Surface Analysis Near and Within the Tipton, Kansas, Tornado on 29 May 2008

370 MONTHLY WEATHER REVIEW VOLUME 139

Surface Analysis near and within the Tipton, Kansas, Tornado on 29 May 2008

BRUCE D. LEE AND CATHERINE A. FINLEY WindLogics, Inc., Grand Rapids, Minnesota

TIMOTHY M. SAMARAS National Technical Systems, Littleton, Colorado

(Manuscript received 13 April 2010, in ﬁnal form 7 September 2010)

ABSTRACT

Data collected by a mesonet within the near-tornado environment and in the Tipton tornado on 29 May 2008 provided a rare opportunity to analyze rear-flank downdraft (RFD) outflow properties closely bounding a tornado and to characterize parcel thermodynamics being ingested into a tornado from the rear-flank downdraft. Parcels moving into the tornado on its right flank had very small negative buoyancy and considerable potential buoyancy. Measurements within and very near the tornado showed similar buoyancy characteristics to the storm inflow. Analyzed surface divergence and videographic evidence indicated that the RFD outflow just to the right and wrapping in front of the tornado was supported by parcels moving out of a narrow downdraft bordering the right flank of the tornado. Surface flow field analysis showed that parcels moved out of the downdraft-associated divergence region and into the right side of, as well as in front of, the tornado. An internal RFD surge boundary was positioned roughly 0.5 km in front of the eastern edge of the analyzed divergence region and implied downdraft. The broader RFD outflow thermodynamic characteristics were consistent with recent research with only small negative buoyancy and substantial potential buoyancy; however, convective inhibition was considerably higher than typically found in other tornadic cases. This latter characteristic was emblematic of the broader storm environment on this day. Parcels making up the RFD outflow originated from low-levels, consistent with recent findings for tornadic rear-flank downdrafts and in contrast to past historical indications for the rear-flank downdraft source region.

1. Introduction datasets within this region exist. Observational studies have shown that rear-flank downdraft (RFD) outflow The ability to understand the processes involved in surrounds or nearly surrounds the tornado (e.g., Brandes tornadogenesis, maintenance, and decay is dependent, in 1977, 1978; Lemon and Doswell 1979; Markowski et al. large part, on obtaining observations in key regions of 2002, hereafter MSR2002; Wurman et al. 2007b); thus, the supercell thunderstorms that historically have been very properties of the near-tornado RFD outflow are espe- difficult to gather. One such area is within roughly 1 km of cially important across the tornado life cycle. In particu- the tornado or tornadogenesis region that contains the air lar, buoyancy characteristics of the RFD outflow may play parcels that ultimately comprise the tornado inflow. With an important role in the development, intensification, the rapid evolution of mobile Doppler radar (Wurman maintenance, and diminution of near-surface rotation et al. 1997; Bluestein and Pazmany 2000; Bluestein and (e.g., Leslie and Smith 1978; Davies-Jones and Brooks Wakimoto 2003), a considerable number of kinematic 1993; Adlerman et al. 1999; Davies-Jones et al. 2001; datasets have been gathered in tornadogenesis or tornado Markowski 2002b; Markowski et al. 2003a, 2008). proximate regions; however, very few thermodynamic The association between supercell thunderstorm RFDs and tornadoes has long been recognized (Markowski 2002b). More recent research has focused on direct mea- Corresponding author address: Dr. Bruce D. Lee, WindLogics, Inc., Itasca Technology Exchange, 201 NW 4th St., Grand Rapids, surements within the RFD outflow by utilizing a mobile MN 55744. mesonet (Straka et al. 1996). The analysis of MSR2002 E-mail: [email protected] and Grzych et al. (2007, hereafter GLF2007) revealed

DOI: 10.1175/2010MWR3454.1

Ó 2011 American Meteorological Society Unauthenticated | Downloaded 10/06/21 05:18 PM UTC FEBRUARY 2011 L E E E T A L . 371 compelling evidence supporting the general conclusion tornado from this storm formed near Glen Elder Dam that tornadic and nontornadic supercells had differing and eventually produced EF-3 damage southwest of RFD thermodynamic characteristics. Specifically, the Jewell, Kansas (NCDC 2008). results of MSR2002 (which are supported by GLF2007) TWISTEX was conducted during May and June of show that tornado likelihood, intensity, and longevity 2008 with a domain that included regions from the upper increase as the near-surface buoyancy, potential buoy- Midwest through the southern Great Plains. The project ancy, and equivalent potential temperature ue increase had a typical complement of three mobile mesonet ve- in the RFD outflow, and as the convective inhibition hicles and a probe vehicle that transported an array of (CIN) in the RFD outflow decreases. HITPR probes and two video probes [see Karstens et al. Although a substantial number of mobile mesonet (2010), their Fig. 3]. The primary objective of the field RFD outflow datasets have been collected and analyzed portion of TWISTEX was to gather thermodynamic and over the past approximately 16 years to determine the kinematic data with a mobile mesonet in the RFD out- association between RFD outflow thermodynamic and flow region near tornadoes and the adjacent RFD gust kinematic character and the tornadic nature of a supercell front (RFDGF) region while concurrently gathering (Markowski 2002a; MSR2002; Lee et al. 2004; Finley and thermodynamic data with the HITPRs in or very near Lee 2004; GLF2007; Hirth et al. 2008), there exist com- tornadoes. The sampling goal was designed such that a paratively few RFD outflow mesonet datasets with sam- combined thermodynamic and kinematic mapping could pling within very close range of a tornado. Additionally, be done in the tornadogenesis and tornado maintenance with the exception of the Verification of the Origins regions while also addressing project objectives involving of Rotation in Tornadoes Experiment (VORTEX; near-surface tornadic flow field analysis with the aid of Rasmussen et al. 1994) deployment on 8 June 1995 near the video probes. Allison, Texas (Winn et al. 1999; MSR2002, see their Fig. 10), no published cases existed before the present 2. Synoptic and mesoscale environment dataset was obtained in which coordinated sampling of the near-tornado RFD outflow by a mobile mesonet was Conditions over northern Kansas and southern undertaken while in situ probes were collecting near and Nebraska on the evening of 29 May were generally very internal tornado observations. favorable for tornadic supercells. The outbreak was as- During the Tactical Weather-Instrumented Sampling sociated with a southwest flow upper-level pattern in In/Near Tornadoes Experiment (TWISTEX), a mobile place over the southwestern and central United States as mesonet and an in situ thermodynamic probe called the shown in the 0100 UTC Rapid Update Cycle (RUC) Hardened In Situ Tornado Pressure Recorder (HITPR; model (Benjamin et al. 2004) 500-mb analysis in Fig. 1. Samaras and Lee 2004) collected data near and within the During the day on 29 May a short wave moved into the tornado that occurred in the vicinity of Tipton, Kansas, central plains, and by evening the right entrance region of on 29 May 2008. Additionally, a seven-camera in situ the jet streak associated with this short wave was located video probe was deployed approximately 10-20 m south over northwest Kansas and southwest Nebraska as shown of the thermodynamic probe to record full 3608 videogra- in Fig. 1. This jet streak was apparent up to the 350-mb phy of the tornado passage. The tornado sampled (here- level. An upstream short wave may be seen over Arizona after called the Tipton tornado) was just one of many and western New Mexico but it is not believed to have tornadoes produced by its parent supercell (NCDC 2008). played a role in influencing the deep-convective environ- The Tipton tornado occurred in open country and re- ment for this case. In conjunction with the central Plains ceived an EF-1 rating with damage relegated to power short wave passage, a strong low-level jet was in place poles and trees during its estimated life of 23 min [based across much of southern and eastern Nebraska and most on TWISTEX tornadogenesis observation and NCDC of Kansas, as seen in the 850-mb vector winds in Fig. 1. A (2008) reported time of dissipation]. With the tornado substantial area within this region extending from west- passing over the in situ probes, with one mesonet station central Kansas to south-central Nebraska had 850-mb positioned 235 m south of the HITPR (roughly 190 m southerly winds exceeding 25 m s21. When coupled with south of the tornado edge) and another mesonet station the surface flow to be presented next, the deep-layer located up to 1 km farther south, the dataset collected vertical wind shear environment over much of Kansas and provides an opportunity to determine the thermody- eastern Nebraska was quite favorable for supercell thun- namic character of the flow getting ingested into the derstorms. tornado from the RFD outflow along the right flank At the surface, an area of low pressure was situated (looking downstream) of the tornado. It is worth noting under the right entrance region of the upper-level jet that just before the Tipton tornado dissipated, the next streak in northwest Kansas (Fig. 2). Strong southerly to

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FIG. 1. Geopotential height field at 500 mb (m, solid lines) and FIG. 2. Surface synoptic analysis at 0100 UTC 30 May 2008 with vector winds at 500 (black arrows) and 850 mb (gray arrows) for mean sea level pressure and subjective frontal positions. Temper- 0100 UTC 30 May 2008 (from RUC analysis). The maximum wind ature (8F), dewpoint (8F), and wind (half barb 5 kt; full barb 10 kt) vectors (m s21) are shown in the lower right. are plotted in the station models. Isobars are shown every 2 mb. The open circle shows the formation location at 2148 UTC of the cell that developed into the Tipton supercell. The black dot locates south-southeasterly winds were present over much of the Tipton supercell at 0100 UTC. the Kansas portion of the warm sector of this cyclone with seasonally high dewpoints and narrowing dewpoint with large convective inhibition. Considerable conditional depressions as the evening progressed over the northern instability was present with surface-based and lowest Kansas area of interest. The cell that would eventually 50-mb mixed parcel convective available potential energy become the cyclic tornadic supercell associated with the (CAPE) of 2291 and 2619 J kg21, respectively. CIN for Tipton tornado (hereafter this storm will be designated surface-based (232 J kg21) or lowest 50-mb (156 J kg21) the Tipton supercell or Tipton storm), formed within a parcel ascent was remarkably high for an environment broken line segment of developing storms at 2148 UTC sustaining tornadic supercells (Rasmussen and Blanchard in far west-central Kansas about 78 km south of Good- 1998). The virtual temperature correction was applied land, as shown in Fig. 2. Initial cell formation with this to the parcel temperature ascent curve as described in broken line commenced much earlier at 1915 UTC near Doswell and Rasmussen (1994) for the CAPE and CIN the dryline. The Tipton storm took on supercell char- calculations. Along with the large CIN, the sounding acteristics (Browning 1964; Rotunno 1993) by 2238 UTC profile below 3 km above ground level (AGL) yielded just south of Oakley, Kansas. By 0100 UTC the supercell no CAPE in this layer for either surface-parcel or lowest had moved into north-central Kansas as indicated in 50-mb parcel ascent. This tornadic supercell case runs Fig. 2 and was located in central Osborne County. Dew- counter to expectations based on Rasmussen (2003) point depressions at this time were only in the 68–98F(38– for typical values of 0–3-km CAPE in environments 58C) range based on indications from the mesonet and supportive of supercells producing significant tornadoes 0100 UTC RUC surface analysis. (middle two quartiles of 0–3-km CAPE in Rasmussen’s To assess the deep convective environment for the sounding distribution range from 18 to 92 J kg21). Fi- Tipton storm, RUC 0100 UTC analysis data were used nally, the surface parcel–based lifting condensation level to create a representative sounding and hodograph for (LCL) was 620 m for this sounding (865 m for lowest central/eastern Osborne County. This approach was pre- 50-mb parcel ascent). Low-LCL environments such as ferred over applying modifications to regional 0000 UTC this have been shown to be associated with significant soundings taken at considerable distances from the storm. tornadoes (Rasmussen and Blanchard 1998; Thompson Based on the sounding and hodograph shown in Fig. 3, the et al. 2003). conditions just preceding the Tipton storm at 0100 UTC The low and deep-layer shear environment for north- were generally quite favorable for supercells, with some central Kansas was very conducive for storm midlevel parameters very favorable for tornadic supercells, pro- and low-level rotation. The hodograph in Fig. 3 has large vided a storm could form or persist within an environment looping clockwise curvature with a profile indicative of

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supercells (Rasmussen 2003; Thompson et al. 2003). The 7.6 EHI should be considered a very large value based on these past studies. In summary, while the majority of key parameters were quite favorable for tornadic supercells, there were a few, such as the CIN and 0–3-km CAPE, that were atypical for tornadic supercells. Note that there were no storms in Kansas south of the Tipton storm through most of the supercell stage of its life, indicating that this storm existed on the very southern margin of where the atmosphere was capable of sustaining deep convection.

3. Data collection and methodology The TWISTEX sampling array on 29 May consisted of two mobile mesonet stations and a probe deployment team with a full complement of six thermodynamic and two video in situ probes. The third mobile mesonet platform was unable to participate because of damage sustained in a previous operation. The mobile mesonet measures temperature, pressure, humidity, and wind ve- FIG. 3. Skew T-logp thermodynamic diagram and hodograph for locity. Time and position were recorded using a global central Osborne County, KS, at 0100 UTC based on RUC analysis. positioning system (GPS). The type of instrumentation Temperature (heavy black line), dewpoint (medium gray line), and mobile mesonet station configuration were based on virtual temperature (black dashed line), and 50-mb mixed layer the design presented by Straka et al. (1996). Some in- parcel ascent path with virtual temperature correction (thick dark gray line) are shown. Altitudes (3100 m) and storm motion vector strumentation differences exist between those specified for the Tipton supercell at 0100 UTC are shown on the hodograph. in Straka et al. and newer mesonet stations such as those used by TWISTEX and Texas Tech University’s Atmo- strong directional and speed vertical wind shear. Deep spheric Science Group, which share the same instrumen- layer 0–6-km vector shear of 30.8 m s21 was more than tation [see Table 1 of Hirth et al. (2008)]. Field procedures sufficient to support supercells, consistent with past mod- were developed such that the GPS could be used for ve- eling and observational research (Weisman and Klemp hicle direction at all times, eliminating the need for a flux 1982; Rasmussen and Blanchard 1998; Thompson et al. gate compass. The field data were collected every 2 s. 2003). To calculate storm-relative helicity (SRH; Davies- A narrow deployment window limited the number of Jones et al. 1990) at 0100 UTC, a cell motion of 2508 at in situ probes used to two HITPRs and one video probe. 15.4 m s21 was determined using Weather Surveillance The rapidly deployable HITPR is aerodynamically Radar-1988 Doppler (WSR-88D; Klazura and Imy 1993) shaped and engineered to obtain thermodynamic obser- level II data from Hastings, Nebraska. In part because of the vations within the harsh tornado environment. The probe marked low-level jet (Bonner 1968) in place (.25 m s21 is outfitted with fast response time sensors that measure between 700 and 1500 m), SRH values were especially pressure, temperature, and humidity. An internal data- large with a 0–1-km SRH of 462 m2 s22 and 0–3-km SRH logger records the sensor readings at 10 Hz. The therm- of 669 m2 s22. Based on the storm environment param- istor and hygristor are located near the top of the cone eter studies of Rasmussen and Blanchard (1998) and just above a set of ventilation holes that surround the Thompson et al. (2003), the 0–3-km SRH was very sup- probe. Details of the design of the HITPR may be found portive of supercells, and the 0–1-km SRH value suggests in Samaras and Lee (2004). Once deployed, the HITPRs enhanced potential for tornadic supercells (Rasmussen taking measurements at about 0.12 m (AGL) effectively 2003; Thompson et al. 2003). The energy helicity index become part of the mesonet. On this day one of the (EHI; Hart and Korotky 1991) helicity–CAPE composite HITPRs failed to record data. The video probe is out- parameter was calculated over the 0–1-km layer and had fitted with seven cameras that provide a tornado-relative a value of 7.6. EHI calculated over the 0–1-km layer has reference for the sampling position of the HITPRs and shown substantial skill in discriminating between envi- provide videography for both vortex structural assess- ronments associated with supercells with significant tor- ment and visual analysis of the near-tornado environ- nadoes and those of nontornadic or only weakly tornadic ment. These probes provide a full 3608 field of view of the

Unauthenticated | Downloaded 10/06/21 05:18 PM UTC 374 MONTHLY WEATHER REVIEW VOLUME 139 evolving and translating near-ground ﬂow-ﬁeld as made from the technique used by MSR2002 whereby the visible by debris and/or condensation as the tornado passes base state was determined by a weighted mean of un- as well as the evolving near-tornado cloud structure. Six contaminated surface airway, aviation routine weather cameras are positioned horizontally, each spanning a 608 report, and Oklahoma Mesonet observations within a horizontal view. The seventh camera is positioned ver- 400-km radius of the storm updraft. Because of hygristor tically. differences between the HITPR and mobile mesonet

Owing to inaccuracies in the mesonet anemometry stations, and noting the sensitivity of ue to relatively during significant vehicle accelerations, velocity data were small differences in water vapor content, we chose not to removed in a similar manner as employed by MSR2002, calculate ue for the HITPR. Thus, graphics containing u9e Markowski (2002a), and GLF2007. The mesonet datasets information will be for the mobile mesonet only. Ad- were also quality-controlled for spurious meteorological ditionally, because of instrument equilibration for the readings and vehicle headings. Biases were removed by HITPR upon deployment, temperature and relative hu- way of intercomparisons between mesonet stations for midity data from the probe were not used until the in- extensive periods when the caravan was in relatively strumentation had equilibrated with site conditions. uniform meteorological conditions and predominantly For portions of the analysis, data points were plotted in transit. relative to Hastings, Nebraska, WSR-88D (KUEX) radar Derived thermodynamic variables were calculated in data using a time-to-space conversion as described by addition to the variables measured directly by the meso- MSR2002. This process put the observational mesonet net. The equivalent potential temperature ue was cal- data into the storm’s positional frame of reference. Five- culated as in MSR2002 and GLF2007 using the formula minute time intervals were used in these time–space derivation of Bolton (1980). In the calculation of the conversion plots consistent with the approximate time for virtual potential temperature uy [see Glickman (2000), a single WSR-88D volume scan; however, aspects of the p. 820], no hydrometeors were assumed to be present. storm kinematic and thermodynamic structure near the These data were collected on storms at a significant hook echo are not in steady state for this length of time, so distance from the nearest WSR-88D site and since the the wings of plotted data carry less credence. The Tipton rain intensity, when encountered by the teams, varied tornado was estimated to move at the same speed and markedly on small space and time scales, we had little direction as the radar-indicated low-level mesocyclone 21 confidence in estimating the liquid water mixing ratio ql (from 2558 at 14.6 m s ) during the approximate center from radar reflectivity. No hail was observed by any of of the observing period (i.e., the time centered around the teams, and when the teams were in precipitation, the 0122:14 UTC). Videography and damage track analysis rainfall never exceeded a qualitative moderate intensity. support this estimate. Time series analysis was used to

Although the exclusion of ql in the formal calculation of identify kinematic and thermodynamic transitions of uy results in less accurate values, only small errors in uy the flow field as concurrently observed by the array. To calculations are expected given the rainfall intensity remove some of the very small time scale fluctuations, experienced. For instance, had the radar reflectivity 6-s data averages were used in the analysis except where been 40 dBZ in this case, an overestimate in uy of about noted. 0.22 K would be incurred from neglecting the ql esti- In accordance with the motion of the Tipton storm, mate from the parameterization of Rutledge and Hobbs teams set up in a north–south observing array along (1984). Kansas Highway 181 (K-181) and in the storm-relative The CAPE and CIN for all the mesonet data points position shown in Fig. 4. The HITPR and video probe were calculated using a sounding developed from RUC were farthest to the north and in the path of the tornado, 0100 UTC analysis data with surface conditions in the with mesonet stations M3 and M2 positioned progres- sounding provided by the mesonet readings. The CAPE sively farther south. and CIN are based on surface parcel ascent and include the virtual temperature correction described in Doswell and Rasmussen (1994). Surface elevation for the sound- 4. Observations and analysis ing was adjusted to be consistent with the elevation of this a. Storm inflow observations and event/deployment event. chronology To obtain the perturbation equivalent potential temperature ue9 and perturbation virtual potential tempera- Arriving ahead of the Tipton storm hook-echo passage ture uy9, base states were determined from mesonet over K-181, teams were able to collect inflow observations station storm inflow observations similar to GLF2007 at a location 8.5 km to the northwest of Tipton, Kansas and Hirth et al. (2008). Note that this method differs (Fig. 4). In a 5-min period just preceding the passage of the

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FIG. 4. Radar reflectivity (0.58) from KUEX at 0122:14 UTC with mesonet deployment location along K-181 shown with bold black line segment. Storm-relative inflow observations along K-181 between 0113:14 and 0118:14 UTC are shown at bottom. Station model displays temperature (8C, top), dewpoint (8C, bottom), and storm-relative winds (half barb 2.5 m s21, full barb 5 m s21, flag 25 m s21). storm’s RFDGF over K-181 at 0119:27 UTC, strong storm- their positions along K-181 for feature-relative data relative inflow winds were approximately 25 m s21 from sampling. Upon arriving at the point along K-181 where the east-northeast. These inflow observations reveal small the tornado was anticipated to cross, the probe deploy- dewpoint depressions of approximately 3.58Candreflect ment team was approximately 1 km east-northeast of the the low LCL storm environment noted in section 2. The tornado, which took on a considerably wider profile than low cloud bases seen at 0116:04 UTC in Fig. 5a are in- that observed approximately 3 min earlier as shown in dicative of this low LCL environment. At this time teams Fig. 5c. were observing increasing low-level rotation as a clear slot The HITPR and video probe deployments were suc- associated with subsiding air in the RFD (Markowski cessful, with the southern portion of the tornado tracking 2002b) wrapped into the south side of the visual low-level over the probes at 0122:53 UTC. A pressure drop of 15 mb mesocyclone. CAPE and CIN for storm-inflow surface- was recorded by the HITPR (Fig. 6), with M3 recording based parcel ascent were 2120 and 304 J kg21, respectively. a nearly 6-mb pressure drop at a distance of 235 m south The notably large inflow CIN and sounding structure seen of the HITPR. Note that M3 took up a stationary position in Fig. 3 are likely responsible for the remarkably sculpted for about 2 min during the time the tornado and accom- and laminar striated appearance of the Tipton storm panying strong RFD winds crossed K-181. Given video from a location well east of the deployment position, as probe evidence of the tornado-relative position of the seen in Fig. 5d. A short-lived tornado was first observed HITPR, and an estimated vortex width of 250 m, M3 was by the deployment teams at 0117:48 UTC. The Tipton located roughly 190 m south of the southern edge of the tornado developed shortly thereafter at 0118:35 UTC. tornado. Note that in the absence of a very close radar to With the trend in the tornado visual appearance indicat- determine the tornado core flow diameter, the 250-m ing a nontransient event (see Fig. 5b for 0118:53 UTC), tornado width was estimated using the tornado trans- both mesonet and probe teams were able to calibrate lation speed in concert with the video probe–derived

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FIG. 5. View of Tipton supercell low-level mesocyclone region and tornado looking (a) west at 0116:04, (b) west at 0118:53, (c) west-southwest at 0121:45, and (d) west at approximately 0130:00 (all times UTC). The image in the last panel is courtesy of Chris Collura. transect duration of the concentrated near-surface de- mesonet data analyses (Finley and Lee 2004; Lee et al. bris annulus. Corroborating width estimates were simi- 2004; Finley and Lee 2008; Hirth et al. 2008) and to recent larly made with the tornado translation speed and the radar observations (Wurman et al. 2007a; Marquis et al. HITPR pressure trace profile. M2 was generally mobile 2008a,b). This surge appears closely juxtaposed with the during this key observing period while sampling the tornado, not unlike the scenarios detailed in Finley and RFD outflow region 1 km south of M3. More details of Lee (2004, 2008) and Marquis et al. (2008a). the pressure traces from the HITPR and M3 along with tornado structure inferences from these data and the video probe recordings may found in Karstens et al. (2010). b. Spatial analysis The most important analysis period is shown in Fig. 7, centered on the KUEX WSR-88D 0122:14 UTC 0.58 base reflectivity scan since key storm features moved over the array within a few minutes either side of this time. The tornado passed over K-181 and the in situ probes at 0122:53 UTC. The mesonet observed the RFDGF passage at 0119:27 UTC and thus began sampling the RFD outflow less than 1 min after the estimated time of tornadogenesis. A comparison between the inflow conditions shown in Fig. 4 and conditions west (behind) the RFDGF in Fig. 7 indicates that the RFDGF represented FIG. 6. Time series of temperature (8C), dewpoint (8C), and a strong kinematic boundary with only meager thermo- pressure (mb) for the HITPR for a 100-s period centered on the time the tornado passed over the HITPR (see legend). The dashed dynamic changes across the boundary. Of kinematic im- portion of the dewpoint trace indicates an instrument equilibration portance, the mesonet data indicated an internal RFD ramp. No moisture information is used from the HITPR during this surge boundary behind the RFDGF similar to other recent equilibration period.

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FIG. 7. Mesonet and probe station data overlaid on Hastings 0122:14 UTC WSR-88D 0.58 base reflectivity imagery with time–space conversion applied. Top inset displays reflectivity and storm motion with inset showing the analysis domain. Mesonet data spans a 5-min period centered on 0122:14 UTC. Probe data (magenta points) spans the time period from 0122:25 to 0124:48 UTC. Tornado drawn to approximate near-surface scale and location at 0122:14 UTC. (top) Station models include temperature (8C, top), dewpoint (8C, bottom), and ground-relative winds along with subjective boundaries. (bottom) Subjective uy9 analysis with station models that 21 include uy9 (K, top), ue9 (K, bottom), and storm-relative winds. Full barb on wind staff is 5 m s and flag is 25 m s21. Data are separated by 14 s. Stations with no staff had wind data removed in quality control. Some overlapping station data (early) were removed for graphical clarity. Ob- servations more than 1 min before or after the reference time have a white bar through the station model center. A 20-s extension of the analysis was used to place the RFDGF.

As shown by MSR2002 and GLF2007, tornado like- was analyzed to assess the buoyancy within the RFD lihood and intensity appear related to thermodynamic outflow, some of which is ingested into the tornado, characteristics of the RFD outflow. Since fluctuations in noting the tornado-relative flow in the second panel of uy from the base state are proportional to buoyancy, u9y Fig. 7. Deficits in uy are quite small (,2 K) from behind

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FIG. 8. As in Fig. 7 (bottom), but a smaller and more highly populated analysis is shown with subjective streamlines. Divergence is hatched within an objectively analyzed region bounded by the mesonet station positions running from south-southwest to east-southeast of the tornado. Data points are separated by 8 s. the initial gust front to just west and southwest of the Given the focused mesonet sampling along and to the tornado. A local maximum in uy lies in the region near right side of the tornado and low-level mesocyclone and within the tornado with values approximating the track, a smaller region with a higher density of plotted storm inflow. The small size of the uy departures near the observations (every 8 s) was analyzed to examine the tornado and to the east, south, and within the first ki- two-dimensional flow field shown in Fig. 8. Over most lometer west of the tornado are generally consistent of the sampled RFD outflow the implied streamlines with those associated with tornado environments by reveal a generally diffluent RFD outflow, with slightly

MSR2002 and GLF2007. Extended analysis of the RFD cooler uy air being advected to the south and east in a for the next 5-min period following that shown in Fig. 7 storm-relative framework. Even the least buoyant RFD revealed no uy deficits larger than 2.7 K in the RFD parcels to the west of the tornado had uy deficits of just outflow region 4 km farther to the west. Although no 2 K, so during this sampling period the RFD outflow observations exist just to the north of the tornado, the proximate to the tornado and low-level mesocyclone tornado appears embedded in air having only weak had only modest negative buoyancy. negative buoyancy. In contrast to the diffluent RFD outflow observed over

The ue deficits within the RFD outflow shown in Fig. 7 a large portion of Fig. 8, a region of confluence exists are also quite modest with values less than 3 K for re- within several hundred meters south and southeast of the gions east, south, and within the first kilometer west of tornado. M3 is close enough to the tornado for the winds the tornado. The ue deficits are generally consistent with to reflect the induced radial inflow as the center of the those found by MSR2002 and GLF2007 for tornadic vortex passed by approximately 320 m to the north. Of

RFDs. Although ue deficits begin to exceed 3 K west of particular interest, the flow analysis of Fig. 8 indicates about 1 km from the tornado, extended analysis of the a narrow, highly diffluent region bounding this confluent RFD for the next 5-min period following that shown in region in a partial arc from south-southwest through east-

Fig. 7 revealed no ue deﬁcits larger than 3.8 K in the southeast and within 1 km of the tornado. A two-pass RFD region 4 km farther west. Barnes objective analysis scheme (Barnes 1964; Koch et al.

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FIG. 9. As in Fig. 7 (bottom), but for CAPE analyzed with station model showing CAPE (J kg21, top) and CIN (J kg21, bottom).

1983) was applied to the time–space converted mesonet has close to neutral buoyancy with the area just east and velocity components in a similar region, confined by the south of the tornado only slightly negatively buoyant. Of mesonet station positions, to interpolate the data to a consequence, the mapping shows parcels having small u9y two-dimensional grid for divergence analysis. As shown in values feeding into the tornado from the near-tornado Fig. 8, this analysis region was largely divergent and peak RFD outflow along its right flank. Advection inferences divergence values were about 2 3 1022 s21. Supporting along the streamlines in Fig. 8 are limited by two- evidence for this partial arc of divergence was found in the dimensionality and the persistence assumption in time– videography provided by the in situ video probe. One of space analysis. In regions impacted by large vertical the six horizontal viewing angles was aligned looking to the motions such as the divergence arc south through southeast, a direction where visibility through the glass shield east of the tornado, the flow moving along the depicted over the camera lens remained relatively free of rain and 3-m streamlines through or out of this region likely pos- debris. Just after the tornado passed over the HITPR, the sess considerable influence from parcels within the strong video clearly showed rapidly descending and evaporating downdraft above this region. Clearly the tornado is in- cloud tags just to the right of the tornado (looking down- gesting air with very little negative buoyancy from the stream), indicative of strong downdraft. In a tornado- region within several hundred meters south through east relative framework the placement of this downdraft of it, as depicted in Fig. 8. During this analysis period, the would align with the arc of divergence in Fig. 8. This region somewhat more negatively buoyant air to the west likely within the outflow also corresponds to an area of modestly does not make it into the tornado undiluted given the greater buoyancy as seen in the uy deficits plotted in Fig. 8, downdraft contributions to the flow nearer the tornado. especially when compared to the RFD outflow air to the Prior RFD outflow characterization studies by MSR2002 west. We will show later in this section that parcels in this and GLF2007 found that RFD outflows of tornadic su- region possessed considerable potential buoyancy. percells usually possessed considerable potential buoyancy Based on the implied flow field near the tornado shown (as did some of the nontornadic supercells, although on in Fig. 8, parcels move from a divergent portion of the average the nontornadic cases had substantially less po- RFD outflow (south-southwest through southeast of the tential buoyancy). To assess the potential buoyancy in this tornado) into a confluent and convergent zone as they case, CAPE was calculated for all the mesonet data move toward and pass around and into the right and front by inserting the mesonet data into the storm inflow side of the tornado. Several hundred meters farther to the sounding. CIN was also calculated with this surface east, parcels move out of a region of divergence and into parcel assumption. As shown in Fig. 9, the RFD outflow the flow field just in front of the tornado (and presumably has substantial CAPE from just behind the RFDGF into the tornado). At this time, the near-tornado region (;2200 J kg21) to the edge of the plotting region 2 km

Unauthenticated | Downloaded 10/06/21 05:18 PM UTC 380 MONTHLY WEATHER REVIEW VOLUME 139 west of the tornado (;1300 J kg21). Although CAPE SRH in discriminating between environments associated values decrease markedly west of the tornado, the outflow with supercells producing significant tornadoes and those parcels in this region still possess considerable potential associated with nontornadic supercells may also be rele- buoyancy. Largely consistent with the u9y and u9e data, vant with respect to the relationship between the low-level a local maximum of CAPE is present in a partial arc vertical shear profile and low-level mesocyclone intensity. 0.5 km south of the tornado in the region that partially c. Parcel origins overlaps the region of divergence. This local maximum extends to the proximate area around and in the tornado. A comparison was made between the RFD outflow ue A surprising characteristic of both the storm inflow and and the inflow sounding vertical profile of ue to de- the relatively ‘‘warm’’ RFD outflow was the large values termine the height on the inflow sounding with equiva- of CIN as seen in Fig. 9. The CIN ranged from about lent ue values to those sampled in the RFD outflow. This 280 J kg21 just behind the RFDGF to around 470 J kg21 height was just about 1 km (AGL) for most of the RFD west of the tornado with values in, near, and just south of outflow west of an approximate point 0.5 km behind the tornado ranging from approximately 330 to 370 J kg21. the RFDGF for the analysis period shown in Figs. 7 and 8. These values stand in contrast with typical values for RFD If the assumption is made that moist entropy was ap- outflow CIN documented in MSR2002 and GLF2007. proximately conserved, it is possible that the height Although some tornado cases cited in MSR2002 had from which the surface RFD outflow parcels originated considerably more RFD outflow CIN, average quadrant was around 1 km. However, to some degree, lateral CIN for significant tornadoes ranged from 214 J kg21 in entrainment should not be ignored, especially in areas quadrant III (generally southwest of the tornado) to of strong vertical accelerations. Since RFD outflow ue 78 J kg21 in quadrant II (generally southeast of the tor- values are only about 2 K less than inflow values, espe- 21 nado). GLF2007 found average CIN values of 82 J kg cially within the local maximum in ue just south of the in the RFD outflow of tornadic supercells. With only tornado in Fig. 8, it is possible that RFD outflow parcels small RFD outflow thermodynamic departures from the originated in the near-surface storm inflow and sub- inflow base state for a good portion of the RFD outflow sequent updraft and then were forced to descend by depicted in Fig. 7, the large CIN values displayed in a downward-directed dynamic pressure gradient force Fig. 9 are not surprising given the storm inflow CIN of or by hydrometeor drag while undergoing some mixing 304 J kg21 noted previously. with air at higher elevations. This could lead to the

The atypically high RFD outflow CIN may have im- slightly lower ue values (when compared to storm inflow) plications for the low-level mesocyclone-scale rotational that were observed at the surface in the RFD outflow. dynamics that need be in place to induce the conver- What appears clear in this case is that the RFD air gence and ascent of low-level parcels through a marked reaching the surface was not predominantly made up of stable layer to support tornadogenesis. Since the storm midlevel, nonupdraft parcels with a cold ue signature. inflow also had large CIN, understanding the flow field Instead, the origins of this air appear to be relatively low configuration that supports low-level mesocyclone de- in the inflow sounding or from near-surface inflow that velopment may have merit. We hypothesize that a very entered the updraft. This result is consistent with the favorable low-level shear environment, as indicated by findings of MSR2002 for tornadic RFD cases. the 0–3-km SRH of 669 m2 s22 and, perhaps more im- d. Time series analysis portantly, by the 0–1-km SRH of 462 m2 s22, fostered low-level mesocyclone development as streamwise vor- To further analyze the kinematic character of the flow ticity was tilted and stretched by the storm updraft. The just south of the tornado track, time series of storm- resultant low-level mesocyclone, if of ample intensity, relative wind direction and speed along with ground- could dynamically impose vertical acceleration sufficient relative speed are displayed for M3 in Fig. 10. As seen for lifting parcels low in the boundary layer through the in this figure and noted previously, storm-relative in- low-level stable layer. In this scenario, the development flow averaged close to 25 m s21. RFDGF passage near of a vigorous low-level mesocyclone provides a mecha- 01:19:25 UTC was accompanied by a large drop in nism for progressively accessing parcels closer to the sur- storm-relative wind speeds given the similarity between face. The general findings of Wicker (1996) regarding the the ground-relative winds and storm motion velocity importance of the environmental low-level vertical shear vectors. Ground-relative wind speeds intensified and profile in low-level mesocyclone intensification may storm-relative westerlies increased as the mesonet en- be particularly relevant. In a broader sense, the research countered an RFD surge at 0121:05 UTC (evident in of Markowski et al. (2003b), Rasmussen (2003), and Fig. 7). At M3’s location, storm-relative winds quickly Thompson et al. (2003) on the importance of 0–1-km backed in response to tornado-induced radial flow, as

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21 FIG. 10. Time series of storm-relative wind speed (m s ), direction (8), and ground-relative wind speed (m s21) for station M3 (see legend). The green circle identifies the peak 2-s ground- relative wind speed. The bold line along the time axis identifies the period the tornado was in progress during the sampling period. The vertical lines A, B, and C identify the respective passage of the RFDGF, the internal RFD surge boundary, and the tornado. seen in Figs. 7 and 8, with directions in the 1808–2108 range concurrent with the tornado passage. Interestingly, the preceding tornado passage (Fig. 10). However, M3’s probe samples its warmest uy during the time of tornado storm-relative winds started backing sharply about 70 s passage with the mobile mesonet also recording uy in- before the tornado passed to the north. The initial flow creases, although much smaller. The spike in uy for the backing was, in part, related to the surface divergence probe is largely due to the pressure drop in and very near mentioned previously that was positioned in an arc from the tornado given the dependence of u on pressure and south-southwest through east-southeast of the tornado. noting that the temperature remained relatively constant Storm-relative winds increased rapidly and veered as the through the tornado. For reference, the HITPR pressure, tornado passed to the north. At 0122:55 UTC, nearly at temperature, and dewpoint are plotted in Fig. 6 for a 100-s the time the tornado center was passing due north of M3, period bounding the approximate midpoint of the tor- the peak 2-s M3 storm-relative wind speed reached nado passage over the HITPR. One might have expected 30 m s21 while the maximum ground-relative speed was u to remain roughly constant and temperature to fall in 44 m s21. During a 20-s period centered on the tornado association with the rapid pressure drop. The only minor passage to the north, the ground-relative wind speed change in temperature for the HITPR during the tornado averaged 39 m s21. After tornado passage, storm-relative transect may be explained in two ways. A leading possi- winds veered from a northwesterly to northerly direction, bility is that the HITPR was sampling the interior of a consistent with the diffluent RFD outflow shown in Fig. 8, two-cell vortex (consistent with observations for the and eventually to a northeasterly direction. Tipton tornado; see Karstens et al. 2010) with parcels at

Time series of u9y for the mesonet shown in Fig. 11 de- least partially reflecting the axial downdraft, thus yielding picts a modest local maximum just behind the RFDGF the absence of falling temperatures associated with the with a subsequent small decrease in parcel buoyancy up pressure drop in the HITPR trace. A second possibility is to the time of tornado passage. Note that only small uy that the response time of the HITPR thermistor was in- deficits of generally less than 1 K were present in the sufficient, for an unknown reason, to reflect a temperature RFD outflow just south of the tornado track up to and drop (if one existed). Perhaps debris loading over a por- through tornado passage. The internal RFD surge tion of ventilation holes during tornado passage reduced boundary delineates a change (lessening) of the local time the circulation through the HITPR. If this second reason rate of change of u9y with values that plateau in the 20.8 were true, the HITPR u9y trace during the tornado transect to 21.0 range before uy rises for all stations roughly in Fig. 11 would look more similar to that of M3. After

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FIG. 11. Time series of (a) uy9 for M2, M3, and HITPR and (b) ue9 for M2 and M3 (see legend). The vertical lines A, B, and C identify the respective passage of the RFDGF, the internal RFD surge boundary, and the tornado. The bold line along the time axis identiﬁes the period the tornado was in progress during the sampling period.

tornado passage the u9y time series for all stations drop, measured just after the RFDGF passage that was tem- consistent with the gradually cooler pool of air noted to porally similar to that observed for uy, ue drops to its the west of the tornado in Figs. 7 and 8. The outflow of lowest pretornado passage level with u9e of about 22.7 K. this event with uy deficits never larger than 2.7 K over Notably, ue then trends upward in the air mass west of the the roughly 10.5-min sampling period was quite warm RFD surge boundary up to the time of tornado passage to compared with the nontornadic RFD group documented the north. Even though ue decreases after tornado pas- in MSR2002. sage, values of u9e generally stay in the 22- to 23.5-K The time series of u9e shown in Fig. 11 also reflect an range through the sampling period. Once again, with environment that is thermodynamically similar to those these modest ue deficits, the RFD outflow was quite warm found by MSR2002 and GLF2007 for RFD outflows compared with the nontornadic RFD group documented accompanying tornadoes. After a ue local maximum in MSR2002.

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5. Summary and discussion field on the left flank of the tornado, as long as a weak buoyancy and potential buoyancy gradient was main- Data collected by a mesonet within the near-tornado tained from roughly southwest through east-southeast of environment and in the Tipton tornado on 29 May 2008 the tornado, parcels entering the right flank and wrap- provided a rare opportunity to analyze RFD outflow ping around the front side of the tornado would have characteristics closely bounding a tornado and to char- minimal negative buoyancy and considerable potential acterize parcel thermodynamics being ingested into a buoyancy. tornado from the RFD. Field sampling was concentrated The thermodynamic mapping prompts a fundamental in a region from the tornado track (HITPR) through question: what process or processes influenced the ther- 1 km south of the track (mobile mesonet). RFD sampling modynamic structure of the near-tornado RFD in the began less than 1 min after the estimated time of torna- Tipton case? With an arc of divergence within 1 km of the dogenesis. As shown in the time–space and time series tornado from south-southwest through east-southeast and analysis, the overall thermodynamic signal of the Tipton videographic evidence of downdraft aligned above this supercell RFD outflow was consistent with the thermo- region, it appears that a narrow downdraft to the right of dynamics of RFD outflow associated with tornadoes as the tornado influenced the near-tornado RFD outflow documented in MSR2002 and GLF2007. Parcels within thermodynamic character. The near-surface flow field in- the RFD outflow to the east, south, and within the first dicated that parcels moved out of this divergence region one kilometer west of the tornado had small uy and ue and into the right side of the tornado. Slightly farther to the deficits of less than 2 K and 3 K, respectively. More spe- southeast, parcels moved out of the divergence arc and into cifically, nearly all observations to the south, east, and the flow field just in front of the tornado (and presumably very near/in the tornado had uy deficits less than 1 K. In into the tornado). Interestingly, the identified internal RFD addition to having only weak negative buoyancy as in- surge boundary was positioned only about 0.5 km in front dicated by the small uy deficits, all RFD parcels sampled of the eastern edge of the calculated divergence region had considerable potential buoyancy. Parcel CAPE ex- and implied downdraft. The connection between the near- ceeded 1600 J kg21 from southwest of the tornado up to tornado downdraft on the tornado’s right flank providing about 2200 J kg21 just behind the RFDGF. RFD outflow thermodynamically ‘‘warm’’ parcels that become part of CIN, which ranged from about 280 J kg21 just behind the the tornado inflow represents a potentially important as- RFDGF to 470 J kg21 west of the tornado, stands in con- sociation from a tornadogenesis and maintenance per- trast to typical values found by MSR2002 and GLF2007 spective. The small u9y associated with this part of the RFD for tornadic cases. The RFD CIN in this case is emblem- outflow suggests that evaporation was likely not driving this atic of the large CIN in the broader storm environment. downdraft. Precipitation experienced by the teams in the

Comparison of ue values within the RFD outflow with region was not heavy, thus reducing the probability that those in the storm inflow sounding indicated that the precipitation drag was largely producing the downdraft. origins of the parcels appear to be relatively low in the A downward-directed dynamic pressure gradient force inflow sounding or from near-surface storm inflow that appears a likely candidate for driving this near-tornado entered the updraft and was forced back to the surface. downdraft. In addition to better understanding the near-

In the former case, ue of RFD parcels matches the inﬂow tornado downdraft driving mechanisms, a topic for future sounding at about 1 km AGL, similar to MSR2002 for research involves understanding the relationship, if any, tornadic events; however, entrainment into the down- between tornado strength and/or size and the intensity and draft prompts a more qualitative assessment of potential size of the downdraft closely bounding the tornado. parcel origin. In the latter case, parcel excursions may The thermodynamic gradients identiﬁed in the 10.5 min have been large with updraft parcels possibly forced of RFD mesonet sampling for the Tipton case were well back down to the surface by a downward-directed dy- delineated and had similar tornado-relative positions to namic pressure gradient force or by hydrometeor drag some datasets in which RFD observations were collected while undergoing some mixing with air at higher eleva- relatively close to a tornado (e.g., MSR2002); however, in tions. This mixing would result in the somewhat cooler cases such as in MSR2002 (see their Fig. 7), Lee et al.

RFD outflow ue than seen in the near-surface inflow. (2004), and Finley and Lee (2008), the local warm pool Analysis of the flow field adjoining the right flank of the within the RFD on the right flank of the tornado was tornado, made possible by very close positioning of M3, much more pronounced. This is due to the introduction indicated that tornado-relative inflow was comprised of of warmer parcels from a surge or pulse in the RFD into RFD outflow parcels with only very weak negative buoy- cooler air already residing in the broader RFD outflow. ancy and substantial potential buoyancy. Although we In the Tipton case, this preexisting relatively cooler air cannot address the thermodynamic character of the flow was not present, or at least, not sampled.

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This study has demonstrated the value of mesonet for RFD evolution sampling. Considerably more RFD RFD observations within the nearest 1 km of a tornado, outflow evolution datasets need to be obtained and ana- including data collection much closer to a tornado. Other lyzed to better understand changes in key processes that documented mesonet datasets of RFDs, while not sam- control tornado maintenance. pling as close to the tornado as that reported herein, have revealed marked thermodynamic and/or kinematic variability on relatively small scales within the RFD (MSR2002; Acknowledgments. We are grateful to all the Finley and Lee 2004; Lee et al. 2004; Hirth et al. 2008; TWISTEX volunteers who helped with data collection. Finley and Lee 2008). Further evidence of likely im- We thank Chris Karstens and Patrick Skinner for their portant processes on small scales and in close proximity contributions in developing data analysis and quality to tornadoes, or tornadogenesis locations, continues to control software. Tony Laubach is recognized for his help grow. As an example, Kosiba and Wurman (2008) recently in managing field operations on this day and for compil- reported on Doppler on Wheels (DOW; Wurman et al. ing the videography of the event. Thanks are also ex- 1997) observations of very small-scale outflow surge phe- tended to two anonymous reviewers whose constructive nomenon within the RFD of the 23 May 2008 Quinter, comments improved the manuscript. Support for the field Kansas, tornadic supercell that appeared associated with campaign was provided by the National Geographic So- the transition of a multiple-vortex low-level mesocyclone ciety. WindLogics, Inc., provided support for the publi- configuration to one with a single tornado. Finley and Lee cation of this research. observed a similar strikingly small outflow surge within the RFD of a multiple-vortex mesocyclone that just pre- ceded one of the tornadogenesis cycles in the 22 May 2008 REFERENCES Hoxie, Kansas, tornadic supercell. The documented in- Adlerman, E. J., K. K. Droegemeier, and R. Davies-Jones, 1999: A ternal RFD outflow variability strongly suggests differ- numerical simulation of cyclic mesocyclogenesis. J. Atmos. ences in the RFD drivers (Markowski 2002b) supporting Sci., 56, 2045–2069. specific portions of the RFD at a given time. A compel- Barnes, S. L., 1964: A technique for maximizing details in numerical weather map analysis. J. Appl. Meteor., 3, 396–409. ling need exists for mesonet RFD sampling that includes Benjamin, S. 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