four X-band radars as well as two operational WSR-88Ds. In this study, the advantages of a mixed-band radar network are demonstrated through a chronological analysis of the event. The two radar networks provided enhanced overall situational awareness. Data from the WSR-88Ds provided 1) clear-air sensitivity, 2) a broad overview of the storm complex, 3) a large maximum unambiguous range, and 4) upper-level scans up to 19.5°. Data from the CASA radars provided 1) high-temporal, 1-min updates; 2) overlapping coverage for dual-Doppler analysis; and 3) dense low-level coverage. The combined system allowed for detailed, dual- and single-Doppler observations of a wind surge, a mesocyclone contraction, and a downburst.

During the Second Verification of the Origins of Rotation in Tornadoes Experiment (VORTEX2), in the spring of 2010, a mobile and pulsed Doppler lidar system [the Truck-Mounted Wind Observing Lidar Facility (TWOLF)] mounted on a truck along with a mobile, phased-array, X-band Doppler radar system [Mobile Weather Radar–2005 X-band, phased array (MWR-05XP)] was used to complement Doppler velocity coverage in clear air near the radar–lidar facility and to provide high-spatial-resolution vertical cross sections of the Doppler wind field in the clear-air boundary layer near and in supercells. It is thought that the magnitude and direction of vertical shear and possibly the orientation and spacing of rolls in the boundary layer have significant effects on both supercell and tornado behavior; MWR-05XP and TWOLF can provide data that can be used to measure vertical shear and detect rolls. However, there are very few detailed, time-dependent and spatially varying observations throughout the depth of the boundary layer of supercells and tornadoes. This paper discusses lidar and radar data collected in or near six supercells. Features seen by the lidar included gust fronts, horizontal convective rolls, and small-scale vortices. The lidar proved useful at detecting high-spatial-resolution, clear-air returns at close range, where the radar was incapable of doing so, thus providing a more complete picture of the boundary layer environment ahead of supercells. The lidar was especially useful in areas where there was ground-clutter contamination. When there was precipitation and probably insects, and beyond the range of the lidar, where there was no ground-clutter contamination, the radar was the more useful instrument. Suggestions are made for improving the system and its use in studying the tornado boundary layer.

Observations from a hybrid phased-array Doppler radar, the Mobile Weather Radar, 2005 X-band, Phased Array (MWR-05XP), were used to investigate the vertical development of tornadic vortex signatures (TVSs) during supercell tornadogenesis. Data with volumetric update times of ∼10 s, an order of magnitude better than that of most other mobile Doppler radars, were obtained up to storm midlevels during the formation of three tornadoes. It is found that TVSs formed upward with time during tornadogenesis for two cases. In a third case, missing low-level data prevented a complete time–height analysis of TVS development; however, TVS formation occurred first near the ground and then at storm midlevels several minutes later. These results are consistent with the small number of volumetric mobile Doppler radar tornadogenesis cases from the past ∼10 years, but counter to studies prior to that, in which a descending TVS was observed in roughly half of tornado cases utilizing Weather Surveillance Radar-1988 Doppler (WSR-88D) data. A comparative example is used to examine the possible effects relatively long WSR-88D volumetric update times have on determining the mode of tornadogenesis.

A novel, rapid-scanning, X-band (3-cm wavelength), polarimetric (RaXPol), mobile radar was developed for severe-weather research. The radar employs a 2.4-m-diameter dual-polarized parabolic dish antenna on a high-speed pedestal capable of rotating the antenna at 180° s−1. The radar can complete a 10-elevation-step volume scan in about 20 s, while maintaining a 180-record-per-second data rate. The transmitter employs a 20-kW peak-power traveling wave tube amplifier that can generate pulse compression and frequency-hopping waveforms. Frequency hopping permits the acquisition of many more independent samples possible than without frequency hopping, making it possible to scan much more rapidly than conventional radars. Standard data products include vertically and horizontally polarized equivalent radar reflectivity factor, Doppler velocity mean and standard deviation, copolar cross-correlation coefficient, and differential phase. This paper describes the radar system and illustrates the capabilities of the radar through selected analyses of data collected in the U.S. central plains during the 2011 spring tornado season. Also noted are opportunities for experimenting with different signal-processing techniques to reduce beam smearing, increase sensitivity, and improve range resolution.

Mobile Doppler radar data, along with observations from a nearby Weather Surveillance Radar-1988 Doppler (WSR-88D), are assimilated with an ensemble Kalman filter (EnKF) technique into a nonhydrostatic, compressible numerical weather prediction model to analyze the evolution of the 4 May 2007 Greensburg, Kansas, tornadic supercell. The storm is simulated via assimilation of reflectivity and velocity data in an initially horizontally homogeneous environment whose parameters are believed to be a close approximation to those of the Greensburg supercell inflow sector. Experiments are conducted to test analysis sensitivity to mobile radar data availability and to the mean environmental near-surface wind profile, which was changing rapidly during the simulation period. In all experiments, a supercell with similar location and evolution to the observed storm is analyzed, but the simulated storm’s characteristics differ markedly. The assimilation of mobile Doppler radar data has a much greater impact on the resulting analyses, particularly at low altitudes (≤2 km), than modifications to the near-surface environmental wind profile. Differences in the analyzed updrafts, vortices, cold pool structure, rear-flank gust front structure, and observation-space diagnostics are documented. An analyzed vortex corresponding to the enhanced Fujita scale 5 (EF-5) Greensburg tornado is stronger and deeper in experiments in which mobile (higher resolution) Doppler radar data are included in the assimilation. This difference is linked to stronger analyzed horizontal convergence, which in turn is associated with increased stretching of vertical vorticity. Changing the near-surface wind profile appears to impact primarily the updraft strength, availability of streamwise vorticity for tilting into the vertical, and low-level vortex strength and longevity.

Polarimetric weather radars significantly enhance the capability to infer the properties of scatterers within a resolution volume. Previous studies have identified several consistently seen polarimetric signatures in supercells observed in the central United States. Nearly all of these studies used data collected by fixed-site S- and C-band radars. Because there are few polarimetric mobile radars, relatively little has been documented in high-resolution polarimetric data from mobile radars. Compared to S and C bands, there has been very limited examination of polarimetric signatures at X band. The primary focus of this paper is on one signature that has not been documented previously and one that has had little documentation at X band. The first signature, seen in at least seven supercell datasets collected by a mobile, X-band, polarimetric radar, consists of a narrow band of locally reduced reflectivity factor ZH and differential reflectivity, typically near the location where the hook echo “attaches” to the main body of the storm echo. No consistent pattern is seen in radial velocity VR or copolar cross correlation ρHV. The small size of this feature suggests a significant heterogeneity in precipitation microphysics, the cause and impact of which are unknown. The greater resolution and the scattering differences at X band compared to other frequencies may make this feature more apparent. The second signature consists of anomalously low ρHV in areas of high ZH along the left section (relative to storm motion) of the bounded weak-echo region. Examples of other polarimetric signatures at X band are provided.