Close-Range Observations of Tornadoes in Supercells Made with a Dual-Polarization, X-Band, Mobile Doppler Radar

Howard B. Bluestein School of Meteorology, University of Oklahoma, Norman, Oklahoma

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Michael M. French School of Meteorology, University of Oklahoma, Norman, Oklahoma

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Robin L. Tanamachi School of Meteorology, University of Oklahoma, Norman, Oklahoma

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Stephen Frasier Microwave Remote Sensing Laboratory, Department of Computer and Electrical Engineering, University of Massachusetts, Amherst, Amherst, Massachusetts

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Kery Hardwick Microwave Remote Sensing Laboratory, Department of Computer and Electrical Engineering, University of Massachusetts, Amherst, Amherst, Massachusetts

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Francesc Junyent Microwave Remote Sensing Laboratory, Department of Computer and Electrical Engineering, University of Massachusetts, Amherst, Amherst, Massachusetts

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Andrew L. Pazmany Microwave Remote Sensing Laboratory, Department of Computer and Electrical Engineering, University of Massachusetts, Amherst, Amherst, Massachusetts

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Abstract

A mobile, dual-polarization, X-band, Doppler radar scanned tornadoes at close range in supercells on 12 and 29 May 2004 in Kansas and Oklahoma, respectively. In the former tornadoes, a visible circular debris ring detected as circular regions of low values of differential reflectivity and the cross-correlation coefficient was distinguished from surrounding spiral bands of precipitation of higher values of differential reflectivity and the cross-correlation coefficient. A curved band of debris was indicated on one side of the tornado in another. In a tornado and/or mesocyclone on 29 May 2004, which was hidden from the view of the storm-intercept team by precipitation, the vortex and its associated “weak-echo hole” were at times relatively wide; however, a debris ring was not evident in either the differential reflectivity field or in the cross-correlation coefficient field, most likely because the radar beam scanned too high above the ground. In this case, differential attenuation made identification of debris using differential reflectivity difficult and it was necessary to use the cross-correlation coefficient to determine that there was no debris cloud. The latter tornado’s parent storm was a high-precipitation (HP) supercell, which also spawned an anticyclonic tornado approximately 10 km away from the cyclonic tornado, along the rear-flank gust front. No debris cloud was detected in this tornado either, also because the radar beam was probably too high.

* Current affiliation: ProSensing, Inc., Amherst, Massachusetts

Corresponding author address: Dr. Howard B. Bluestein, School of Meteorology, University of Oklahoma, 120 David L. Boren Blvd., Suite 5900, Norman, OK 73072. Email: hblue@ou.edu

Abstract

A mobile, dual-polarization, X-band, Doppler radar scanned tornadoes at close range in supercells on 12 and 29 May 2004 in Kansas and Oklahoma, respectively. In the former tornadoes, a visible circular debris ring detected as circular regions of low values of differential reflectivity and the cross-correlation coefficient was distinguished from surrounding spiral bands of precipitation of higher values of differential reflectivity and the cross-correlation coefficient. A curved band of debris was indicated on one side of the tornado in another. In a tornado and/or mesocyclone on 29 May 2004, which was hidden from the view of the storm-intercept team by precipitation, the vortex and its associated “weak-echo hole” were at times relatively wide; however, a debris ring was not evident in either the differential reflectivity field or in the cross-correlation coefficient field, most likely because the radar beam scanned too high above the ground. In this case, differential attenuation made identification of debris using differential reflectivity difficult and it was necessary to use the cross-correlation coefficient to determine that there was no debris cloud. The latter tornado’s parent storm was a high-precipitation (HP) supercell, which also spawned an anticyclonic tornado approximately 10 km away from the cyclonic tornado, along the rear-flank gust front. No debris cloud was detected in this tornado either, also because the radar beam was probably too high.

* Current affiliation: ProSensing, Inc., Amherst, Massachusetts

Corresponding author address: Dr. Howard B. Bluestein, School of Meteorology, University of Oklahoma, 120 David L. Boren Blvd., Suite 5900, Norman, OK 73072. Email: hblue@ou.edu

1. Introduction

Polarimetric radars have been used to discriminate among various types of hydrometeors, owing to their different shapes and composition (e.g., Zrnic and Ryzhkov 1999; Straka et al. 2000; Bringi and Chandrasekar 2001). It has been proposed that polarimetric radars can also distinguish between tornadic debris, which is assumed to consist of irregularly shaped and randomly oriented particles, and hydrometeors, which are more regularly shaped and more systematically oriented (Ryzhkov et al. 2005).

Ryzhkov et al. (2005), using S-band (10-cm wavelength) radars having half-power beamwidths of 1.9° by 0.9°, and 0.9° (the effective beamwidths are actually slightly wider, as a result of the rotation of the antenna), respectively, analyzed volume-scan data collected in supercells in central Oklahoma on 3 May 1999, 8 May 2003, and 9 May 2003, when tornadoes were reported at ranges of 45–60, 20, and 35–55 km, respectively. They found signatures of radar reflectivity factor Z between 45 and 55 dBZ and differential reflectivity (ZDR) less than 0.5 dB, which they interpreted as representing debris. They also found relatively low values of the cross-correlation coefficient (ρhv) of less than 0.8. Although they argued that a ρhv signature might be the best indicator of a tornado signature because it is not affected by errors in radar calibration as ZDR is, they also pointed out that ρhv might be in error if the differential phase varies within the radar resolution volume.

Beginning in 2001, a group from the School of Meteorology at the University of Oklahoma (OU) and a group from the Department of Electrical and Computer Engineering at the University of Massachusetts, Amherst (UMass) operated a mobile, X-band (3-cm wavelength) radar for general storm surveillance and to probe tornadoes and tornadic storms at close range in the southern Great Plains. In spring, 2002, the radar first had polarimetric and Doppler capability (Pazmany et al. 2003). In spring, 2004, data were collected at close range in tornadoes on two days. In two instances, the tornado and its associated debris cloud and condensation funnel were clearly visible from the site of the radar.

The purpose of this paper is to detail the analyses of the polarimetric Doppler radar collected and to correlate them with visual aspects of the tornado, so as to determine if there is a debris signature that is distinguishable from a hydrometeor signature. This study extends that of Ryzhkov et al. (2005) in that more cases are added and in that an X-band radar was used; more importantly, the radar was located much closer to the tornadoes, so that the spatial resolution was finer. Also, since in some instances the tornado was visible, airborne debris could be correlated with features in the radar analyses. A description of the mobile radar system used and the way the data were collected and processed are detailed in section 2. Case studies for tornadoes on two days are discussed in section 3. The study is summarized and the major findings and implications are given in section 4.

2. Description of the mobile radar system and data collection

The mobile, polarimetric, X-band radar system (UMass X-Pol) was designed and built by graduate students and faculty at the Microwave Remote Sensing Laboratory (MIRSL) at UMass, as a less-expensive, lower power, yet reliable, polarimetric alternative to the Doppler on Wheels (DOW; Wurman et al. 1997). A polarimetric version of the DOW, the X-band Polarimetric radar on Wheels (X-POL), is used by the University of Connecticut (Anagnostou et al. 2004), but to the best of our knowledge, no analyses of tornadoes based on data from it have been reported in the literature. While UMass X-Pol is of great value in field experiments (e.g., Kramar et al. 2005), its design and construction was itself a valuable educational experience for the participating students at UMass. Like other ground-based mobile Doppler radars (Bluestein et al. 2001), it is mounted on a truck.

Details about the radar system are found in Pazmany et al. (2003) and Junyent et al. (2004). The radar system was developed from a magnetron-based marine radar transceiver manufactured by Raytheon. It was modified to transmit equal-power vertically (V) and horizontally (H) polarized pulses. Radar volumes were oversampled every 37.5 m (the radial resolution was actually 150 m) and then boxcar averaged over 75 m, every 37.5 m. A second receiver was added so that ZDR and differential phase shift (Kdp) could be computed without having to use an expensive, high-power, transmit-receive switch. The cross-correlation coefficient ρhv and Doppler velocity are also computed, the latter being calculated from coherent-on-receive pulse-pair measurements.

Since radars that operate in the X-band are highly susceptible to attenuation when there is heavy precipitation, it is anticipated that there could be errors in estimates of ZDR when the radar beam passes through heavy rain, as a result of differential attenuation. While attenuation can be corrected for using, for example, the “self-consistent” method of Bringi et al. (2001), no attempts were made to do so in this study.

A staggered pulse repetition frequency was implemented to extend the maximum unambiguous velocity, while retaining a relatively long maximum unambiguous range (Zrnic and Mahapatra 1985). The radar transceiver, dual-polarized antenna, and pedestal are mounted on the truck platform; the data acquisition, positioner controller, and display systems are inside the crew cabin. Further details about the characteristics of the radar system are shown in Table 1.

The data processing system in 2004 allowed for two modes of data collection. In “surveillance mode,” low data-rate radar reflectivity data only were displayed and recorded, potentially out to relatively long range (as far as 120 km). In this mode, the radar operator could get a general idea of the intensity, size, shape, and locations of storms on a plan position indicator (PPI) display. In “data-collection mode,” intermediate frequency (IF) radar data were streamed directly to disk for ranges out to 30 km. The data were not displayed in real time, owing to data bandwidth and processor limitations. The parameters H and V reflectivity (ZH and ZV), ZDR, KDP, ρhv, and Doppler velocity mean and standard deviation (Doviak and Zrnic 1984; Bringi and Chandrasekar 2001) were computed after field operations. Thus, surveillance mode was used when positioning the radar truck and in determining whether or not it was worthwhile to begin recording all the radar variables. During data collection (in both modes), the antenna scanned 360° at constant elevation angle. When raw data were being recorded, it was not possible to monitor any of the data on the PPI display. [Every 5 or 10 min or so, the radar operator would switch back to surveillance mode to ensure that the storm features being recorded were still noted on the PPI display at a desired range and azimuth; thus, there were many instances of several minute gaps in data collection (of all the radar variables).] Data were stored in wedge-shaped segments and for certain antenna-rotation speeds certain segments would not be stored, owing to aliasing. Finally, during 2004 the storage space on the radar computer was severely limited, so that at times older data would have to be deleted during field operations, in order to make room for new data, and therefore the radar operator had to be very conservative when making the choice to operate in data-collection mode. (The data-processing system was improved in 2005 and most of the aforementioned impediments to data collection have since been removed.) The processed data collected in both surveillance and data-collection modes were converted into a format compatible for use with the National Center for Atmospheric Research’s (NCAR) SOLO software (Oye et al. 1995), so that radar images could be displayed, edited, and otherwise manipulated.

Field operations were conducted as in past years (Bluestein 1999; Kramar et al. 2005). Storms and tornadoes were scanned at an elevation angle as close to the ground as possible, but above most intervening trees, buildings, etc. Unlike the data collected by Ryzhkov et al. (2005) using fixed-site radars, the data collected by UMass X-Pol were usually less subject to beam blockage near the ground at azimuths along which tornadoes were located, owing to our efforts to keep the view of storm features as unobstructed as possible, thanks to the mobility of the radar truck. However, the antenna was not as elevated as the fixed-site S-band radars used by Ryzhkov et al. (2005) were, and no efforts were made to correct for any blockage at all as they had done. Also, efforts were not made to level the radar truck, so there may be some unknown amount of vertical excursions in the PPI scans. The antenna rotation speed was as rapid as possible so that the data were oversampled in azimuth just enough that there was at least one beam for every 1.25° in azimuth.

In addition to the data collected by UMass X-Pol, data were also collected by the UMass millimeter-wavelength (W-band), mobile Doppler radar (Bluestein et al. 2005), by two DOWs (Kosiba et al. 2005), and by an infrared digital thermal camera (Tanamachi et al. 2006), analyses of which are reported elsewhere.

3. Case studies

a. Spatial resolution of the data collected in the cases

During the 2004 field experiment, which ran from late April to early June, data were collected in tornadoes at ranges varying from ∼4 km on 12 May to ∼8–14 km on 29 May. Resolution volumes therefore varied from slightly greater (taking into account smearing due to antenna rotation) than ∼70 m × 70 m × 150 m on 12 May to slightly greater than ∼175 m × 175 m × 150 m on 29 May.

The elevation angles of the antenna were approximately 2.5° and 3° on 12 May, and 5.1° and 4.8° on 29 May; thus, the approximate height of the center of the radar beam on 12 May was ∼175–210 m AGL and on 29 May was ∼675 m–1.25 km AGL. Since the radar platform was not leveled, the aforementioned heights could be in error to an unknown extent. The higher heights of the beam on 29 May were a product of the longer range to the tornado and the higher elevation angles, which were necessary so that the radar beam was aimed above distant trees. Since from experience we believe that the elevation angles could have been in error by as much as 2°, the uncertainty in the height of the center of the beam was ∼±140 and ±450 m on 12 and 29 May 2004, respectively.

For comparison, the height of the center of the beam in the data presented by Ryzhkov et al. (2005), which was at 0° for data collected on 3 May 1999 and 0.5° elevation angle for data collected on 8 and 9 May 2003, varied from as low as ∼30–175 m AGL when the tornado was at 20-km range to ∼435 m AGL when the tornado was at 50-km range. Thus, the heights above the ground of their data were comparable to those of ours in two cases, but lower than ours in the other. The radial resolution of the data was 240 m on 3 May 1999 and 267 m (A. Ryzhkov 2005, personal communication) on 8 and 9 May 2003. Thus, the resolution volumes of the data were, at best, ∼315 m × 315 m × 265 m (on 8 May 2003). Thus, although most of the data collected by UMass X-Pol were at a much finer spatial resolution (∼70 m × 70 m × 150 m on 12 May 2004), data were collected only at one elevation angle (as close to the ground as possible), while the data described by Ryzhkov et al. (2005) were collected in a deep volume. Thus, the dataset detailed in this paper lacks the vertical continuity nicely shown by them.

b. Tornadoes on 12 May 2004 in southern Kansas

1) The “Attica” tornado

A supercell that formed in south-central Kansas, near the intersection of the dryline and an outflow boundary, around 1730–1800 CDT (UTC is 5 h later) on 12 May 2004, spawned a series of tornadoes (Fig. 1), two of which were probed in data collection mode by the UMass X-Pol. One tornado, which formed just east of Attica, Kansas, produced damage rated by the National Weather Service (NWS) as F2 (see Fujita 1981 for the Fujita scale). This tornado tracked to the north-northwest and was highly visible from the radar truck (Fig. 2), only ∼4 km to its east. Owing to its motion normal to the road on which the radar truck was parked, the storm-intercept crew was able to remain at one location for the entire life of the tornado.

A storm-scale perspective of the radar reflectivity pattern of the 12 May supercell at the time of the Attica tornado (2001:09 CDT; times for radar images here and that follow are valid for the time of the beginning of the scan) is seen in Fig. 3c. A hook echo coiled up at its tip and a low-reflectivity notch north and northeast of the hook echo are connected to the main body of the storm, which lies to the north and northeast. Before the tornado had formed, an appendage of radar echo on the southwest side of the storm (∼45 dBZ) narrowed into a thin band of much weaker radar reflectivity (Fig. 3a), which curved around in a counterclockwise manner, culminating in a narrow ring of a 30-dBZ echo; the band of weaker radar reflectivity then curved back around in a clockwise direction, giving the appearance of a ring having two spiral bands (Fig. 3b). It is likely that the ring marks the debris in what looked like a dust whirl on the ground (not shown), in advance of the development of the tornado. Neither polarimetric nor Doppler velocity data were available at this time. The appearance and horizontal dimensions of the ring looked very much like those of a dust devil (Bluestein et al. 2004a; cf. Fig. 4). The location of the ring was at an inflection point along the line and probably marked the intersection of the hook echo with the rear-flank gust front, as in a tornado in Nebraska discussed by Bluestein et al. (2003; cf. Figs. 5 and 6) and in a tornado in Kansas discussed by Tanamachi et al. (2007).

The radar reflectivity pattern associated with the tornado was a ring of moderately intense echo (∼40 dBZ) about 625 m in diameter, surrounding a weaker region of echo (∼15–25 dBZ) ∼200 m in diameter; beyond the ring of intense echo there were spiral bands of reflectivity of ∼40–50 dBZ (Fig. 4a). Such features are common in tornadoes when viewed by radars at close range (e.g., Fujita 1981; Bluestein et al. 1993; Wurman et al. 1996; Wakimoto et al. 1996; Wurman and Gill 2000; Bluestein and Pazmany 2000; Bluestein et al. 2003, 2004b; Alexander and Wurman 2005; Dowell et al. 2005).

The region encompassing the ring of moderately intense echo was collocated with quasi-circular regions of relatively low ZDR and ρhv (Fig. 4b). These regions of ZDR below ∼0.5 dB (coded green and white) and ρhv below 0.5 (coded green) were ∼900 m in diameter. The first spiral band outside the inner ring was characterized by ZDR of ∼1.5–3 dB. The Doppler velocity couplet associated with the tornado vortex was ∼325 m in diameter, while the diameter of winds in excess of ∼25 m s−1 was ∼800 m in diameter (Fig. 4c). While F2 damage was reported with this tornado, the relatively weak winds sensed by the radar may have been a result of the relatively high elevation angle, such that the center of the radar beam was ∼175 m, well above the tops of nearby trees, and possibly above the level of the highest wind speed (e.g., Wurman and Gill 2000; Alexander and Wurman 2005; Bluestein et al. 2005). It also possible that the Doppler wind data displayed in Fig. 4c were not collected when the tornado was of F2 intensity. The wind speeds measured by UMass X-Pol were consistent with those measured by DOW3 (more information available online at www.cswr.org/dataimages/rotate/12May2004.html) at approximately the same time. To further illustrate how radar reflectivity, ZDR, and Doppler velocity were correlated, the aforementioned parameters are plotted in Fig. 5 (ρhv is not shown) through the tornado, at the constant range of the center of the tornado. A region of negative values of ZDR is clearly evident encompassing the core of the tornado at 2001:09 CDT; the ring of moderate reflectivity is less easily discernible. The extremely low values of ZDR at the center of the vortex (<−2 dB) in the face of relatively high reflectivity (∼30 dBZ) may indicate that that there were relatively few pieces of highly reflective debris there and that they were not randomly distributed.

From the known distance of the vortex signature from UMass X-Pol (and from the nearby UMass W-band radar) and still photographs of the tornado taken from the site of the radar truck, the dimensions of the visible dust/debris cloud and condensation funnel were estimated photogrammetrically (Fig. 2). The width of the most opaque portion of the symmetrical portion of the dust/debris cloud was ∼525 m, while the dust/debris cloud also extended off to the south (to the left in Fig. 2) another 300 m or so. There is thus qualitative agreement between the visible dust/debris (cross-sectional diameter in the viewing plane of ∼525–825 m), and the regions of low ZDR and low ρhv that mark the tornado (cross-sectional diameter in the viewing plane of ∼900 m), though the latter extends out farther than the ring of most intense echo (∼625 m in diameter). The distance from the center of the tornado to the center of the ring of the most intense debris echo (∼310 m) is greater than the distance to the radius of maximum wind speed (∼160 m). It is thus concluded that the quasi-circular ring of intense reflectivity probably was composed of debris (Z ∼ 40 dBZ and ZDR ≤ 0.5 dB) that were centrifuged radially outward (Snow 1984; Dowell et al. 2005), while the spiral bands were composed of hydrometeors (Z ∼ 40–50 dBZ and ZDR ∼ 1.5–3 dB). However, since the sizes of the scatterers making up the debris cloud were not known, the centrifuging hypothesis cannot be tested in this case. Wurman et al. (1996) and Wurman and Gill (2000) had also suggested that, based on nonpolarimetric radar observations, the inner ring of high reflectivity probably represents debris, while the outer spiral bands represent raindrops.

It is evident from the photograph shown in Fig. 2 that the debris cloud extended from the ground up to as high as ∼600 m AGL. Wurman and Gill (2000) estimated the height of the debris cloud in a tornado to be ∼700 m or greater. Unfortunately data were not collected at higher-elevation angles in our case so that variations of ZDR, ρhv, and Z could not be correlated as a function of height. Since Ryzhkov et al. (2005) found a column of low ZDR (<0 dB) as deep as 2 km in one case, it is believed that it is reasonable to assume that the column of debris would have been associated with a similar column of low ZDR.

2) The tornado southwest of Harper

This tornado was rated only at F0, but had a much longer damage path than the Attica tornado (Fig. 1). Unlike the Attica tornado, this tornado moved to the east-northeast, and the storm-intercept crew therefore had to move periodically, to the east and then north to avoid getting too close to it. The tornado, though weak, still had a formidable-looking debris cloud (Fig. 6). Since Fig. 6 was made from a video frame capture and there were uncertainties in the focal length of the lens and the location, it was not possible to photogrammetrically analyze the visible debris/cloud width, etc., as in Fig. 2.

A storm-scale perspective of the radar reflectivity pattern of the 12 May supercell at the time of the tornado seen in Fig. 6 is shown in Fig. 7. A hook echo, the tip of which contains a small ring surrounding a pinhole, and a low-reflectivity notch north and northeast of the hook echo, are connected to the main body of the storm, which lies to the north and northeast.

As in the Attica tornado, the tornado seen in Fig. 6 was collocated with a ring of moderate reflectivity (∼30 dBZ) and a vortex signature (brown to green couplet) with Doppler velocities as high as only ∼25 m s−1 (Figs. 8a,c). In the 16 s between scans, the reflectivity pattern changed noticeably: the pinhole seen at 2021:28 CDT had filled in by 2021:44 CDT and the reflectivity just to the east of the tornado in a spiral band had increased from ∼40 dBZ (brown) to ∼45 dBZ (pink).

Regions of ZDR < 0.5 dB and ρhv < 0.5 ∼250–325 m across spanned across the center of the tornado (Figs. 8b,c) and covered a region having Z ∼ 20–30 dBZ, save for the pinhole at 2021:28 CDT. At 2021:28 CDT it covered the region of the 30-dBZ ring (Fig. 8a). The diameter of the most intense winds in the tornado Doppler-velocity couplet was ∼250 m. The ring (∼330 m in diameter) is therefore thought to have been composed of debris, some of it centrifuged outward, rather than hydrometeors. In addition, there were narrow, curved bands of ZDR < 0.5 dB and ρhv < 0.5 in the spiral band to the northeast of the tornado at 2021:28 CDT and to the east of it at 2021:44 CDT. It is suspected that these bands were also composed of debris, since they had Z < 45 dBZ; however, it is also possible that they were composed of a relatively small number of hailstones. At 2021:44 CDT a region of anomalously high ZDR (brown) ∼4 dB and low ρhv (green) ∼0.5 appeared just to the west of the tornado. The change from just 16 s earlier was marked. It was associated with the tip of the spiral band noted earlier, and the appearance of higher reflectivities in it. We attribute this region to the sudden onset of large raindrops, perhaps associated with a “rain curtain” spiraling around the tornado, or to the sudden lofting of debris.

As in the Attica tornado, radar reflectivity, ZDR, and Doppler velocity (but not ρhv) were plotted through the tornado, at the constant range of the center of the tornado (Fig. 9). A region of near-zero ZDR is clearly evident encompassing the core of the tornado at 2021:28 and 20021:44 CDT.

c. Tornadoes on 29 May 2004 in central Oklahoma

1) The “Geary–Calumet” tornado

A supercell that formed in western Oklahoma amidst convective storms that had begun near the dryline, tracked eastward and produced several tornadoes, one whose damage track began to the northeast of Geary and northwest of Calumet, Oklahoma (R. Smith, NWS, Norman, 2005, personal communication). (More detailed tornado track information can be found online at http://www.cswr.org/dataimages/rotate/geary-summary-2004-0711fp.pdf.) The “Geary–Calumet” tornado referred to in this section was named Tornado E in the aforementioned Web site. The damage path of the cyclonic tornado in Fig. 10 actually reflects the track of one of a number of tornadoes and/or parent mesocyclones. A detailed discussion of the evolution of all the vortices is beyond the scope of this study. The NWS rated the damage along the region of the tornado to the northwest of Calumet as F1, almost F2 intensity. This tornado was mostly hidden from view behind precipitation, but was visible to at least one storm chaser who was positioned in a favorable location with respect to it (J. Petrowski 2005, personal communication). From a vantage point north of Calumet, the storm was visible to the west-northwest as a striated cylinder having a flared-out base on its southern end (Fig. 11a). To the north of the base the sky was relatively bright. The storm looked very much like a high-precipitation (HP; Doswell et al. 1990; Moller et al. 1994) supercell (Fig. 11b) in that there was an extensive, dark-appearing area of precipitation underneath much of the base.

The radar depiction of the storm (Fig. 12a), however, deviated from the idealized depiction of an HP supercell (Fig. 12b). As in the HP model, the 29 May storm had an area of heavy precipitation behind the rear-flank gust front, south of the inflection point in the boundary that passed along the rear edge of the updraft. In the model, however, the area of precipitation to the rear of the rear-flank gust front is part of the main storm echo and curves such that the leading, eastern edge is concave, while in the 29 May storm the region of precipitation was convex, bulging forward; in addition, there was a narrow notch of weak reflectivity that curved around the leading edge of the precipitation, and this precipitation band circled an echo-weak hole or eye, that was connected to the main body of the storm’s radar echo by a narrow band. Perhaps the reason why the 29 May storm deviated from the model was that the real storm had a much more intense cyclonic circulation (a tornado or tornado cyclone) than any cyclonic circulation implied in the model. The weak-echo notch seems to have been associated with the brighter area seen in Fig. 11, on the extreme right-hand (northeastern) side of the cloud base.

The radar reflectivity pattern associated with the tornado/parent circulation–mesocyclone northwest of Calumet and northeast of Geary was an echo-weak hole ∼1.5 km wide at 1948:12 and 1950:07 CDT (Fig. 13a), and ∼1 km wide at 1955:37 and 1957:17 CDT (Fig. 14a). The hole was embedded within a region of ∼30-dBZ echo; higher reflectivities (∼45 dBZ) were found to the southeast of the hole. At 1948:12 and 1950:07 CDT the width of the Doppler velocity vortex signature couplet was ∼1.75 km (Fig. 13b) and had contracted to ∼1.25 km at 1955:37 and 1957:17 CDT. Maximum Doppler velocities were ∼28 m s−1 in the approaching direction; as in the Attica tornado, Doppler velocities were less than what one would expect in a tornado [according to the DOW Web site (http://www.cswr.org/dataimages/rotate/geary-summary-2004-0711fp.pdf) a tornado did not appear until 1952 CDT]. However, since the height of the radar beam was at least 925 m AGL, it is likely that the maximum wind speeds were at lower elevations and therefore not resolved. Because there was a lack of visual documentation of a tornado owing to an opaque region of precipitation, and because the vortex signature was so broad, the circulations at 1948:12 and 1950:07 CDT might be best characterized as strong mesocyclones.

The region within the core of the tornado/mesocyclone, which encompassed the weak-echo hole, ZDR was <0.5 dB (Figs. 13b and 14b, light green region), except for an appendage of ZDR ∼ 2, which coincided with a curved band of reflectivity of ∼20 dBZ that protruded inside the weak-echo hole at 1950:07 CDT (Figs. 13a,b, right side). It is possible that this feature was associated with precipitation that had been advected into the center of the circulation by a smaller scale vortex embedded within the hole. There is some evidence in the Doppler velocity field that there may have been a few submesocyclone-scale vortices (Fig. 13c, right side). However, this conclusion is not certain because the radar reflectivity pattern was weak at the edge of the hole where the smaller-scale vortices may have been and close inspection of the raw, unedited Doppler velocity data did not unambiguously support the objective unfolding algorithm’s analysis shown in the figure. It is not unlikely, though, that such a wide circulation could have been associated with multiple vortices (Davies-Jones et al. 2001), since widening of a vortex core can be associated with an increase in swirl ratio and a transition to a multiple vortex regime. There was no unambiguous association of a circular region of low ZDR, however, and the tornado, as in the analyses of the tornadoes on 12 May 2004. In this case, a band of low ZDR (<0.5 dB) (light green) from the rear (west) connected with the region of low ZDR near the core of the tornado. Part of this band of low ZDR was significantly negative (as low as ∼−4 dB; purple) at 1948:12 and 1950:07 CDT. Ryzhkov et al. (2005) have also found negative values of ZDR in a tornadic supercell; they attribute negative values to “a certain degree of vertical common orientation of the scatterers” or to “their large size.” In this case, it is more likely that the anomalously negative values of ZDR were a result of differential attenuation because the radar beam would have passed through a region of high reflectivity (>50 dBZ; bright pink in Fig. 13a) before reaching the region of low ZDR. Further evidence that these values of ZDR were unrealistically low and due to differential attenuation is found in observations of ρhv (cf. Figs. 13b,c); values of ρhv are much too high to indicate debris. At 1955:37 and 1957:17 CDT there appeared to be a band of higher ZDR ∼ 2 dB wrapping around the region of weaker ZDR ∼ 0.5 dB or less (light green) that coincided with the center of the tornado. However, from observations of ρhv (cf. Figs. 14a–c) it is seen that a region of ρhv < 0.5, which would be indicative of debris, has a diameter that is approximately the same as that of the weak-echo hole and is coincident with it, and so is therefore not likely significant.

2) The anticyclonic “Calumet” tornado

Following the deployment of UMass X-Pol to collect data just north of Calumet, the storm-intercept team retreated to a location just south of Calumet for safety, as the storm approached (Fig. 10). In a surveillance scan at 2003:54 CDT it is seen that the eye/weak-echo hole associated with the tornado northeast of Geary was still evident (Fig. 15a). At a low-elevation angle, where there was considerable blocking from utility poles and trees, an anticyclonic hook echo was seen ∼10 km to the south-southeast of the eye. The weak-echo notch seen ∼10–11 min earlier (Fig. 12) was still noted. About 1 min later, at 2004:58 CDT, the hole and anticyclonic hook are better seen (Fig. 15b), as the antenna had been elevated enough so that the radar beam was not being blocked by intervening utility poles and trees. The radar imagery in Figs. 15a,b look remarkably like that associated with the Grand Island, Nebraska, tornado of 3 June 1980 (Fujita 1981), in which a weak-echo hole was associated with a cyclonic tornado, and an anticyclonic hook, about 5 km away along the rear-flank gust front, was associated with an anticyclonic tornado. A similar flow pattern was inferred in a supercell in Iowa on 13 June 1976, which also produced a cyclonic–anticyclonic tornado pair (Brown and Knupp 1980).

Unfortunately, owing to a limitation of the signal-processing software, sector scans containing both the remains of the cyclonic tornado and the anticyclonic tornado in the 29 May storm were recovered in data-collection mode only at 2008:41 CDT (Fig. 16). It is seen that the remains of the cyclonic tornado were embedded within an area of a ∼45–50-dBZ echo surrounding the center of the vortex (Fig. 16a), which was associated with a vortex signature couplet of only ∼20–25 m s−1 of shear across the vortex couplet (Fig. 16c). The vortex was embedded within a region of ZDR ∼ 1–2 dB (Fig. 16b), which is indicative of precipitation, not debris. Furthermore, the ρhv in the region of the vortex was ∼0.9, which is too high to indicate debris. Since data from a DOW indicated much stronger cyclonic shear (see online at http://www.cswr.org/dataimages/rotate/geary-summary-2004-0711fp.pdf), it is very likely the UMass radar beam sampled a volume well above the ground, where wind speeds associated with the tornado were much weaker.

A region of divergent, anticyclonic shear of ∼18 m s−1 over 1 km (Figs. 16c,f) was located near the anticyclonic hook (Fig. 16a), more clearly seen in Fig. 15, several minutes earlier. No evidence of debris could be found in either ZDR (Fig. 16b), which was too high, or in ρhv, which was also too high. Because there was no indication of debris and because the anticyclonic shear was too low, it is likely that as in the case of the cyclonic tornado to the north, the radar beam was above the column of strongest winds in the anticyclonic tornado.

At 2010 CDT an anticyclonic tornado appeared (Fig. 17) to the northeast of UMass X-Pol as a bright, sunlit condensation funnel (Fig. 10). A video taken by one of the authors (R. Tanamachi), who was located much closer to the tornado than UMass X-Pol, shows anticyclonically rotating clouds around the condensation funnel. In fact, the attention of our storm-intercept team was focused on the area of the remains of the cyclonic tornado to the north. It was a surprise to us that a new tornado formed where it did, because we were looking for a new cyclonic circulation and did not see one. The anticyclonic tornado discussed here is named Tornado F (see online at http://www.cswr.org/dataimages/rotate/geary-summary-2004-0711fp.pdf); another anticyclonic tornado named Tornado G was noted after UMass X-Pol stopped collecting data.

The visual evidence (Fig. 17), other storm chasers’ accounts (e.g., see online at www.siue.edu/~jfarley/Chase%205-29-04.htm), the anticyclonic hook echo just a few minutes earlier, and Fujita’s (1981) and Brown and Knupp’s (1980) analyses also support the notion that the tornado in Fig. 17 was anticyclonic and had developed where the anticyclonic hook echo had been. More details concerning the anticyclonic tornadoes will likely appear in studies elsewhere that detail analyses of DOW and Shared Mobile Atmospheric Research and Teaching Radar (SMART-R) data.

4. Summary and discussion

Data in several supercell tornadoes in the Southern Plains, that had been collected by an X-band, dual-polarization, Doppler radar, were analyzed. It was found that regions of precipitation could be distinguished from regions of debris.

In one case (the “Attica” tornado), a debris ring was clearly evident as ZDR < 0.5 and ρhv < 0.5 dB and Z ∼ 40 dBZ, which was coincident with a Doppler signature of a cyclonic vortex. This finding was supported qualitatively by photogrammetric analyses of images of the tornado from the radar truck.

In another case (the tornado southwest of Harper, Kansas), an area of debris was indicated as a region (not a ring) of ZDR < 0.5 dB, ρHV < 0.5, and Z ∼ 20–30 dBZ (or less, with a narrow weak-echo hole at one time). In this tornado, a curved band of debris was noted near the tornado as a curved band of ZDR < 0.5 dB, ρHV < 0.5, and Z ∼ 45 dBZ.

In the “Geary–Calumet” tornado, which was not visible owing to intervening precipitation, the tornado was marked by a region of ZDR < 0.5 dB and Z ∼ 25–30 dBZ, outside of the inner, weak-echo hole. At one time, a band of low (ZDR < 0.5 dB) and in one instance anomalously low, negative ZDR (∼−4 dB) was located just to the southwest of the tornado in a region of moderately large Z (∼30 dBZ). However, since ρhv was >0.5, it is likely that these regions did not have any debris; the anomalously low values of ZDR were probably due to differential attenuation. When attenuation is significant, it might be preferable to use ρhv rather than ZDR to locate debris, since it is not sensitive to attenuation; it is, however, sensitive to differential phase, which is more likely in radar volumes that are not filled uniformly with the same type of scatterers, the same-sized scatterers, or are simply not completely filled with scatterers at all; it is expected that differential phase would be a problem in volumes where there are sharp gradients in reflectivity and/or when volumes are relatively large, such as at longer ranges. In future studies using dual-polarization radar data at X-band, it may be prudent to use both ZDR and ρhv to distinguish not just debris from hydrometeors, but also different types of hydrometeors from each other. Future studies of dual-polarization, X-band radar data in tornadic storms might also benefit from corrections for attenuation using the “self-consistent” method of Bringi et al. (2001), as has been adapted for X-band data by Park et al. (2005a) and used by Park et al. (2005b).

It is therefore concluded, in accord with Ryzhkov et al. (2005), that dual-polarization radar data can help identify tornadoes by locating clouds of debris just above the ground. The analyses of data presented in this paper also support the identification of debris bands outside the center of a tornado.

The authors encourage more data be collected in and near tornadoes with mobile, dual-polarization, Doppler radars, along with detailed photographic documentation. In particular, more rapidly scanning and phased-array radars (e.g., Wurman and Randall 2001; PopSefanija et al. 2005) with full volume scans need to be used to obtain more complete analyses, which show the full three-dimensional structure of debris clouds.

Finally, since an anticyclonically rotating vortex–tornado, indicated as an anticylonic hook echo and in Doppler velocity as an anticyclonic shear signature, were found, it is hoped that numerical modelers can isolate the appropriate environmental conditions necessary for such a feature and simulate a supercell with a cyclonic–anticyclonic surface vortex–tornado couplet like those in the Geary–Calumet supercell. This rare type of storm is important because it can produce anticyclonic tornadoes in a region not focused on by spotters.

Acknowledgments

This work was supported by NSF Grant ATM-0241037 to OU and ATM-0242166 to UMass. The authors thank Mark Laufersweiler (OU) for his computer assistance. Rick Smith at the NWS in Norman, Oklahoma, provided damage-survey information for the 29 May storm. We are grateful to two anonymous reviewers for their comments, one of who located information from the DOW and SMART-R radars posted on public Web sites. The second author operated the radar in the field. Mark Laufersweilwer (OU) and Curtis Alexander (OU) assisted with computer- and graphics-related issues. Discussions with Guifu Zhang were helpful. Part of this work was done while the first author was a visiting scientist in the Mesoscale and Microscale Meteorology (MMM) Division at the National Center for Atmospheric Research (NCAR) in Boulder, Colorado. NCAR is supported by the National Science Foundation.

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Fig. 1.
Fig. 1.

Tornado tracks and estimated F-scale rating of each tornado on 12 May 2004, as determined by the NWS, Wichita, KS. Also shown are two of the deployment sites (R1 and R3) of the UMass mobile, X-band radar when data were being collected, for the second and fourth tornado.

Citation: Monthly Weather Review 135, 4; 10.1175/MWR3349.1

Fig. 2.
Fig. 2.

Selected images of the Attica tornado, during its mature stage, at (a) 2002 and (b) 2003 CDT 12 May 2004. The view is to the west from a location approximately 5 km east of Attica, KS. The approximate dimensions of the opaque debris cloud, condensation funnel, and height of the cloud base, as determined from photogrammetric analysis, are as indicated. (Photographs courtesy of H. Bluestein)

Citation: Monthly Weather Review 135, 4; 10.1175/MWR3349.1

Fig. 3.
Fig. 3.

Depiction of the radar reflectivity (dBZ, color coded at the left) of the Attica tornado, from data collected by the UMass, mobile, X-band Doppler radar (UMass X-Pol) at (a) 1951:49 CDT 12 May 2004, in “surveillance” mode; the entire parent supercell is shown; beam blockage is noted to the northeast (thick solid line with arrows on either end); (b) at 1951:49 CDT 12 May 2004, but inset for area around the tornado; slightly different color-scale increment from that used in (a); and (c) at 2001:09 CDT 12 May 2004, in “data-collection” mode; much of the parent supercell is shown. Range markings (white dashed lines) are shown in (a) every 4 km (range plotted in km/4), (b) every 250 m, and (c) every 1 km. The elevation angle was ∼2.5°.

Citation: Monthly Weather Review 135, 4; 10.1175/MWR3349.1

Fig. 4.
Fig. 4.

Features of the tornado on 12 May 2004, just east of Attica, KS, as depicted by UMass X-Pol, but on a small scale just for the area in the vicinity of the tornado. (a) Radar reflectivity factor Z (dBZ), (b) differential reflectivity ZDR, (c) cross-correlation coefficient ρhv, and (d) Doppler velocity V (m s−1) at left (right) 2001:09 (2001:23) CDT. Range markers are displayed every 250 m [except in (b), where they are given every 200 m]; range marker values are given in km, but are truncated/rounded (so, e.g., 4.2 km is actually 4.25 km and 4.8 km is actually 4.75 km). Color codes for the scale of the parameters are shown at the bottom. Dot indicates the approximate location of the center of the tornado. Enlarged color scales are reproduced along the sides.

Citation: Monthly Weather Review 135, 4; 10.1175/MWR3349.1

Fig. 5.
Fig. 5.

Radial profiles of radar reflectivity (dBZ), Doppler velocity (m s−1), and differential reflectivity ZDR (dB × 10), during the mature stage of the Attica tornado. Profiles are from samples taken at a constant range, passing through the center of the tornado (denoted as a dot in Fig. 4). The distance from the vortex center is negative (positive) to the left (right) of the radar view of the tornado at (a) 2001:09 and (b) 2001:23 CDT.

Citation: Monthly Weather Review 135, 4; 10.1175/MWR3349.1

Fig. 6.
Fig. 6.

Image of the tornado subsequent and to the east of the Attica, KS, tornado (the fourth tornado) at approximately 2022 CDT. Image is from a frame captured from a video. The view is to the southwest (cf. Fig. 1). (Image courtesy of H. Bluestein)

Citation: Monthly Weather Review 135, 4; 10.1175/MWR3349.1

Fig. 7.
Fig. 7.

Same as in Fig. 3, but for the tornado seen in Fig. 6, at 2021:28 CDT; the range markers are given in km. The elevation angle was ∼3°. Radar was in “data-collection” mode.

Citation: Monthly Weather Review 135, 4; 10.1175/MWR3349.1

Fig. 8.
Fig. 8.

Same as in Fig. 4, but for the tornado seen in Fig. 6, at left (right) 2021:28 (2021:44) CDT; (a) a dot is not plotted at 2021:28 so that the pinhole of light green at the center can be seen.

Citation: Monthly Weather Review 135, 4; 10.1175/MWR3349.1

Fig. 9.
Fig. 9.

Same as in Fig. 5, but for the tornado seen in Fig. 6 at (a) 2021:28 and (b) 2021:44 CDT.

Citation: Monthly Weather Review 135, 4; 10.1175/MWR3349.1

Fig. 10.
Fig. 10.

Approximate tornado tracks of one of several cyclonic tornadoes near Geary, OK (to the west, off the map), and Calumet, OK, on 29 May 2004, as determined by the NWS, Norman, OK, and from tracks determined by a DOW radar. Also shown are the deployment sites (R1 and R2) of the UMass X-Pol. Base map adapted from Mapquest. Approximate times are given along the tracks in CDT as YYZZ, where the time in hours (as “19” or “20”) is not shown, while YY and ZZ represent minutes and seconds, respectively. The second track was for one of several, rare, anticyclonic tornadoes. Damage in the track of the cyclonic tornado was rated as F1, almost F2.

Citation: Monthly Weather Review 135, 4; 10.1175/MWR3349.1

Fig. 11.
Fig. 11.

(a) Photograph of the supercell near Geary, OK, on 29 May 2004, viewed from the east, 1.6 km north of Calumet, OK, at approximately 1947 CDT; wide-angle (17 mm) view is to the west (cf. Fig. 10). If there were a tornado present in the storm when (a) was taken, it would have been in the lower-right-hand sector of the image (denoted by “circulation”), hidden by precipitation. According to observations from the DOW (see online at http://www.cswr.org/dataimages/rotate/geary-summary-2004-0711fp.pdf), a tornado (noted “E”) did not appear until 1952 CDT. (b) Idealized depiction of the cloud features in an HP supercell, from approximately the same vantage point as in (a). From Moller et al. (1994). (Photograph courtesy of H. Bluestein)

Citation: Monthly Weather Review 135, 4; 10.1175/MWR3349.1

Fig. 12.
Fig. 12.

(a) Storm-scale depiction of the radar reflectivity (dBZ) in the Geary–Calumet supercell at 1943:12 CDT 29 May 2004. Data were collected by UMass X-Pol in “surveillance mode.” Range markings are given (in black) in km, every 2 km; the white range markers are shown every 2 km, with range plotted in km/4. The arrow points to a band of reflectivity that connects the main body of the storm to the “eye” and rear-flank gust front in the lower-left quadrant of the image. The “notch” of low reflectivity corresponds to the bright area seen in the lower-right portion in Fig. 11a. The lack of data in a narrow swath to the north of the radar is due to beam blockage. (b) Idealized plan-view depiction of radar-echo distribution, anvil edge, storm-induced surface boundaries, and storm updraft in an HP supercell. From Moller et al. (1994).

Citation: Monthly Weather Review 135, 4; 10.1175/MWR3349.1

Fig. 13.
Fig. 13.

Same as in Fig. 4, but for a mesocyclone northwest of Calumet, OK, at the left (right) 1948:12 (1950:07) CDT 29 May 2004, and with range markers shown every 500 m. Circle indicates a possible small-scale vortex signature.

Citation: Monthly Weather Review 135, 4; 10.1175/MWR3349.1

Fig. 14.
Fig. 14.

Same as in Fig. 13, but for a tornado at 1955:37 (1957:17) CDT.

Citation: Monthly Weather Review 135, 4; 10.1175/MWR3349.1

Fig. 15.
Fig. 15.

(a) Same as in Fig. 12, but at (a) 2003:54 and (b) 2004:58 CDT. A rare, anticyclonic hook echo is noted. Two swaths of significant beam blockage are indicated by the thick solid-line segments with arrows at either end. (c) (above and to the left) WSR-57 radar reflectivity image of cyclonic and anticyclonic tornadoes to the northwest of Grand Island, NE, on 3 Jun 1980; from the NWS, Grand Island, NE; (below and to the right) estimate of streamlines about cyclonic and anticyclonic tornadoes at 2040 m and 810 m AGL, respectively, at 2108 CDT. From Fujita (1981). Corresponding weak-echo “eyes” and anticyclonic hooks are connected by lines.

Citation: Monthly Weather Review 135, 4; 10.1175/MWR3349.1

Fig. 16.
Fig. 16.

Radar depiction from UMass X-Pol of a cyclonic–anticyclonic tornado–mesocyclone–mesoanticyclone pair at 2008:41 CDT 29 May 2004. (a) Radar reflectivity factor Z (dBZ), (b) differential reflectivity ZDR (dB), (c) Doppler velocity V (m s−1), and (d) cross-correlation coefficient ρhv. Circles in (a) and (c) denote the region of the cyclonic–vortex signature. (e) Expanded view of radar reflectivity factor in (a); color scale is different to enable the reader to see the pattern more easily. (f) Expanded view of Doppler velocity in (c); color scale is different to enable the reader to see the pattern more easily; circle marks location of anticyclonic vortex signature. (g) Expanded view of cross-correlation coefficient in (d). Range markers are displayed every 500 m; range marker values are given in km. Color codes for the scale of the parameters are shown at the top. In (a), the color scale is the same as that in Fig. 4a. In (b), light green is 0 dB, yellow is from ∼1.5–∼2.5 dB, and medium green is from ∼−1.4 to ∼−2.5 dB. In (c), brown is ∼10–12.5 m s−1, medium green is ∼−7.5 m s−1. In (d), the color scale is the same as that in Fig. 4c. In (e), yellow–brown are ∼32–40 dBZ. In (f), pink represents ∼11–13 m s−1, medium green ∼−7 m s−1.

Citation: Monthly Weather Review 135, 4; 10.1175/MWR3349.1

Fig. 17.
Fig. 17.

Photograph of an anticyclonic tornado to the east-northeast of Calumet, OK, at 2010 CDT, viewed to the northeast from 1.1 km south of Calumet (cf. Fig. 10). (Photograph courtesy of H. Bluestein)

Citation: Monthly Weather Review 135, 4; 10.1175/MWR3349.1

Table 1.

Characteristics of the UMass X-Pol.

Table 1.
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  • Fig. 1.

    Tornado tracks and estimated F-scale rating of each tornado on 12 May 2004, as determined by the NWS, Wichita, KS. Also shown are two of the deployment sites (R1 and R3) of the UMass mobile, X-band radar when data were being collected, for the second and fourth tornado.

  • Fig. 2.

    Selected images of the Attica tornado, during its mature stage, at (a) 2002 and (b) 2003 CDT 12 May 2004. The view is to the west from a location approximately 5 km east of Attica, KS. The approximate dimensions of the opaque debris cloud, condensation funnel, and height of the cloud base, as determined from photogrammetric analysis, are as indicated. (Photographs courtesy of H. Bluestein)

  • Fig. 3.

    Depiction of the radar reflectivity (dBZ, color coded at the left) of the Attica tornado, from data collected by the UMass, mobile, X-band Doppler radar (UMass X-Pol) at (a) 1951:49 CDT 12 May 2004, in “surveillance” mode; the entire parent supercell is shown; beam blockage is noted to the northeast (thick solid line with arrows on either end); (b) at 1951:49 CDT 12 May 2004, but inset for area around the tornado; slightly different color-scale increment from that used in (a); and (c) at 2001:09 CDT 12 May 2004, in “data-collection” mode; much of the parent supercell is shown. Range markings (white dashed lines) are shown in (a) every 4 km (range plotted in km/4), (b) every 250 m, and (c) every 1 km. The elevation angle was ∼2.5°.

  • Fig. 4.

    Features of the tornado on 12 May 2004, just east of Attica, KS, as depicted by UMass X-Pol, but on a small scale just for the area in the vicinity of the tornado. (a) Radar reflectivity factor Z (dBZ), (b) differential reflectivity ZDR, (c) cross-correlation coefficient ρhv, and (d) Doppler velocity V (m s−1) at left (right) 2001:09 (2001:23) CDT. Range markers are displayed every 250 m [except in (b), where they are given every 200 m]; range marker values are given in km, but are truncated/rounded (so, e.g., 4.2 km is actually 4.25 km and 4.8 km is actually 4.75 km). Color codes for the scale of the parameters are shown at the bottom. Dot indicates the approximate location of the center of the tornado. Enlarged color scales are reproduced along the sides.

  • Fig. 5.

    Radial profiles of radar reflectivity (dBZ), Doppler velocity (m s−1), and differential reflectivity ZDR (dB × 10), during the mature stage of the Attica tornado. Profiles are from samples taken at a constant range, passing through the center of the tornado (denoted as a dot in Fig. 4). The distance from the vortex center is negative (positive) to the left (right) of the radar view of the tornado at (a) 2001:09 and (b) 2001:23 CDT.

  • Fig. 6.

    Image of the tornado subsequent and to the east of the Attica, KS, tornado (the fourth tornado) at approximately 2022 CDT. Image is from a frame captured from a video. The view is to the southwest (cf. Fig. 1). (Image courtesy of H. Bluestein)

  • Fig. 7.

    Same as in Fig. 3, but for the tornado seen in Fig. 6, at 2021:28 CDT; the range markers are given in km. The elevation angle was ∼3°. Radar was in “data-collection” mode.

  • Fig. 8.

    Same as in Fig. 4, but for the tornado seen in Fig. 6, at left (right) 2021:28 (2021:44) CDT; (a) a dot is not plotted at 2021:28 so that the pinhole of light green at the center can be seen.

  • Fig. 9.

    Same as in Fig. 5, but for the tornado seen in Fig. 6 at (a) 2021:28 and (b) 2021:44 CDT.

  • Fig. 10.

    Approximate tornado tracks of one of several cyclonic tornadoes near Geary, OK (to the west, off the map), and Calumet, OK, on 29 May 2004, as determined by the NWS, Norman, OK, and from tracks determined by a DOW radar. Also shown are the deployment sites (R1 and R2) of the UMass X-Pol. Base map adapted from Mapquest. Approximate times are given along the tracks in CDT as YYZZ, where the time in hours (as “19” or “20”) is not shown, while YY and ZZ represent minutes and seconds, respectively. The second track was for one of several, rare, anticyclonic tornadoes. Damage in the track of the cyclonic tornado was rated as F1, almost F2.

  • Fig. 11.

    (a) Photograph of the supercell near Geary, OK, on 29 May 2004, viewed from the east, 1.6 km north of Calumet, OK, at approximately 1947 CDT; wide-angle (17 mm) view is to the west (cf. Fig. 10). If there were a tornado present in the storm when (a) was taken, it would have been in the lower-right-hand sector of the image (denoted by “circulation”), hidden by precipitation. According to observations from the DOW (see online at http://www.cswr.org/dataimages/rotate/geary-summary-2004-0711fp.pdf), a tornado (noted “E”) did not appear until 1952 CDT. (b) Idealized depiction of the cloud features in an HP supercell, from approximately the same vantage point as in (a). From Moller et al. (1994). (Photograph courtesy of H. Bluestein)

  • Fig. 12.

    (a) Storm-scale depiction of the radar reflectivity (dBZ) in the Geary–Calumet supercell at 1943:12 CDT 29 May 2004. Data were collected by UMass X-Pol in “surveillance mode.” Range markings are given (in black) in km, every 2 km; the white range markers are shown every 2 km, with range plotted in km/4. The arrow points to a band of reflectivity that connects the main body of the storm to the “eye” and rear-flank gust front in the lower-left quadrant of the image. The “notch” of low reflectivity corresponds to the bright area seen in the lower-right portion in Fig. 11a. The lack of data in a narrow swath to the north of the radar is due to beam blockage. (b) Idealized plan-view depiction of radar-echo distribution, anvil edge, storm-induced surface boundaries, and storm updraft in an HP supercell. From Moller et al. (1994).

  • Fig. 13.

    Same as in Fig. 4, but for a mesocyclone northwest of Calumet, OK, at the left (right) 1948:12 (1950:07) CDT 29 May 2004, and with range markers shown every 500 m. Circle indicates a possible small-scale vortex signature.

  • Fig. 14.

    Same as in Fig. 13, but for a tornado at 1955:37 (1957:17) CDT.

  • Fig. 15.

    (a) Same as in Fig. 12, but at (a) 2003:54 and (b) 2004:58 CDT. A rare, anticyclonic hook echo is noted. Two swaths of significant beam blockage are indicated by the thick solid-line segments with arrows at either end. (c) (above and to the left) WSR-57 radar reflectivity image of cyclonic and anticyclonic tornadoes to the northwest of Grand Island, NE, on 3 Jun 1980; from the NWS, Grand Island, NE; (below and to the right) estimate of streamlines about cyclonic and anticyclonic tornadoes at 2040 m and 810 m AGL, respectively, at 2108 CDT. From Fujita (1981). Corresponding weak-echo “eyes” and anticyclonic hooks are connected by lines.

  • Fig. 16.

    Radar depiction from UMass X-Pol of a cyclonic–anticyclonic tornado–mesocyclone–mesoanticyclone pair at 2008:41 CDT 29 May 2004. (a) Radar reflectivity factor Z (dBZ), (b) differential reflectivity ZDR (dB), (c) Doppler velocity V (m s−1), and (d) cross-correlation coefficient ρhv. Circles in (a) and (c) denote the region of the cyclonic–vortex signature. (e) Expanded view of radar reflectivity factor in (a); color scale is different to enable the reader to see the pattern more easily. (f) Expanded view of Doppler velocity in (c); color scale is different to enable the reader to see the pattern more easily; circle marks location of anticyclonic vortex signature. (g) Expanded view of cross-correlation coefficient in (d). Range markers are displayed every 500 m; range marker values are given in km. Color codes for the scale of the parameters are shown at the top. In (a), the color scale is the same as that in Fig. 4a. In (b), light green is 0 dB, yellow is from ∼1.5–∼2.5 dB, and medium green is from ∼−1.4 to ∼−2.5 dB. In (c), brown is ∼10–12.5 m s−1, medium green is ∼−7.5 m s−1. In (d), the color scale is the same as that in Fig. 4c. In (e), yellow–brown are ∼32–40 dBZ. In (f), pink represents ∼11–13 m s−1, medium green ∼−7 m s−1.

  • Fig. 17.

    Photograph of an anticyclonic tornado to the east-northeast of Calumet, OK, at 2010 CDT, viewed to the northeast from 1.1 km south of Calumet (cf. Fig. 10). (Photograph courtesy of H. Bluestein)

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