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    Time–height profile of the magnitude of the largest Doppler velocity value within each TVS (adjusted for TVS motion). Dots indicate data points and dashed lines represent the limits of data collection. Velocity shears below the TVS detectability level are lightly shaded. The black region at bottom center is the diameter (using ordinate scale) of the Union City tornado funnel near the cloud base. [From Brown et al. (1978).]

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    Reflectivity (dBZ) from the WSR-88D at 0.55° elevation angle in Dodge City, KS, at (a) 0021 and (b) 0149 UTC 24 May 2008; in Cheyenne, WY, at (c) 2143 and (d) 2229 UTC 5 Jun 2009; and near Oklahoma City, OK, at (e) 2049 and (b) 2057 UTC 24 May 2011. In (b)–(f), the location of the MWR-05XP during its deployments is indicated by a white circle [in (a), the deployment had not yet begun]. Range rings are every 10 km. In (a)–(b), (c)–(d), and (e)–(f) the images are centered at the same location.

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    The MWR-05XP (a) scanning the Goshen County, WY, tornado at 2201 UTC 5 Jun 2009 and (b) being set up prior to it obtaining data of the El Reno tornado at 2041 UTC 24 May 2011. (Both photographs © Michael French.)

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    A series of MWR-05XP radial velocity (m s−1) PPI scans at 1.0° elevation angle before, during, and after the formation of the E–P tornado at (a) 0152:03, (b) 0154:53, (c) 0157:56, (d) 0201:01, (e) 0204:05, (f) 0206:55, (g) 0210:01, and (h) 0213:04 UTC 24 May 2008. White circles enclose the TVS associated with the E–P tornado. The blue circle in (a) encloses an anticyclonic VS. The black circles in (c),(d) enclose a VS that dissipated at ∼0202 UTC and was not associated with the tornado. The color-coded maximum VS/TVS GTG ΔV values (m s−1) are listed next to the outlined VS/TVSs. Range rings are every 5 km. All images are centered at the same location. The approximate center beam height at the location of the VS/TVS in (a),(c), and (g) is 300, 250, and 300 m ARL, respectively.

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    As in Fig. 4, but at (a) 0154:53, (b) 0159:49, (c) 0204:48, and (d) 0209:45 UTC at (left) 9.8° and (right) 17.1° elevation angle. The black and red circles enclose VSs that were not associated with the E–P tornado. The approximate center beam height at the location of the VS in (b) at 9.8° elevation angle is 2.6 km ARL and in (d) at 17.1° elevation angle is 6.1 km ARL. Note the different radial velocity scale from that used in Fig. 4.

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    (a) Time–height series of the formation of the E–P TVS using the baseline criteria discussed in the text, (b) time series of maximum ΔV (m s−1) in the E–P TVS, and (c) the distance (km) between the maximum inbound and outbound radial velocities in the E–P mesocyclone–tornado cyclone from 0159:49 to 0208:48 UTC at 1.0°, 2.5°, and 3.9° elevation angle. In (a), the time of TVS formation in data from 5.4° elevation angle also is shown. In (b) and (c), a simple 1–2–1 filter in time was applied to smooth the curves. Approximate beam heights at the TVS center are indicated in the top-left-hand corner of (b).

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    MWR-05XP radial velocity (m s−1) PPI scans at 9.8° elevation angle at (a) 0159:08, (b) 0200:05, (c) 0201:01, and (d) 0202:11 UTC and at 18.5° elevation angle at (e) 0200:05, (f) 0203:36, (g) 0206:55, and (h) 0210:041 UTC 24 May 2008. Circles enclose VSs that formed and dissipated during the time the E–P TVS was continuously identifiable in data from the lowest ∼1 km. Each VS is outlined in a different colored circle to highlight any VS temporal continuity. The white circle in (h) encloses the TVS likely associated with the E–P tornado. The color-coded maximum VS/TVS GTG ΔV values (m s−1) are listed next to the outlined VS/TVSs. Range rings are every 2 km. Images in (a)–(d) and (e)–(h) are centered at the same location. The approximate center beam height at the location of the VSs–TVSs in (a),(b),(c),(d),(f),(g), and (h) is 2.6, 2.7, 2.8, 2.8, 6.5, 6.6, and 6.9 km ARL, respectively.

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    MWR-05XP radial velocity (m s−1) PPIs at several elevation angles (1.0°–18.5°) at (a) 2146:21, (b) 2148:20, and (c) 2150:36 UTC 5 Jun 2009. The VSs are enclosed by black circles. In (c), the Goshen County TVS at 1.0° elevation angle is enclosed by a white circle. Range rings are every 2 km. All images are centered at the same location. Approximate heights at the center of the domain range from 350 m at 1.0° to 3.5 km at 9.8° to 6.7 km ARL at 18.5° elevation angle.

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    MWR-05XP radial velocity (m s−1) PPI scans at (a) 1.0°, (b) 11.2°, and (c) 20.0° elevation angle at 2157:19 UTC when the TVS associated with the Goshen County tornado (white circles) was identifiable in data from all 14 elevation angles in MWR-05XP data. Range rings are every 1 km. All images are centered at the location of the TVS at that level. The TVS GTG ΔV values (m s−1) are listed next to the outlined TVS. The approximate center beam height at the location of the TVS in (a),(b), and (c) is 0.3, 3.0, and 5.5 km ARL, respectively.

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    MWR-05XP radial velocity (m s−1) PPI scans at (a) 1.0°, (b) 8.3°, (c) 14.1°, and (d) 20.0° elevation angle during the time period that a TVS was first identified at each level in MWR-05XP data. White circles enclose the TVS associated with the Goshen County tornado. Range rings are every 1 km. Images from a particular level are centered at the same location. The TVS GTG ΔV values (m s−1) are listed next to the outlined TVS. The approximate center beam height at the location of the TVS in (a),(b),(c), and (d) is 0.3, 2.3, 4.1, and 5.8 km ARL, respectively.

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    (a) Time–height series of the formation of the TVS associated with the Goshen County tornado using the baseline criteria discussed in the text. The black markers indicate the time and approximate height that the TVS is first identified at each of the 14 elevation angles used in MWR-05XP data collection. The dotted vertical gray line marks the approximate time of tornadogenesis according to data from other mobile Doppler radars. The TVS criteria are changed in (b)–(h). In (b), a TVS must have three consecutive sets of gates (in range) with ΔV > 20 m s−1. In (c) and (d), the TVS ΔV threshold is raised to 25 and 30 m s−1, respectively. In (e) and (f), the maximum amount of time the criteria cannot be met while a signature is still considered a TVS is increased to 60 and 120 s, respectively. Finally, in (g) and (h), the criteria changes in (d),(e) and (d),(f), respectively, are combined.

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    As in Fig. 10, but for the El Reno tornado on 24 May 2011. The approximate center beam height at the location of the TVS in (a),(b),(c), and (d) is 0.3, 3.3, 6.0, and 7.8 km ARL, respectively. The radial velocity scale is different from that used in Fig. 10.

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    As in Fig. 11, but for the El Reno tornado. Note that data collection was up to a 40° elevation angle (26 levels rather than 14 levels) for this case.

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    Vertical profiles of the maximum, GTG differential velocity, ΔV (m s−1), at a few times (corresponding to radar volume scan times) during tornado development for the idealized, empirically determined models on which the (a) descending and (b) nondescending classification is based. The altitude zpeak of the peak differential velocity ΔVpeak within a volume scan, and altitude zlow of the differential velocity ΔVlow at the lowest elevation angle, within the same volume scan, are indicated in (a). (From T99.)

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    Vertical profiles of TVS ΔV in four successive volume scans from the KCYS WSR-88D on 5 Jun 2009 during the formation of the Goshen County tornado. Two additional volume scans, beginning at 2139:10 and 2143:44 UTC, respectively, also were included in the analysis but no vertically or temporally continuous TVSs were identified. TVSs were identified using the T99 criteria.

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    Radial velocity (m s−1) PPI scans from KCYS at (a) 0.55°, (b) 0.94°, (c) 1.38°, (d) 1.86°, (e) 2.48°, (f) 3.16°, (g) 4.05°, (h) 5.14°, (i) 6.46°, (j) 8.05°, and (k) 10.06° elevation angle from the last volume scan before formation of the Goshen County tornado. White rectangles enclose the TVSs that meet the T99 criteria discussed in the text. Black rectangles enclose TVSs with relaxed criteria allowing the ΔV threshold to be met over four gates rather than two. The yellow rectangles enclose TVSs under the relaxed criteria with no height continuity requirement. Dotted yellow rectangles outline the approximate areas where the relaxed-criteria TVS does not display height continuity. The aqua and light green rectangles show the location of the only identified TVSs at 0.55° elevation angle in the next two volume scans, 2153:09 and 2157:43 UTC, respectively; however, the former signature lacked height continuity. The black arrow is the approximate direction of storm motion as used in Markowski et al. (2012a). The white, aqua, and green circles mark the approximate location of the Goshen County TVS from MWR-05XP data at 1.0° elevation angle at 2152:03, 2153:12, and 2157:19 UTC, respectively. Range rings are every 1 km. All images are centered at the same location. The TVS ΔV values (m s−1) are listed next to the outlined TVSs. The approximate center beam height at the center of the domain in (a)–(k) is 0.8, 1.3, 1.7, 2.3, 2.9, 3.7, 4.7, 5.8, 7.2, 9.0, and 11.0 km ARL, respectively.

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    Radial velocity (m s−1) PPI scans (a),(b) at 5.1° elevation angle from the KCYS WSR-88D at 2151:12 and 2155:46 UTC, respectively and (c)–(h) at 18.5° elevation angle from the MWR-05XP at 2151:10, 2151:28, 2151:46, 2152:03, 2152:19, and 2152:36 UTC, respectively during the formation of the Goshen County tornado. White circles enclose WSR-88D TVSs in (a),(b) and a MWR-05XP VS in (c)–(h) that are referenced in the text. The dotted circle in (g) outlines the weakened shear signature (no longer a VS) and (h) outlines the approximate area where the shear signature is located in (g). Red (black) circles enclose an additional VS (areas of transient cyclonic shear) in MWR-05XP data. Range rings are every 1 km. Images in (a),(b) and (c)–(h) are centered at the same location. The GTG ΔV values (m s−1) of the VS/TVSs discussed in the text are listed next to the outlined VS/TVSs. The approximate center beam height at the center of the domain in (a),(b) and (c)–(h) is 5.9 and 6.2 km ARL, respectively.

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    Time series of observed VSs at 18.5° elevation angle in MWR-05XP data from 2151:10 to 2155:46 UTC, a time period spanning successive KCYS scans at 5.1° elevation angle. The TVS associated with the Goshen County tornado is marked by a black square. The dotted vertical gray lines indicate the times that the KCYS 5.1° elevation angle scans began.

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    Time series of observed VSs at (a) 17.1°, (b) 14.1°, and (c) 3.9° elevation angle in MWR-05XP data from 2148:20 to 2157:19 UTC. The time period covers that of two consecutive KCYS volume scans during the formation of the Goshen County tornado. The TVS associated with the Goshen County tornado is marked by a black diamond. VS/TVSs exhibiting vertical continuity use consistent markers and labels. The dotted vertical gray lines indicate the times that the KCYS volume scans began.

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    An illustration, based loosely on the Goshen County supercell, of how enhanced levels of vertical vorticity within a mid- and low-level mesocyclone might appear as a descending incipient tornado in WSR-88D data. The black bar indicates a tornado and the dotted horizontal line marks the level of free convection. Gray (dark green) shading represents mesocyclone-scale (tornadic) vertical vorticity. Light green shading highlights areas (top) of locally enhanced vertical vorticity as discussed in the text and (bottom) where the T99 TVS criteria are met based on the given vertical vorticity distribution. The black dots indicate the approximate center beam locations from a WSR-88D scanning a storm 60 km away using VCP 212.

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Reexamining the Vertical Development of Tornadic Vortex Signatures in Supercells

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  • 1 School of Meteorology, University of Oklahoma, Norman, Oklahoma
  • | 2 ProSensing, Inc., Amherst, Massachusetts
  • | 3 Naval Postgraduate School, Monterey, California
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Abstract

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.

Current affiliation: NOAA/National Severe Storms Laboratory, Norman, Oklahoma.

Corresponding author address: Michael M. French, NOAA/National Severe Storms Laboratory, National Weather Center, 120 David L. Boren Blvd., Norman, OK 73072. E-mail: michael.french@noaa.gov

Abstract

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.

Current affiliation: NOAA/National Severe Storms Laboratory, Norman, Oklahoma.

Corresponding author address: Michael M. French, NOAA/National Severe Storms Laboratory, National Weather Center, 120 David L. Boren Blvd., Norman, OK 73072. E-mail: michael.french@noaa.gov

1. Introduction

It has been surmised that tornadogenesis results when near-surface convergence acts on existing (i.e., prior to the tornado) vertical vorticity, amplifying it to tornadic levels (e.g., Ward 1972). However, less understood is the development of rotation close to the surface prior to its contraction to the tornadic scale as well as the processes leading to the final contraction in some storms and not others (e.g., Davies-Jones 1982; Davies-Jones et al. 2001; Markowski and Richardson 2009). Several mechanisms that would allow for tornado-strength vertical vorticity to reach the surface from aloft have been suggested. For example, a dynamic pipe effect (DPE; Leslie 1971) acting on strong rotation above the surface is one proposed mechanism. In the DPE, there is existing rotation in cyclostrophic balance (i.e., rotating flow behaving like an impermeable pipe) above the surface. At the lower end of the vortex, there is a rotationally induced negative pressure perturbation and associated rising air and convergence. The convergence acts to enhance vertical vorticity at this lower level until it too reaches cyclostrophic balance. The process may continue until a vortex reaches the surface (e.g., Smith and Leslie 1978).

The DPE is of particular interest to operational forecasters. Compared to tornado development in which strong rotation forms first at or near the surface, tornado-scale rotation that forms above the surface first and descends to the ground is more likely to be sampled by the network of Weather Surveillance Radar-1988 Doppler (WSR-88D) because of its initial location at storm midlevels. In addition, the DPE process provides additional lead time for a forecaster to issue a warning prior to tornadogenesis.1 Circumstantial observational evidence of a DPE was presented in the case of the 24 May 1973 Union City, Oklahoma, tornado, in which 25 min prior to tornadogenesis, strong radial wind shear was identified in adjacent azimuths, a feature that initially was named a gate-to-gate tangential shear signature (GGS; Burgess et al. 1975) and eventually a tornadic vortex signature (TVS; Brown et al. 1978). Additional observations and results from radar simulators confirmed that the TVS is consistent with tornado-scale rotation (e.g., Brown and Lemon 1976; Wilson et al. 1980; Dunn 1990; Burgess et al. 1993; Brown and Wood 2012). In the Union City tornado, the TVS was identified first at midlevels; it built downward with time, reaching close to the surface approximately coincident with the time of tornadogenesis (Fig. 1; Brown et al. 1978).

Fig. 1.
Fig. 1.

Time–height profile of the magnitude of the largest Doppler velocity value within each TVS (adjusted for TVS motion). Dots indicate data points and dashed lines represent the limits of data collection. Velocity shears below the TVS detectability level are lightly shaded. The black region at bottom center is the diameter (using ordinate scale) of the Union City tornado funnel near the cloud base. [From Brown et al. (1978).]

Citation: Monthly Weather Review 141, 12; 10.1175/MWR-D-12-00315.1

In observations from Vasiloff (1993), among others, and numerical simulations from Trapp and Fiedler (1995), it was found that TVSs/tornadoes also could form uniformly over a large depth of the atmosphere or ascend, rather than descend, during tornadogenesis. Using WSR-88D data, Trapp et al. (1999, hereafter T99), found that tornadoes were not preceded by a descending TVS in nearly half of their sample of 52 tornadoes. Instead, the TVSs were detected near the surface first and built upward with time or azimuthal shear increased in the lowest levels (or even up to midlevels) “nearly simultaneously.” Other recent examples of TVSs that formed in this manner are found in Collins et al. (2000), Burgess et al. (2002) (a “tornado cyclone signature”), and Dunn and Vasiloff (2001). WSR-88D data were used in the two former cases and Terminal Doppler Weather Radar (TDWR) data (∼2.5-min volumes) were used in the latter case.

Initially, authors expressed uncertainty about the role of the midlevel TVS in supercell tornadogenesis (e.g., Rotunno 1986, 1993). Subsequently, Trapp and Davies-Jones (1997) used numerical and analytical models to propose two different modes of tornadogenesis. In mode I tornadogenesis, an “embryonic” tornado forms 3–4 km aloft and slowly descends to the surface via the DPE. In mode II tornadogenesis, the embryonic tornado forms in the lowest levels rapidly. Neither mesocyclone detection nor mesocyclone strength are credible indicators of tornado formation (e.g., Burgess and Lemon 1990; Trapp et al. 2005), so the lack of a TVS before mode II tornadogenesis occurs inherently makes tornado warning issuance in such cases extremely difficult.

Over the past ∼15 years, the advent of mobile Doppler radar platforms has allowed for the tornadogenesis mode to be investigated using data with higher spatial and temporal resolution. For example, single-Doppler tornadogenesis data from a 3-cm wavelength (X-band) Doppler on Wheels (DOW; Wurman et al. 1997) was briefly discussed in Wurman et al. (2007b). Notable increases in axisymmetric vertical vorticity (AVV) developed suddenly in the lowest 1 km and also in the 4–5-km layer at the time of tornadogenesis. In addition, a recent climatology of high-resolution DOW tornado observations by Alexander (2010) included five additional tornadogenesis cases (one of which also was discussed in Alexander and Wurman 2005). In all five cases, it was found that (i) a rapid, low-level, horizontal scale contraction occurred simultaneously in height2 and (ii) AVV of the vortex signature intensified to tornadic strength in the lowest 2 km. There was no evidence in the six combined cases that a TVS descended to the surface as in mode I tornadogenesis. In other mobile Doppler radar observational studies of tornadogenesis, volumetric data were not analyzed (e.g., Bluestein et al. 2003; Wurman et al. 2007a), so the vertical directionality of tornado-scale rotation development could not be determined.

The recent lack of mode I tornadogenesis cases in the literature may be the result of a small sample of tornadogenesis cases or insufficient storm sampling at mid- and upper levels by mobile Doppler radars. Another possibility is that the lack of mode I tornadogenesis observations results from shortcomings in previous observing platforms used to sample the tornadogenesis process. For example, in discussing mode II tornadogenesis observations made with TDWR data, Vasiloff (2001) noted that the corresponding WSR-88D tornadogenesis data could be interpreted as containing a descending TVS (mode I tornadogenesis). The author commented “[The discrepancy in the mode of tornadogenesis] indicates that the relatively large time gaps between WSR-88D scans may result in errors in determining whether or not tornadoes are descending or nondescending.” Observational data utilizing volumetric, ∼10-s updates (i.e., the advective time scale of a common tornado; Bluestein et al. 2010) of mode I tornadogenesis would establish that previous mode I tornadogenesis observations likely resulted from real processes and were not artifacts of insufficient storm sampling.

The Mobile3 Weather Radar, 2005 X-band, Phased Array (MWR-05XP; Bluestein et al. 2010) is an electronic scanning, ground-based, mobile, X-band Doppler radar. The MWR-05XP scans electronically in elevation and, to a limited extent (a 6°–8° sector), in azimuth. Resulting volumetric data update times from 1° to 20° or 1° to 40° in elevation angles are ∼(5–15) s and data from a given azimuth are collected almost simultaneously in elevation. Since 2007, the MWR-05XP has been used to obtain volumetric data in supercells and tornadoes. In three cases, volumetric data were obtained in supercells while tornadogenesis was occurring.4 The manner in which the MWR-05XP obtains data allows for the height evolution of the radial velocity field during tornadogenesis to be explored every ∼10 s up to storm midlevels. As a result, the data can be used to examine whether mode I or mode II tornadogenesis was occurring. Other examples of “rapid scan” weather radars include the stationary National Weather Radar Testbed (Zrnic et al. 2007) and two mobile systems: the Rapid-Scan DOW and the Rapid-Scanning, X-band, Polarimetric Doppler radar (RaXPol; Pazmany et al. 2013).

In section 2 of this study, the MWR-05XP and the datasets under examination are briefly described. Section 3 includes detailed observations of tornadogenesis in three cases using data from the MWR-05XP. In section 4, the radar observations are summarized and put in the context of previous observations of tornadogenesis to motivate further discussion and examination of the mode I tornadogenesis process.

2. Data

a. The MWR-05XP

Data for this study were obtained by the MWR-05XP in the springs of 2008, 2009, and 2011. The MWR-05XP utilizes a hybrid antenna with electronic scanning in elevation and both electronic and mechanical scanning in azimuth. In 2008, to attain the dwell time necessary to gather enough independent samples, the azimuthal rotation rate was decreased such that the antenna rotated ∼1° during the time samples were obtained at each elevation angle. Back scanning, electronically scanning in the opposite direction of mechanical scanning, was implemented in 2009 to allow the radar system enough time to collect independent samples at each azimuth utilizing faster antenna rotation rates. At least 10 independent samples were obtained for all datasets used in this study.

A radar center frequency near 9.5 GHz was used in data collection. The peak power of the MWR-05XP is ∼15 kW. The pulse duration (τ) is ∼1 μs, so the gate length is 150 m oversampled by a factor of 2. The half-power beamwidth (sampling interval) is 1.8° (1.5°) in azimuth and 2.0° (1.5°) in elevation. The pulse repetition frequency (PRF) can be varied within deployments, but typically is 2500–5000 Hz. Common maximum unambiguous ranges are 30, 45, or 60 km and Nyquist velocities are 20–37 m s−1.

The greatest strength of the MWR-05XP is its ability to collect rapid, volumetric updates of weather targets with a sufficient number of independent samples. For the datasets used in this study, volumetric update times are O(10 s) for elevation angle ranges of 1°–20° or 1°–40°, which represents a 6–8-km-thick layer for the datasets examined. However, the spatial resolution of the MWR-05XP is coarse compared to other Doppler, weather radars, fixed or mobile. At typical ranges of 5 (15) km, the MWR-05XP azimuthal gate spacing is ∼160 (∼470) m, so tornado core flow is only well resolved in very large (>1.5-km diameter) tornadoes (Carbone et al. 1985). The MWR-05XP still can be used to assess the bulk properties of tornadoes by assessing azimuthal shear signatures, as is commonly done using WSR-88D data (e.g., Brown et al. 1978; T99).

Other data problems are common to X-band radar systems, including signal attenuation, velocity aliasing, and occasional beam smearing. Also, in 2008–11, the MWR-05XP did not have levelers. In an error analysis performed by French (2012), vertical positional errors were found to be ±750 m maximum for a truck roll angle of ±3° and echoes within 20 km of the truck. It is not necessary to know the exact heights of MWR-05XP radar observations for the analyses performed herein. Rather, the conclusions that are made depend on the differences between observations made at several levels. To mitigate the effects of using data from an unleveled radar, observations from below 2.5 km above radar level (ARL; hereafter, all heights given in the text are approximate values ARL) and above 4.5 km will be emphasized for storm low and midlevels, respectively. For additional information about the MWR-05XP, see Bluestein et al. (2010). For a complete positional error analysis, see French (2012).

b. Datasets

ProSensing, Inc., provided reflectivity and radial velocity plan position indicator (PPI) data in Universal Format corrected for pointing angle. Data were translated to DORADE format and viewed and edited using the Solo-II software package (Oye et al. 1995). The DORADE Radar Editing Algorithms, Detection, Extraction, and Retrieval (DREADER) software was used to remove noise and isolated data points. Ground clutter was not observed in the datasets at the approximate locations of tornadogenesis. Velocity aliasing was present in most of the data used in this study. Automatic dealiasing algorithms performed poorly in areas of tight radial velocity gradients (i.e., tornadoes), so most scans were manually dealiased.

Details about the three datasets used in this study are provided in Table 1. The formation of tornadoes from (i) 23 May 2008 in Ellis County, Kansas; (ii) 5 June 2009 in Goshen County, Wyoming; and (iii) 24 May 2011 near El Reno, Oklahoma was investigated. For the first dataset, the MWR-05XP was located about 15 km north of Hays, Kansas. The storm was large in areal extent and was the result of the merger of two supercells (Figs. 2a,b). The low-level mesocyclone and associated hook echo in the southern flank of the combined storm tracked northeastward passing ∼10 km west of the radar location. The entirety of the deployment took place at night, so there were no visual sightings of any tornadoes by the MWR-05XP crew. There are no photographs of the radar at the deployment site, but, in the recollection of the radar team, the radar was relatively level and located in the middle of a hybrid dirt-paved road. The tornado formed northwest of the radar between Ellis and Plainville, Kansas (hereafter the E–P tornado), completely embedded in precipitation in the northern part of the combined storm.

Table 1.

Summary of several characteristics of MWR-05XP datasets used in this study. The tornado durations are estimates from Storm Data except for the formation time of the Goshen County tornado, which was altered to match detailed VORTEX2 data. A second deployment during the El Reno tornado on 24 May 2011 is not included.

Table 1.
Fig. 2.
Fig. 2.

Reflectivity (dBZ) from the WSR-88D at 0.55° elevation angle in Dodge City, KS, at (a) 0021 and (b) 0149 UTC 24 May 2008; in Cheyenne, WY, at (c) 2143 and (d) 2229 UTC 5 Jun 2009; and near Oklahoma City, OK, at (e) 2049 and (b) 2057 UTC 24 May 2011. In (b)–(f), the location of the MWR-05XP during its deployments is indicated by a white circle [in (a), the deployment had not yet begun]. Range rings are every 10 km. In (a)–(b), (c)–(d), and (e)–(f) the images are centered at the same location.

Citation: Monthly Weather Review 141, 12; 10.1175/MWR-D-12-00315.1

The second dataset was obtained on 5 June 2009, during year one of the Second Verification of the Origins of Rotation in Tornadoes Experiment (VORTEX2; Wurman et al. 2012). The complete life cycle of a tornado (Goshen County, hereafter the GC tornado; Figs. 2c,d) was captured by the MWR-05XP. The deployment location was a paved road leading to an abandoned missile silo. The road was relatively level and the deployment location was one of the better sites encountered by the MWR-05XP crew in 2009 (Fig. 3a). Several other studies have examined the GC supercell (e.g., Wakimoto et al. 2011; Markowski et al. 2012a,b; Atkins et al. 2012; Wurman et al. 2012; 2013; Kosiba et al. 2013). However, the MWR-05XP dataset is unique in that it contains the only rapid-scan radar data of the GC supercell obtained during tornadogenesis.

Fig. 3.
Fig. 3.

The MWR-05XP (a) scanning the Goshen County, WY, tornado at 2201 UTC 5 Jun 2009 and (b) being set up prior to it obtaining data of the El Reno tornado at 2041 UTC 24 May 2011. (Both photographs © Michael French.)

Citation: Monthly Weather Review 141, 12; 10.1175/MWR-D-12-00315.1

The third dataset examined was collected on 24 May 2011 near El Reno. The MWR-05XP began data collection around the time a violent tornado formed (Fig. 2e) and the deployment was shortened because of impending heavy precipitation (Fig. 2f). During data collection, the truck was located on the east side of a paved road that again was approximately level (Fig. 3b). Data of the El Reno tornado also were obtained by the companion RaXPol, in which radial velocities over 115 m s−1 were observed associated with the tornado (Pazmany et al. 2013).

3. Observations of tornadogenesis

The main question addressed in this section is “Is there evidence in MWR-05XP volumetric data of midlevel TVSs that descended to the surface at the times of tornadogenesis?” Objective criteria were established to differentiate between a vortex signature (VS) and a TVS so that a TVS “origin” time could be determined at each level. The TVS origin times were then used to determine the vertical directionality of TVS development. To be categorized as a TVS, a cyclonic shear signature first had to match up with the time and location of a known tornado (i.e., from damage surveys and visual sightings). Then, the radial velocity field within the shear signature had to contain (i) gate-to-gate (GTG) ΔV ≥ 20 m s−1 (in which ΔV is the largest magnitude GTG difference between the maximum and minimum radial velocities in the shear signature) and (ii) local maxima–minima (in range or azimuth) over a diameter no greater than 2 km. Criterion (ii) was used to ensure better that the radial wind shear thresholds met in (i) resulted from rotation rather than deformation (i.e., at a minimum, there was a radial velocity “couplet”). Next, a scan time was established in which the TVS criteria were met at all levels in MWR-05XP data. Last, in data from each elevation angle, the TVS origin time was determined by working backward from this scan time until the shear signature no longer met the TVS criteria for at least 30 s continuously.5 Any VSs referenced in the text met all of the TVS criteria except for the requirement that they be associated with a known tornado.

The TVS criteria are based on those previously used in T99 and Alexander (2010), but with modifications. The low GTG ΔV threshold is similar to the 15 m s−1 GTG ΔV cutoff used in T99 and is reflective of the relatively coarse spatial resolution of the MWR-05XP data (i.e., in that tornadoes were not well resolved). In addition, as in T99, the primary focus here is in determining how TVSs developed rather than whether TVSs existed. In T99, a TVS was required to meet the ΔV threshold in three range-consecutive sets of gates. This requirement was not adopted here because median tornado diameters are only ∼300 m (Alexander 2010), and small tornadoes might be overlooked in the analysis. Other studies utilizing mobile Doppler radar data (e.g., Marquis et al. 2012; Kosiba et al. 2013) have used a minimum ΔV of 40 m s−1 over a diameter less than 2 km as a vortex criterion, as established in Alexander (2010). However, the data used in those studies came from systems with a radar beamwidth half that of the MWR-05XP (0.9° vs 1.8°, respectively) and azimuthal sampling typically five times as fine (0.3° vs 1.5°, respectively). Increasing radar spatial degradation of a tornado signature is associated with lower maximum TVS radial velocities and ΔVs (e.g., Brown et al. 1978; Wood and Brown 1997), so a lower ΔV threshold than the 40 m s−1 threshold used in those studies is warranted here. In addition, the use of GTG ΔV precludes use of AVV (twice the maximum ΔV divided by the distance between the maximum and minimum radial velocities) as a TVS criterion because the diameter of a TVS is an indeterminable overestimate of the actual vortex diameter (e.g., Brown et al. 1978).

The criteria above allowed for a measure of objectivity to be used in determining when vortices began. However, though objectively defined and based on past work, the criteria are still imperfect indicators of tornadic rotation. For example, the use of these criteria is not an indication that ΔV ≥ 20 m s−1 definitely indicates a tornado. Therefore, the criteria as defined above are referred to here as the “baseline” criteria and an emphasis will be placed on the impact that the exact criteria have on the analysis results when discussing the vertical directionality of TVS formation.

a. Ellis–Plainville tornado on 23 May 2008

The E–P tornado was rated EF1 based on damage to some trees and power lines. The tornado was estimated as beginning at 0208 UTC (hereafter all times in UTC) in Storm Data. The analysis of the mode of TVS formation is somewhat qualitative for the E–P tornado because of major attenuation in volumetric MWR-05XP data, which adversely affected data quality at some levels. The progression of the radial velocity field prior to, during, and after 0208 is shown at heights of 300 m (1.0° elevation angle; Fig. 4), 2.5 km (9.8° elevation angle; Fig. 5, left), and 6 km (17.1° elevation angle; Fig. 5, right). At 0152:03, there were remnants of a near-ground, shallow anticyclonic vortex as well as broad, weak cyclonic shear (Fig. 4a). Cyclonic rotation steadily increased after 0150 (Fig. 4b) until a VS could be identified by 0157:56 (Fig. 4c). The VS subsequently dissipated at ∼0202 (not shown) and a second VS had formed by 0201:01 (Fig. 4d; see below). Ground-relative radial velocities had reached over 40 m s−1 by 0204:05 (Fig. 4e,f) as the second VS became larger and stronger. Radial velocities subsequently decreased but were still strong (Figs. 4g,h) before the second VS moved westward out of view of the radar.

Fig. 4.
Fig. 4.

A series of MWR-05XP radial velocity (m s−1) PPI scans at 1.0° elevation angle before, during, and after the formation of the E–P tornado at (a) 0152:03, (b) 0154:53, (c) 0157:56, (d) 0201:01, (e) 0204:05, (f) 0206:55, (g) 0210:01, and (h) 0213:04 UTC 24 May 2008. White circles enclose the TVS associated with the E–P tornado. The blue circle in (a) encloses an anticyclonic VS. The black circles in (c),(d) enclose a VS that dissipated at ∼0202 UTC and was not associated with the tornado. The color-coded maximum VS/TVS GTG ΔV values (m s−1) are listed next to the outlined VS/TVSs. Range rings are every 5 km. All images are centered at the same location. The approximate center beam height at the location of the VS/TVS in (a),(c), and (g) is 300, 250, and 300 m ARL, respectively.

Citation: Monthly Weather Review 141, 12; 10.1175/MWR-D-12-00315.1

Fig. 5.
Fig. 5.

As in Fig. 4, but at (a) 0154:53, (b) 0159:49, (c) 0204:48, and (d) 0209:45 UTC at (left) 9.8° and (right) 17.1° elevation angle. The black and red circles enclose VSs that were not associated with the E–P tornado. The approximate center beam height at the location of the VS in (b) at 9.8° elevation angle is 2.6 km ARL and in (d) at 17.1° elevation angle is 6.1 km ARL. Note the different radial velocity scale from that used in Fig. 4.

Citation: Monthly Weather Review 141, 12; 10.1175/MWR-D-12-00315.1

It is not known if the first cyclonic VS was associated with a tornado, so its vertical development is not studied here. However, the location of the second VS was consistent with that from the surveyed E–P tornado. In addition, the radial winds within the second VS at 300 m were at EF1 strength within 5 min of the tornadogenesis time estimated in the damage survey. Despite the lack of visual sightings, the collocation of surveyed damage with radar data provides confidence that the second VS was associated with the E–P tornado (hereafter E–P TVS). The formation of the E–P TVS by ∼0201 and the large, near-surface radial velocities that developed at ∼0204 (Fig. 4e) are circumstantial evidence that tornadogenesis occurred between 0200 and 0204.

Data from heights of 2.5 and 6 km are shown to illustrate the progression of the radial velocity field aloft in the storm (Fig. 5). Even 10 min before tornadogenesis, there was only broad cyclonic shear aloft (Fig. 5a). By 0159:49 (Fig. 5b), shear had increased notably and there was a VS at 2.5 km, but only mesocyclone-scale cyclonic rotation at 6 km. As with the first cyclonic VS at 300 m, the VS at 2.5 km had dissipated by 0202 and was not associated with the E–P tornado. After 0204, and the likely time of tornadogenesis, there was no identifiable cyclonic shear at 2.5 km and still only broad rotation at 6 km (Fig. 5c). Several minutes later (Fig. 5d), there was a VS at 6 km, but again no identifiable cyclonic shear at 2.5 km. The lack of any cyclonic shear signatures at 2.5 km likely was caused by data loss from attenuation. Shortly after 0204, any VSs were located out of the maximum range of usable reflectivity and radial velocity data from 5.4° to 15.6° elevation angle (e.g., Figs. 5c,d, left). Nonetheless, there were no VSs identified in data from 2.5 km associated with the E–P tornado between 0200 and 0204.

Despite the missing data, TVS development was examined in the lowest 1.5 km using MWR-05XP data from 1.0°, 2.5°, 3.9°, and 5.4° elevation angle. At 0204:05, a vertically continuous TVS was identified at these levels (e.g., Fig. 4e). After 0204, data quality worsened considerably at 5.4° elevation angle, but the TVS still could be followed at the other three levels, so it is likely that the cyclonic shear signature was representative of the tornado at this level. Working backward from 0204:05, the approximate TVS formation time was determined at the four levels (Fig. 6a). The TVS formed at ∼0201 in the lowest 1.5 km and the approximate time of formation was not sensitive to the exact TVS criteria (not shown). The TVS was identifiable after 0201 in the lowest three levels, a time during which TVS ΔV steadily increased (Fig. 6b). Also, there was a general decrease in the distance between the maximum and minimum radial velocities (Δx) in the mesocyclone–tornado cyclone (Fig. 6c), consistent with a scale contraction in the lowest 1 km.

Fig. 6.
Fig. 6.

(a) Time–height series of the formation of the E–P TVS using the baseline criteria discussed in the text, (b) time series of maximum ΔV (m s−1) in the E–P TVS, and (c) the distance (km) between the maximum inbound and outbound radial velocities in the E–P mesocyclone–tornado cyclone from 0159:49 to 0208:48 UTC at 1.0°, 2.5°, and 3.9° elevation angle. In (a), the time of TVS formation in data from 5.4° elevation angle also is shown. In (b) and (c), a simple 1–2–1 filter in time was applied to smooth the curves. Approximate beam heights at the TVS center are indicated in the top-left-hand corner of (b).

Citation: Monthly Weather Review 141, 12; 10.1175/MWR-D-12-00315.1

The lack of data from 1.5 to 6 km prevents an exact determination of the vertical directionality of TVS formation at these levels. Instead, the time of TVS formation near the surface (∼0201) is emphasized, as it was at these times that there were data available from 1.5 to 6.0 km. During tornadogenesis in the lowest 1.5 km (Fig. 6a), no consistent TVS was observed in data from 1.5 to 6 km (e.g., Figs. 7a–d). At and above 6 km, where there were continuous data, there was observed broad cyclonic shear likely indicative of a midlevel mesocyclone within which there were several transient VSs (e.g., Figs. 7f,g). After 0210, a consistent TVS was identified in data from 17.1° to 20.0° elevation angle (e.g., Fig. 7h) until data also were lost at these levels at ∼0220 (not shown). In summary, there was no continuous TVS observed from 1.5 to 6 km at the TVS origin time in the lowest 1.5 km and there was no consistent TVS observed above 6 km in MWR-05XP data until several minutes after TVS development occurred in the lowest 1.5 km. As a result, there is no evidence that mode I tornadogenesis occurred in this case.

Fig. 7.
Fig. 7.

MWR-05XP radial velocity (m s−1) PPI scans at 9.8° elevation angle at (a) 0159:08, (b) 0200:05, (c) 0201:01, and (d) 0202:11 UTC and at 18.5° elevation angle at (e) 0200:05, (f) 0203:36, (g) 0206:55, and (h) 0210:041 UTC 24 May 2008. Circles enclose VSs that formed and dissipated during the time the E–P TVS was continuously identifiable in data from the lowest ∼1 km. Each VS is outlined in a different colored circle to highlight any VS temporal continuity. The white circle in (h) encloses the TVS likely associated with the E–P tornado. The color-coded maximum VS/TVS GTG ΔV values (m s−1) are listed next to the outlined VS/TVSs. Range rings are every 2 km. Images in (a)–(d) and (e)–(h) are centered at the same location. The approximate center beam height at the location of the VSs–TVSs in (a),(b),(c),(d),(f),(g), and (h) is 2.6, 2.7, 2.8, 2.8, 6.5, 6.6, and 6.9 km ARL, respectively.

Citation: Monthly Weather Review 141, 12; 10.1175/MWR-D-12-00315.1

b. Goshen County tornado on 5 June 2009

The GC tornado was rated EF2 by the NWS based on a damage survey, though the rapid-scan DOW recorded near ground radial velocities in excess of 70 m s−1 (Kosiba et al. 2013). A funnel cloud near the ground was not observed until ∼2202. However, data of the vortex from the higher-spatial-resolution DOW6–7 reached the Alexander (2010) ΔV tornado thresholds about 10 min before a condensation funnel formed, at ∼2152 (e.g., Wakimoto et al. 2011; Kosiba et al. 2013). In studying tornadogenesis, it is assumed that tornado formation occurred at 2152. After 2200, a near-ground cyclonic shear signature in MWR-05XP data was collocated with the visible funnel and tracked for ∼30 min.

An overview of what the radial velocity field looked like prior to tornado formation is shown in Fig. 8. Regions in which cyclonic shear meets the VS requirements are outlined in black circles. At 2146:21 (Fig. 8a), there was no obvious low-level mesocyclone, perhaps owing to a lack of scatterers in the region (a low-level mesocyclone was identified after 2130 in dual-Doppler analyses; Markowski et al. 2012a). Cyclonic shear indicative of a midlevel mesocyclone is seen in data from higher elevation angles, and several VSs are identified. At 2148:20, there was broad rotation in MWR-05XP data in the lowest 3 km, but there were no VSs (Fig. 8b). Aloft (above 3 km), there was broad, strong cyclonic shear and vertically continuous VSs. However, the VSs were almost all different from the ones shown 2 min earlier, the latter of which all had dissipated.

Fig. 8.
Fig. 8.

MWR-05XP radial velocity (m s−1) PPIs at several elevation angles (1.0°–18.5°) at (a) 2146:21, (b) 2148:20, and (c) 2150:36 UTC 5 Jun 2009. The VSs are enclosed by black circles. In (c), the Goshen County TVS at 1.0° elevation angle is enclosed by a white circle. Range rings are every 2 km. All images are centered at the same location. Approximate heights at the center of the domain range from 350 m at 1.0° to 3.5 km at 9.8° to 6.7 km ARL at 18.5° elevation angle.

Citation: Monthly Weather Review 141, 12; 10.1175/MWR-D-12-00315.1

Approximately 90 s prior to the estimated time of tornadogenesis (2150:36; Fig. 8c), there was increased broad, cyclonic shear and there were intermittent VSs in the lowest 3 km. At 350 m, the GC TVS is identified using the baseline criteria (white circle in Fig. 8c). Also, from 3 to 7 km, many of the VSs identified are the same as those from 2 min prior (Fig. 8b). However, above the lowest-observed level, none of the VSs shown in Fig. 8c were associated with the GC tornado (see below). In this case, VSs became more frequent closer to the ground as tornadogenesis neared, but the life span of individual VSs, as determined by the objective criteria, was highly variable at all levels. During the tornadogenesis period, mesocyclone ΔV generally increased, and a low-level scale contraction was observed (∼2154; not shown).

MWR-05XP data of the TVS in the area of the tornado were obtained at all 14 elevation angles every ∼9 s during the tornadogenesis period, so the vertical directionality of TVS formation can be determined. At 2157:19, there was a TVS in data from all levels scanned by the MWR-05XP (e.g., Fig. 9). Subsequently, MWR-05XP data collection ceased for ∼2.5 min to change the scanning sector. In addition, the tornado temporarily weakened at low levels after 2157 (cf. Fig. 6a in Kosiba et al. 2013), which could have adversely affected the TVS origin times determined using the baseline criteria. Working backward in time from 2157:19, the location and strength (ΔV) of the TVS were recorded at each level until the shear signature no longer met the criteria for a TVS (e.g., Fig. 10). At most levels, the TVS origin occurred when the ΔV cutoff was not met within local velocity maxima–minima. A time–height series of the formation of the vortex signature using the earliest TVS observation at each level is shown (Fig. 11a). The TVS is identified first at the lowest-observed level and then at progressively higher levels. The upward progression of the TVS initially was slow, but occurred much faster upward above 1 km. The TVS took about 4.5 min to reach from 300 m to 5.8 km in height, but only ∼100 s to reach from 1.1 to 5.8 km. The first identification of the TVS at the lowest-observed level occurred ∼90 s prior to the estimated tornadogenesis time (e.g., Fig. 8c).

Fig. 9.
Fig. 9.

MWR-05XP radial velocity (m s−1) PPI scans at (a) 1.0°, (b) 11.2°, and (c) 20.0° elevation angle at 2157:19 UTC when the TVS associated with the Goshen County tornado (white circles) was identifiable in data from all 14 elevation angles in MWR-05XP data. Range rings are every 1 km. All images are centered at the location of the TVS at that level. The TVS GTG ΔV values (m s−1) are listed next to the outlined TVS. The approximate center beam height at the location of the TVS in (a),(b), and (c) is 0.3, 3.0, and 5.5 km ARL, respectively.

Citation: Monthly Weather Review 141, 12; 10.1175/MWR-D-12-00315.1

Fig. 10.
Fig. 10.

MWR-05XP radial velocity (m s−1) PPI scans at (a) 1.0°, (b) 8.3°, (c) 14.1°, and (d) 20.0° elevation angle during the time period that a TVS was first identified at each level in MWR-05XP data. White circles enclose the TVS associated with the Goshen County tornado. Range rings are every 1 km. Images from a particular level are centered at the same location. The TVS GTG ΔV values (m s−1) are listed next to the outlined TVS. The approximate center beam height at the location of the TVS in (a),(b),(c), and (d) is 0.3, 2.3, 4.1, and 5.8 km ARL, respectively.

Citation: Monthly Weather Review 141, 12; 10.1175/MWR-D-12-00315.1

Fig. 11.
Fig. 11.

(a) Time–height series of the formation of the TVS associated with the Goshen County tornado using the baseline criteria discussed in the text. The black markers indicate the time and approximate height that the TVS is first identified at each of the 14 elevation angles used in MWR-05XP data collection. The dotted vertical gray line marks the approximate time of tornadogenesis according to data from other mobile Doppler radars. The TVS criteria are changed in (b)–(h). In (b), a TVS must have three consecutive sets of gates (in range) with ΔV > 20 m s−1. In (c) and (d), the TVS ΔV threshold is raised to 25 and 30 m s−1, respectively. In (e) and (f), the maximum amount of time the criteria cannot be met while a signature is still considered a TVS is increased to 60 and 120 s, respectively. Finally, in (g) and (h), the criteria changes in (d),(e) and (d),(f), respectively, are combined.

Citation: Monthly Weather Review 141, 12; 10.1175/MWR-D-12-00315.1

To gauge the sensitivity of the analysis to the baseline TVS criteria, the time series was analyzed using different TVS requirements (Figs. 11b–h). Requiring a TVS to have three consecutive sets of gates with GTG ΔV ≥ 20 m s−1 (Fig. 11b), similar to the TVS criteria used for WSR-88D data in T99, pushed upward progression of TVS formation later in time, particularly at the lowest levels. More stringent ΔV cutoffs of 25 (Fig. 11c) and 30 (Fig. 11d) m s−1 also shifted the upward progression of TVS formation later in time at most levels. Increasing the length of time the criteria cannot be met and the signature still be considered a TVS (Figs. 11e,f) had little effect on the analysis. Finally, combining the more stringent ΔV cutoffs with the more lenient time cutoffs (Figs. 11g,h) resulted in later TVS formation times in the lowest 4 km. In all iterations of the time series, the TVS associated with the GC tornado is identified first close to the surface and last at midlevels, reaching up to 5.8 km in 2.5–4.5 min. The relatively close agreement between the initial TVS identification times from all eight time series and the estimated tornadogenesis time from other mobile Doppler radars provides increased confidence that the methodology employed here to determine the vertical directionality of TVS formation is appropriate. For pertinent details on factors that may have specifically led to tornadogenesis occurring in the GC supercell, see Kosiba et al. (2013).

c. El Reno tornado on 24 May 2011

Based on a damage survey and radar observations from RaXPol, the El Reno tornado was rated EF5 and estimated to have formed at 2050. MWR-05XP data collection began at 2049:30, so there are no volumetric data of the pretornadogenesis phase of the supercell. As a result, only the vertical directionality of TVS development was examined. A strong, near-ground cyclonic shear signature in MWR-05XP data was identified at the approximate location where the El Reno tornado was observed by the RaXPol team and where damage was surveyed. The first MWR-05XP deployment ended at 2056:04, a time at which the TVS associated with the tornado could be identified in data from all 26 elevation angles used in data collection (1°–40° every 1.5°; not shown).

The TVS was followed backward in time from 2056:04 for all 26 levels until the criteria were not met at each level (e.g., Fig. 12). In a time series of the TVS origin time and height (Fig. 13a), it again can be seen that the estimated origin time is later at higher levels, which is evidence of upward TVS development. However, in this case, in the lowest 2.5 km, the TVS was followed backward in time until the beginning of the deployment,6 so the vertical directionality of TVS development could not be determined at the lowest levels. Nonetheless, at 4–6 (6–8) km, the TVS formed at least 1–3 (3–5) min after formation in the lowest 2.5 km. Time series with the same altered criteria as those used for the GC tornado (Figs. 13b–h) all similarly show upward TVS development taking 4–6 min to reach close to 8 km, but shifted later or earlier in time to varying degrees depending on whether the TVS criteria are more or less stringent. In the lowest 5 km, the TVS origin time occurred when the baseline ΔV criterion was met but distinct maxima–minima could not be identified in a large area of locally enhanced inbound flow (e.g., Fig. 12b). Above 5 km, the TVS origin time occurred when the ΔV threshold was not met (e.g., Fig. 12d). Again, there is close agreement at low levels between the TVS origin time and the estimated tornadogenesis time, though this may simply be indicative of a data collection period that began only ∼30 s before the estimated time of tornadogenesis.

Fig. 12.
Fig. 12.

As in Fig. 10, but for the El Reno tornado on 24 May 2011. The approximate center beam height at the location of the TVS in (a),(b),(c), and (d) is 0.3, 3.3, 6.0, and 7.8 km ARL, respectively. The radial velocity scale is different from that used in Fig. 10.

Citation: Monthly Weather Review 141, 12; 10.1175/MWR-D-12-00315.1

Fig. 13.
Fig. 13.

As in Fig. 11, but for the El Reno tornado. Note that data collection was up to a 40° elevation angle (26 levels rather than 14 levels) for this case.

Citation: Monthly Weather Review 141, 12; 10.1175/MWR-D-12-00315.1

4. Summary and discussion

Rapidly-updating volumetric data of the supercell tornadogenesis process were analyzed for three cases: 23 May 2008, 5 June 2009, and 24 May 2011. There is no evidence in any of the datasets that a TVS built downward from midlevels in the storm prior to or at the time of tornadogenesis. At the approximate formation time of the E–P tornado, there was a consistent TVS in data only at the lowest-observed levels. Furthermore, a rapid low-level-scale contraction was observed (Fig. 6c), consistent with the nondescending tornadogenesis cases discussed in Alexander (2010). In the GC and El Reno supercells, the objectively defined TVS built upward with time during tornadogenesis (Figs. 11 and 13). There are now at least nine documented cases utilizing high spatial and/or temporal resolution mobile Doppler radar data of tornado formation; the mode I tornadogenesis process was not observed in any of them.7 However, despite the three additional cases investigated here, the sample size of volumetric mobile Doppler radar cases of tornadogenesis is still far too small to determine whether the relative frequency of mode I tornadogenesis is different from that determined in T99. Nonetheless, if high temporal resolution data of tornadogenesis are available, it is worth examining whether corresponding WSR-88D data have the volumetric update times necessary to determine accurately the mode of tornadogenesis for those cases.

T99 presented objective criteria to determine whether tornadogenesis was considered descending or nondescending using single-Doppler WSR-88D data. TVSs had to meet the criteria discussed in section 3 and were manually identified to assure that their approximate location matched that of damage from confirmed tornadoes. A parameter was developed that used the TVS data to quantify the difference between the authors' empirical models of mode I (Fig. 14a) and mode II (Fig. 14b) tornadogenesis. The parameter is defined as
e1
where ΔVpeak is the largest TVS ΔV within a volume, occurring at height zpeak. The quantity ΔVlow is the maximum ΔV at the lowest elevation angle in which a TVS was identified; the height of that ΔV observation is zlow. A TVS was said to be descending if
  1. S ≥ 2.25 m s−1 km−1 and

  2. zpeak ≥ 3.0 km

in at least one volume scan prior to tornadogenesis or else the TVS was categorized as nondescending.
Fig. 14.
Fig. 14.

Vertical profiles of the maximum, GTG differential velocity, ΔV (m s−1), at a few times (corresponding to radar volume scan times) during tornado development for the idealized, empirically determined models on which the (a) descending and (b) nondescending classification is based. The altitude zpeak of the peak differential velocity ΔVpeak within a volume scan, and altitude zlow of the differential velocity ΔVlow at the lowest elevation angle, within the same volume scan, are indicated in (a). (From T99.)

Citation: Monthly Weather Review 141, 12; 10.1175/MWR-D-12-00315.1

To evaluate the utility of WSR-88D data in determining the tornadogenesis mode for the cases discussed in section 3, pretornadogenesis WSR-88D data of the E–P and GC supercells were examined (no comparative MWR-05XP pretornadogenesis data were obtained in the El Reno supercell).8 In both cases, evidence (MWR-05XP data) was presented that tornadoes developed first near the surface and then built upward with time. For the E–P tornado, inspection of the WSR-88D data was affected by poor data quality. The closest radars (KDDC and KUEX) both were over 150 km away from the approximate location of tornadogenesis, so there were no near-ground radar observations, azimuthal spatial resolution was coarse, and midlevel radial velocities were noisy (not shown). As a result, WSR-88D data during the formation of the E–P tornado were not analyzed either. Similarly, T99 did not analyze cases in which tornadoes were detected beyond 150 km in range from the radar in question.

In contrast, the GC supercell was located ∼60 km away from KCYS during tornadogenesis. Radial velocity data from three volume scans both before and after tornadogenesis were inspected.9 At each level, it was first determined if there was a qualifying TVS using the same criteria as those used in T99.10 If there was a TVS, then ΔV values were recorded along with approximate center beam heights. The progression of the TVSs identified in the six volume scans is shown in Fig. 15. In volume scans beginning at 2139:10 and 2143:44, no TVSs were identified (a midlevel TVS was observed several minutes earlier); the ΔV threshold occasionally was met, but shear signatures were isolated and lacked time and height continuity (not shown). In the subsequent four volumes, including the one immediately preceding tornadogenesis (2148:17), the maximum TVS ΔV was found at midlevels. In the lowest 4 km, there initially was either no TVS or a TVS with a relatively small ΔV. As a result, the progression of the TVS ΔV field more closely resembles the T99 empirical model of mode I (descending) tornadogenesis (Fig. 14a) rather than that for mode II tornadogenesis (Fig. 14b).

Fig. 15.
Fig. 15.

Vertical profiles of TVS ΔV in four successive volume scans from the KCYS WSR-88D on 5 Jun 2009 during the formation of the Goshen County tornado. Two additional volume scans, beginning at 2139:10 and 2143:44 UTC, respectively, also were included in the analysis but no vertically or temporally continuous TVSs were identified. TVSs were identified using the T99 criteria.

Citation: Monthly Weather Review 141, 12; 10.1175/MWR-D-12-00315.1

To evaluate the above comparison numerically, the S parameter was calculated using the volume scan beginning at 2148:17 (Fig. 16). The T99 TVS criteria were not met in data from 0.55° elevation angle (no height continuity, Fig. 16a) nor from 0.94°–1.86° and above 8.05° elevation angle (ΔV threshold not met; Figs. 16b–d,k). TVSs were identified in data from 2.48°–8.05° elevation angle (Figs. 16e–j, white rectangles). The maximum TVS ΔV was 45 m s−1 at 4.05° elevation angle (height of 4.7 km) and the lowest TVS ΔV was 20.5 m s−1 at 2.48° elevation angle (height of 3 km). Using the above observations in Eq. (1) gives an S value of ∼14.4 m s−1 km−1 and a zpeak of 4.7 km, indicating a descending TVS. Furthermore, including the TVSs that lacked height continuity at 0.55° elevation angle (maximum ΔV of 34.5 m s−1) in the analysis does not change the underlying result (S of ∼2.7 m s−1 km−1, same zpeak).

Fig. 16.
Fig. 16.

Radial velocity (m s−1) PPI scans from KCYS at (a) 0.55°, (b) 0.94°, (c) 1.38°, (d) 1.86°, (e) 2.48°, (f) 3.16°, (g) 4.05°, (h) 5.14°, (i) 6.46°, (j) 8.05°, and (k) 10.06° elevation angle from the last volume scan before formation of the Goshen County tornado. White rectangles enclose the TVSs that meet the T99 criteria discussed in the text. Black rectangles enclose TVSs with relaxed criteria allowing the ΔV threshold to be met over four gates rather than two. The yellow rectangles enclose TVSs under the relaxed criteria with no height continuity requirement. Dotted yellow rectangles outline the approximate areas where the relaxed-criteria TVS does not display height continuity. The aqua and light green rectangles show the location of the only identified TVSs at 0.55° elevation angle in the next two volume scans, 2153:09 and 2157:43 UTC, respectively; however, the former signature lacked height continuity. The black arrow is the approximate direction of storm motion as used in Markowski et al. (2012a). The white, aqua, and green circles mark the approximate location of the Goshen County TVS from MWR-05XP data at 1.0° elevation angle at 2152:03, 2153:12, and 2157:19 UTC, respectively. Range rings are every 1 km. All images are centered at the same location. The TVS ΔV values (m s−1) are listed next to the outlined TVSs. The approximate center beam height at the center of the domain in (a)–(k) is 0.8, 1.3, 1.7, 2.3, 2.9, 3.7, 4.7, 5.8, 7.2, 9.0, and 11.0 km ARL, respectively.

Citation: Monthly Weather Review 141, 12; 10.1175/MWR-D-12-00315.1

There is a caveat to consider in strictly using the T99 TVS criteria in the above calculation. When the authors developed their TVS criteria, “super resolution,” oversampling WSR-88D radial velocity data in azimuth by a factor of 2, had not been implemented. Brown et al. (1978) found that the opposing velocity peaks in a TVS should be separated by one beamwidth. As a result, a vortex couplet in adjacent gates as viewed in legacy data now may be found spread among three or four azimuths in oversampled data (Brown and Wood 2012). To account for the possibility that GTG ΔV values were being depressed by oversampling, the T99 TVS criteria were relaxed at low levels such that the ΔV maxima and minima were no longer restricted to adjacent gates, but rather to within three gates (i.e., radial velocity maxima and minima were allowed to be separated by two gates in between).

When the low-level radial velocity data were reexamined, there were TVSs identified from 0.55°–1.3° elevation angles (Figs. 16a–c, black rectangles). However, the lowest TVS ΔV (31.5 m s−1) leads to an S of ∼3.3 m s−1 km−1, a value that still categorizes the TVS as descending. Two additional signatures identified at 0.55° elevation angle met the ΔV threshold under the relaxed criteria and would change the S value result to nondescending (Fig. 16a, yellow rectangles). However, both signatures lacked height (e.g., Figs. 16b,c, dotted yellow rectangles) and time (dotted aqua and light green rectangles in Fig. 16a) continuity. More importantly, all of TVSs in data from 0.55° elevation angle (at both 2148:34 and 2153:09) were located at a relatively far distance from the location of the GC tornado as approximated using MWR-05XP TVS data (circles in Fig. 16a), so they likely were not associated with the GC tornado. Regardless of whether the specious signatures are considered as part of the analysis, the strongest continuous TVSs were found at midlevels at and after tornadogenesis in the GC supercell.

T99 openly discussed the imperfect nature of the criteria they used in determining the tornadogenesis mode using WSR-88D data. For example, they discussed the relatively low ΔV threshold used for TVS identification and the development of an objective criteria based on a subjective classification scheme. Furthermore, it should not necessarily be surprising that data from a radar system utilizing 30 times the volumetric temporal resolution of the WSR-88D provides evidence of shortcomings in their criteria [as noted earlier, in Vasiloff (2001) even a halving of the volumetric update time resulted in a clearer identification of the tornadogenesis mode]. Instead, it is more instructive to investigate what the WSR-88D was observing at midlevels and how that relates to the underlying theory of a midlevel incipient tornado that descends to the surface via the DPE.

KCYS data from 2.4°–8.0° elevation angle were matched up with MWR-05XP data to explore the strong midlevel TVS in more detail. For example, one of the strongest TVSs was located at 5.1° elevation angle (height of 5.9 km) in KCYS data at 2151:12 (Fig. 17a). In qualitatively matching heights, the corresponding MWR-05XP scan is from 18.5° elevation angle (Fig. 17c) where the strongest VS also was located at a height of 5.9 km (the evolution described below was similar in data from the levels above and below 18.5° elevation angle). In the next KCYS scan at that level (2155:46; Fig. 17b), a TVS was located in approximately the same location relative to the storm. However, by the time of the 2155:46 KCYS scan, the TVS associated with the GC tornado had been tracked for over a minute at 18.5° elevation angle in MWR-05XP data (Fig. 11). The ∼4.5 min between KCYS scans at the location of the midlevel TVS encompassed the approximate formation time of the GC TVS at that level. The MWR-05XP dataset allows for a unique look at how the midlevel WSR-88D TVS evolved between volume scans.

Fig. 17.
Fig. 17.

Radial velocity (m s−1) PPI scans (a),(b) at 5.1° elevation angle from the KCYS WSR-88D at 2151:12 and 2155:46 UTC, respectively and (c)–(h) at 18.5° elevation angle from the MWR-05XP at 2151:10, 2151:28, 2151:46, 2152:03, 2152:19, and 2152:36 UTC, respectively during the formation of the Goshen County tornado. White circles enclose WSR-88D TVSs in (a),(b) and a MWR-05XP VS in (c)–(h) that are referenced in the text. The dotted circle in (g) outlines the weakened shear signature (no longer a VS) and (h) outlines the approximate area where the shear signature is located in (g). Red (black) circles enclose an additional VS (areas of transient cyclonic shear) in MWR-05XP data. Range rings are every 1 km. Images in (a),(b) and (c)–(h) are centered at the same location. The GTG ΔV values (m s−1) of the VS/TVSs discussed in the text are listed next to the outlined VS/TVSs. The approximate center beam height at the center of the domain in (a),(b) and (c)–(h) is 5.9 and 6.2 km ARL, respectively.

Citation: Monthly Weather Review 141, 12; 10.1175/MWR-D-12-00315.1

At 2151:12, there were several areas of cyclonic rotation and one clearly defined TVS in KCYS data (Fig. 17a). Similarly, there were numerous areas of cyclonic rotation and a VS (white circle) within the larger-scale mesocyclone in MWR-05XP data from the same approximate time and height (Fig. 17c). In the subsequent minute, however, the VS in MWR-05XP data dissipated rapidly. After the VS strengthened noticeably at 2151:28 (Fig. 17d), it began to weaken in the next ∼30 s (Fig. 17e,f). The VS then underwent further weakening (Fig. 17g) before it no longer could be identified or tracked (Fig. 17h). Each of the VSs identified between 2151:10 and 2155:46 in MWR-05XP data from 18.5° elevation angle were tracked using the MWR-05XP VS criteria discussed previously (Fig. 18). In the MWR-05XP data, all VSs and areas of cyclonic shear identified from 2151 to 2152 completely dissipated within 70 s. A little over 2 min later, new VSs formed, one of which was the TVS associated with the GC tornado. As a result, the two TVSs observed by KCYS in successive volume scans (2151:12 and 2155:46) likely resulted from two different underlying features, only the latter of which was the GC tornado.

Fig. 18.
Fig. 18.

Time series of observed VSs at 18.5° elevation angle in MWR-05XP data from 2151:10 to 2155:46 UTC, a time period spanning successive KCYS scans at 5.1° elevation angle. The TVS associated with the Goshen County tornado is marked by a black square. The dotted vertical gray lines indicate the times that the KCYS 5.1° elevation angle scans began.

Citation: Monthly Weather Review 141, 12; 10.1175/MWR-D-12-00315.1

In section 3b, the variable nature of observed pretornadic midlevel VSs was discussed (e.g., Fig. 8). When viewing the evolution of individual VSs over a short time period at 18.5° elevation angle (Fig. 18), it can be seen that the VSs only lasted a few minutes each. To examine further the frequency of pretornadic midlevel (low level) VSs for the GC supercell, MWR-05XP data from 17.1° and 14.1° (3.9°) elevation angle were inspected over period of time spanning two KCYS volume scans during the approximate formation time of the GC tornado (Fig. 19). The VS markers are the same if a VS exhibited height continuity (for this case, within 1-km range and 5° azimuth). Over a dozen different VSs were identified, several of which did not exhibit height continuity and all of which (outside of the GC TVS) lasted for less than 3 min. For the GC supercell, pretornadic VSs were ubiquitous at midlevels but lacked temporal and spatial (in the vertical) continuity. Only at the time of tornadogenesis was a consistent (in time and height) TVS seen, first at low levels and then at progressively higher levels (Fig. 11).

Fig. 19.
Fig. 19.

Time series of observed VSs at (a) 17.1°, (b) 14.1°, and (c) 3.9° elevation angle in MWR-05XP data from 2148:20 to 2157:19 UTC. The time period covers that of two consecutive KCYS volume scans during the formation of the Goshen County tornado. The TVS associated with the Goshen County tornado is marked by a black diamond. VS/TVSs exhibiting vertical continuity use consistent markers and labels. The dotted vertical gray lines indicate the times that the KCYS volume scans began.

Citation: Monthly Weather Review 141, 12; 10.1175/MWR-D-12-00315.1

Despite the appearance of a dominant midlevel TVS prior to tornadogenesis in KCYS data, the sudden formation and dissipation of VSs in MWR-05XP data provide strong evidence that the midlevel WSR-88D TVS at ∼2151 was not an incipient tornado. Furthermore, the DPE cannot occur in vortices that constantly form and dissipate; rather a vertically and temporally continuous vortex is necessary for a tornado to build downward in this manner. Likewise, it may be tempting to view separate MWR-05XP VSs as resulting from continuous vorticity maxima that underwent large changes in intensity (e.g., VS 171a and the GC TVS in Figs. 19a,b). While this possibility cannot be ruled out, it is unlikely in this case because (i) of the small VS/TVS ΔV threshold and (ii) longer-lived VSs tended to form (dissipate) on the front (rear) side of the larger-scale mesocyclone, indicative of a type of VS cycling process. For this case, the observational evidence does not support the explanation of the DPE as the dynamical connection between the midlevel descending TVS in WSR-88D data and tornadogenesis.

For the GC case, the following is speculated (Fig. 20): areas of locally strong cyclonic vertical vorticity formed and dissipated within the midlevel mesocyclone over short periods of time. As the low-level mesocyclone strengthened, these enhanced areas of rotation became more common closer to the surface, and soon thereafter, tornadogenesis occurred. However, the relatively coarse spatial and temporal resolution of WSR-88D data is such that the separate and distinct areas of enhanced rotation (or even the larger-scale mesocyclone) appeared to be a temporally and vertically continuous TVS that descended to the surface near the time of tornadogenesis. In MWR-05XP data of the GC supercell, there were large changes in VS ΔV that occurred over a period of time an order magnitude faster than the WSR-88D volumetric update time (e.g., Figs. 17c–h). In such cases, it may not be suitable to use WSR-88D data to identify the mode of tornadogenesis.

Fig. 20.
Fig. 20.

An illustration, based loosely on the Goshen County supercell, of how enhanced levels of vertical vorticity within a mid- and low-level mesocyclone might appear as a descending incipient tornado in WSR-88D data. The black bar indicates a tornado and the dotted horizontal line marks the level of free convection. Gray (dark green) shading represents mesocyclone-scale (tornadic) vertical vorticity. Light green shading highlights areas (top) of locally enhanced vertical vorticity as discussed in the text and (bottom) where the T99 TVS criteria are met based on the given vertical vorticity distribution. The black dots indicate the approximate center beam locations from a WSR-88D scanning a storm 60 km away using VCP 212.

Citation: Monthly Weather Review 141, 12; 10.1175/MWR-D-12-00315.1

The causes of tornado formation first near the ground are beyond the scope of this paper. However, the sudden acceleration of TVS growth in the GC supercell above ∼2 km is of interest. One speculative possibility is that tornado growth upward occurred much faster after the vortex reached the approximate level of free convection (cf. Fig. 2 in Markowski et al. 2012a), when air parcels no longer were reliant only on vertical pressure gradient forces for upward motion. Unfortunately, because data collection in the El Reno supercell began just prior to the estimated tornadogenesis time, it is not known well how TVS growth progressed in the lowest 2.5 km.

To the authors' knowledge, the GC supercell dataset is currently the only one with rapid-scan, volumetric data of storm midlevels prior to tornadogenesis to be analyzed. As a result, the above hypothesis applies only to this single case. It is hoped that the MWR-05XP and other rapid-scan radars can obtain additional volumetric observations of supercells prior to tornadogenesis and use them to determine definitively the frequency with which mode I tornadogenesis occurs. Until then, there is no argument put forth here that a descending TVS should not be used as a tool in the tornado warning process by local NWS offices. Rather, we encourage forecasters to use a variety of tools beyond the appearance of a descending midlevel TVS in determining the likelihood of tornadogenesis occurring. Ideally, in the future, a large sample of rapid-scan, volumetric supercell observations can be used to relate patterns in mid- and low-level mesocyclone evolution to tornadogenesis.

Acknowledgments

The authors thank Philip Chilson, Richard Doviak, and Alan Shapiro, who reviewed early drafts of this work within the first author's Ph.D. dissertation at the University of Oklahoma. Thanks also to Alex Schenkman, Matthew Kumjian, Lance Leslie, Lou Wicker, Jeff Snyder, and Curtis Alexander for useful discussions regarding the results of this research. The latter also provided significant computational assistance in the use of the DREADER software package. We are grateful to Jana Houser, Paul Buczynski, Randy George, and the VORTEX2 crews, particularly Josh Wurman, David Dowell, and Erik Rasmussen, for their assistance in data collection. Three anonymous reviewers helped to focus and clarify several major points in this manuscript. This study was supported by NSF Grants ATM-0637148 and ATM-0934307.

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1

Consistent with Glickman (2000), a tornado is strictly defined here as being in contact with the surface. Therefore, even if there is tornado-scale rotation above the surface, tornadogenesis cannot occur until that rotation is at the ground.

2

In an additional case observed by the Rapid-Scan DOW (Wurman and Randall 2001), a tornado was observed to contract at six levels simultaneously during tornadogenesis (Wurman et al. 2008).

3

Incorrectly referred to as “meteorological” in Bluestein et al. (2010).

4

In this study, only observations of supercell tornadogenesis are discussed. Most past observations of mode I tornadogenesis, including those in T99, were made in supercells. In contrast, mode II tornadogenesis has been observed frequently in several different storm types.

5

Following individual TVSs was relatively straightforward because of the high temporal resolution of the MWR-05XP, which made it unlikely that separate TVSs would be misconstrued as one continuous TVS. Nonetheless, the possibility that the TVS origin times from MWR-05XP data predate the actual times of tornado formation cannot be ruled out.

6

The lack of pretornadogenesis data for this case means the possibility that a TVS descended to low levels prior to the start of MWR-05XP data collection cannot be ruled out. In such a scenario, the TVS would have had to weaken aloft and then reform upward from low levels. To the authors' knowledge, such an evolution has not been documented previously. In addition, RaXPol data of the same case, in which data collection began ∼30 min prior to tornadogenesis, also do not show a descending TVS (J. Houser 2013, personal communication).

7

Dowell and Bluestein (2002) discuss the possibility that vertical vorticity retrieved in a pseudo-dual-Doppler analysis may have descended to low levels prior to tornadogenesis in their case. However, a local maximum in vorticity also was observed at low levels prior to tornadogenesis and a time–height analysis of GTG shear could not be undertaken because of irregularly spaced data. As a result, the mode of tornadogenesis in their case is not categorized definitively here.

8

Data of the El Reno supercell were obtained by RaXPol beginning at ∼2020, initially using 2–3-min volumetric updates and eventually using ∼20-s volumetric updates; ∼3 min prior to the formation of the EF5 tornado, the dissipation of another tornado was observed, which would have complicated any comparisons to WSR-88D data.

9

In T99, the authors treated the beginning time of the volume scan in which tornadogenesis occurred as the time of tornadogenesis. In this case, the tornado formed at ∼2152. The volume scan beginning at 2148:34 was not completed until 2152:33, which may have been after tornadogenesis. As a result, under the T99 guidelines, the volume scan under consideration here may not be considered as occurring prior to tornadogenesis. Nonetheless, the entire volume of the storm was scanned prior to 2152.

10

We continue to refer to vortex signatures that meet the T99 criteria in WSR-88D data as TVSs. In MWR-05XP data, vortex signatures still must meet the baseline criteria discussed in the text to be labeled as such.

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