Observations of the Surface Boundary Structure Within the 23 May 2007 Perryton, Texas, Supercell

3730 MONTHLY WEATHER REVIEW VOLUME 139

Observations of the Surface Boundary Structure within the 23 May 2007 Perryton, Texas, Supercell

PATRICK S. SKINNER,CHRISTOPHER C. WEISS, AND JOHN L. SCHROEDER Texas Tech University, Lubbock, Texas

LOUIS J. WICKER National Severe Storms Laboratory, Norman, Oklahoma

MICHAEL I. BIGGERSTAFF University of Oklahoma, Norman, Oklahoma

(Manuscript received 9 December 2010, in ﬁnal form 5 May 2011)

ABSTRACT

In situ data collected within a weakly tornadic, high-precipitation supercell occurring on 23 May 2007 near Perryton, Texas, are presented. Data were collected using a recently developed fleet of 22 durable, rapidly deployable probes dubbed ‘‘StickNet’’ as well as four mobile mesonet probes. Kinematic and thermodynamic observations of boundaries within the supercell are described in tandem with an analysis of data from the Shared Mobile Atmospheric Research and Teaching Radar. Observations within the rear-flank downdraft of the storm exhibit large deficits of both virtual potential temperature and equivalent potential temperature, with a secondary rear-flank downdraft gust front trailing the mesocyclone. A primarily thermodynamic boundary resided across the forward-flank reflectivity gradient of the supercell. This boundary is characterized by small deficits in virtual potential temperature coupled with positive perturbations of equivalent potential temperature. The opposing thermodynamic perturbations appear to be representative of modified storm inflow, with a flux of water vapor responsible for the positive perturbations of the equivalent potential temperature. Air parcels exhibiting negative perturbations of virtual potential temperature and positive perturbations of equivalent potential temperature have the ability to be a source of both baroclinically generated streamwise horizontal vorticity and greater potential buoyancy if ingested by the low-level mesocyclone.

1. Introduction visually manifested by a cloud-free region or clear slot, wraps around the updraft and forms a sharp kinematic The long-standing conceptual model of the near-surface and thermodynamic gradient along its leading edge with structure of a supercell thunderstorm presented in Lemon the storm inflow. However, the leading edge of the FFD and Doswell (1979) displays two distinct downdrafts outflow is typically less discernible visually and is de- within the storm (their Fig. 7): a rear-flank downdraft scribed by Lemon and Doswell as ‘‘forming a relatively (RFD) coincident with a hook echo in radar reflectivity weak discontinuity at the surface on the forward and [see Markowski (2002a) for a recent review] and a right flanks of the radar echo.’’ broader forward-flank downdraft (FFD) resulting from In the years since the publication of Lemon and Doswell latent chilling through evaporation and melting as well (1979), varying representations of FFD boundaries have as hydrometeor loading within the precipitation core of been presented in in situ, Doppler radar, and three- the supercell downwind of the updraft. The RFD, often dimensional numeric cloud model studies. For exam- ple, Dowell and Bluestein (2002a) found only subtle wind shifts located roughly north of the cyclonic me- Corresponding author address: Patrick Skinner, Dept. of Wind Science and Engineering, Texas Tech University, 10th and Akron, socyclone in their pseudo-dual-Doppler analysis of the Lubbock, TX 79409. 8 June 1995 McLean, Texas, tornadic supercell while Beck E-mail: [email protected] et al. (2006) found no discernible wind shift near the

DOI: 10.1175/MWR-D-10-05078.1

Ó 2011 American Meteorological Society Unauthenticated | Downloaded 10/05/21 06:23 AM UTC DECEMBER 2011 S K I N N E R E T A L . 3731 surface, but a strong, convergent wind shift approxi- These findings contrast those of several numerical mately 1 km above ground level (AGL) in a nontornadic modeling studies, which found baroclinically generated supercell. Additionally, a dual-Doppler study by Frame horizontal vorticity along sharp buoyancy gradients et al. (2009) found minimal wind shifts along the leading within the forward flank to be an important contributor edge of the forward flank and parcel trajectories that to low-level mesocyclogenesis when air parcels in the originated in storm inflow and pass through the forward region are stretched as they accelerate into the updraft, flank from southeast to northwest. then are subsequently tilted into the vertical and further The majority of three-dimensional numeric cloud stretched within the updraft (Klemp and Rotunno 1983; model simulations of supercells denote the leading edge Rotunno and Klemp 1985; Wicker and Wilhelmson 1995; of the FFD outflow using the 21-K perturbation isen- Adlerman et al. 1999). However, as noted by Klemp and trope of either potential temperature u or virtual po- Rotunno (1983), Dowell and Bluestein (2002a), and tential temperature uv, which is typically located along Shabbott and Markowski (2006), significant streamwise or ahead of the forward, right flank of the simulated vorticity can be generated along weaker buoyancy gra- surface rainwater field (Rotunno and Klemp 1982; dients given a long parcel residence time. Low-level Klemp and Rotunno 1983; Rotunno and Klemp 1985; mesocyclogenesis would then proceed in a similar man- Davies-Jones and Brooks 1993; Wicker and Wilhelmson ner to that shown in numerical modeling studies, but 1995; Adlerman et al. 1999). Several simulations have with more potentially buoyant air parcels entering the produced wind shifts within the forward flank running low-level updraft, allowing more substantial tilting of approximately northward from the cyclonic low-level streamwise horizontal vorticity into the vertical and mesocyclone (Rotunno and Klemp 1985; Wicker and subsequent stretching. Wilhelmson 1995; Adlerman et al. 1999) but these were The 2007 Multiple Observations of Boundaries in the not collocated with the 21-K perturbation isotherm, Local-Storm Environment experiment (MOBILE-07), which is similar to the radar-observed gust front struc- conducted by Texas Tech University (TTU), utilized ture presented in Romine et al. (2008). Additionally, a fleet of 22 unmanned, durable, rapidly deployable recent simulations by Beck and Weiss (2008) have pro- surface stations and a four-probe mobile mesonet with duced multiple, transient kinematic and thermodynamic an emphasis on collecting observations within the for- boundaries within the forward flank rotating cycloni- ward flank of supercells. The primary research goals of cally around the center of circulation. the project were to The contrasting representation of boundaries within d identify the prevalence and characteristics of kine- the forward-flank regions of supercells highlights the matic and thermodynamic boundaries within the for- need for high-resolution in situ surface observations ward flank of supercells and within the region, such as those obtained with the de- d sample the thermodynamic deficits within the forward ployment of a mobile mesonet (Straka et al. 1996) in the flank of tornadic and nontornadic supercells to add to Verification of the Origin of Rotation in Tornadoes the dataset presented by Shabbott and Markowski Experiment (VORTEX; Rasmussen et al. 1994) and (2006). subsequent field campaigns. The mobile mesonet has vastly increased the available direct surface datasets This study will focus on observations collected within from within supercells, which were previously limited to a weakly tornadic, high-precipitation (HP) supercell fortuitous passages over fixed observation sites (Johnson near Perryton, Texas, occurring on 23 May 2007. A de- et al. 1987; Dowell and Bluestein 1997). Mobile mesonet scription of the instrumentation, data collection, and data have primarily been utilized to study the RFD analysis methods will be presented in section 2. Analysis (Markowski et al. 2002; Markowski 2002b; Grzych et al. of the 23 May 2007 case will follow in section 3, focusing 2007; Hirth et al. 2008) with only one study focused on on observations within the forward flank, and discussion, the forward flank (Shabbott and Markowski 2006). In conclusions, and future recommendations will be pre- their study, Shabbott and Markowski (2006) focused on sented in section 4. observations made within the FFD outflow of 12 tornadic and nontornadic supercells and did not note dis- 2. Methodology tinct wind shifts within the region. a. Definition of the forward-flank reflectivity gradient A shared finding from mobile mesonet studies is that (FFRG) small thermodynamic deficits of virtual potential temperature and equivalent potential temperature ue rela- This paper roughly follows the definition of the for- tive to near-surface inflow values in both the RFD and ward flank put forth by Shabbott and Markowski (2006) FFD result in an increased likelihood of tornadogenesis. as ‘‘the region of low-level radar reflectivity on the forward

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is broadly defined herein as the region to the right of a line bisecting the forward-flank precipitation shield (Fig. 1). b. StickNet Due to the limitations of using manned instrumentation, such as a mobile mesonet, in hazardous environ- ments, the TTU Atmospheric Science Group and Wind Science and Engineering Research Center have developed a suite of ruggedized surface-observing probes dubbed ‘‘StickNet’’ (Schroeder and Weiss 2008; Weiss and Schroeder 2008). The 2007 incarnation of Project MOBILE marked the first mass deployments of StickNet in a supercell environment. Twenty-two probes were available to MOBILE-07 utilizing two different instrument packages (Table 1). Ten ‘‘type A’’ probes (Fig. 2a) are equipped to measure temperature, relative humidity, pressure, wind speed, and wind direction at either 1-, 5-, or 10-Hz sampling intervals. Two additional type A probes were only equipped to measure wind speed and direction in MOBILE-07. The remaining 10 type B FIG. 1. Schematic illustrating the near-surface representation of the forward-flank (hatched) and forward-flank reflectivity gradient probes (Fig. 2b) utilize a Vaisala WXT510 ‘‘all in one’’ (crossed). [Adapted from Shabbott and Markowski (2006)]. instrument that produces measurements of temperature, relative humidity, barometric pressure, wind speed, and wind direction, as well as estimates of accumulated side of a line drawn orthogonal to the major axis of the rainfall and hail size at a 1-Hz interval. Instrumentation echo, through the radar-observed circulation center’’ for each probe is mounted on a Crane Tall TriMax tripod (their Fig. 4). Shabbott and Markowski (2006) used the and data are collected via Laboratory Virtual Instrumen- term ‘‘FFD outflow’’ to describe air parcels within the tation Engineering Workbench (LabVIEW) programs forward flank. In this study, forward-flank reflectivity running on a National Instruments Compact FieldPoint gradient (FFRG) is utilized in place of FFD outflow module housed within a Campbell Scientific data ac- as observations along the right-forward flank of the quisition (DAQ) box. An external battery box can be Perryton supercell indicate air parcels in the region are affixed to each probe for long-term (greater than 18 h) better characterized as modified storm inflow. The FFRG deployments. All probes are equipped with a flux-gate

TABLE 1. StickNet instrument speciﬁcations for each platform type. [Adapted from Weiss and Schroeder (2008).]

Component Model Type Output Accuracy GPS receiver SnyPaQ/E-M12 with SMA-35 A, B Time, lat (decimal 8), antenna lon (decimal 8), alt (m) Compass KVH C100 A, B 08–360860.58 All in one Vaisala WXT510 B Temperature (T): 2528 to 1608C T: 60.38C RH: 0%–100% RH: 63% Barometric pressure (BP): 600–1100 hPa BP: 60.5 hPa Wind speed (WS): 0–60 m s21 WS: 60.3 m s21 Wind direction (WD): 08–3608 WD: 638 Rain: mm Rain: 65% day21 Hail: hits per centimeter squared Temperature/RH sensor (1) R. M. Young 41382 A T: 2508 to 1508C T: 60.38C RH: 0%–100% RH: 62% Temperature/RH sensor (2) Campbell Scientiﬁc HMP45C A T: 2408 to 1608C T: 60.38C RH: 0%–100% RH: 62% Pressure Vaisala PTB101B A 600–1060 hPa 61.5 hPa Wind R. M. Young 05103V A WS: 0–100 m s21 WS: 61 WD: 08–3608 WD: 638

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FIG. 2. (a) Photograph of a StickNet type A platform: 1) RM Young 5106 Wind Monitor, 2) flux gate compass, 3) Campbell Scientific T/RH HMP45C, 4) data acquisition box, 5) external battery mounting pipe, and 6) mounting point for staking tripod to the ground. (b) Photograph of a StickNet type B platform: as in (a), but with 7) a Vaisala WXT 510 replacing 1) and 3). compass that allows wind direction values to be cor- trailer and carried by pickup trucks using custom-fitted rected to magnetic north, a GPS antenna/receiver that racks to permit greater maneuverability. records probe position and synchronizes observation In MOBILE-07 a one-road deployment strategy was times, and a bubble level to permit rapid leveling of the utilized for StickNet. The two trailers (identified as SN3 probe during deployment. and SN4) would deploy a coarse array with approxi- For a short-term (thunderstorm) deployment, a team mately 3-km probe spacing centered about a point iden- of two people will extend, secure, and level the tripod, tified to be the approximate location at which the storm attach the DAQ box and plug in connections for the updraft will cross the array. The trailers move only south- flux-gate compass and instrumentation package, anchor ward during the deployment to ensure a safe exit from the probe using 0.5-m stakes, then turn on the power and the storm environment. As the updraft approaches the ensure data are being collected. A single probe can be coarse array, a finescale array is laid out by the two pickup expected to be fully deployed in under 2 min. Spring trucks (SN1 and SN2) with 0.8-km spacing between load tests conducted by the TTU Wind Science and probes. These more nimble vehicles are capable of mov- Engineering Research Center suggest that a probe ing northward to adapt to changes in the estimation of deployed in a long-term configuration (which includes the updraft passage over the array without risking their the external battery box) is capable of withstanding safety. a maximum 3-s wind gust of 63 m s21. c. Mobile mesonet In transit, the probes are housed in two trailers out- fitted to store 12 probes apiece. The DAQ box and ex- A four-probe mobile mesonet was also operated in ternal battery are connected to one 10-bank and one MOBILE-07 in association with the Department of 5-bank battery recharger in the fore of the trailer allowing Atmospheric, Oceanic, and Space Sciences at the Uni- them to be recharged in transit. Additionally, each DAQ versity of Michigan. The TTU mobile mesonet design is box is networked to a local Ethernet hub, which allows based on Straka et al. (1996) with small alterations as data to be downloaded following each deployment. Dur- described by Hirth et al. (2008). The deployment strategy ing deployments, five probes are removed from each of the mobile mesonet was to perform transects across

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TABLE 2. StickNet instrumentation malfunctions on 23 May 2007. of wind direction. Consistent wind direction biases were introduced in five StickNet probes during the Perryton Probe ID Issue Corrective action deployment by errors in the flux-gate compass readings. 03A Temperature and RH Thermodynamic data The flux-gate compass of two probes (08A and 15B) erroneously low discarded 08B No compass reading Wind data corrected failed, resulting in erroneous wind directions with re- 13A Temperature and RH Thermodynamic data spect to magnetic north. Each of these probes was lo- erroneously high discarded cated between two fully functioning probes that observed 15A No compass reading Wind data corrected similar wind directions throughout the deployment. By 21A Temperature and RH Thermodynamic data calculating the mean wind direction for the fully func- erroneously low discarded tioning probes over a 10-min period of nearly consistent wind direction, the erroneous probe data were corrected the right-forward flank of the storm. Due to potentially using the mean value. Though large biases in wind di- hazardous conditions and low visibility within the for- rection of 70.28 (236.38) were corrected in probes 08B ward flank, one probe remained equatorward of the (15A), the mean wind direction between the fully storm within the storm inflow. The ‘‘field coordinator’’ functioning probes varied by less than 108 during the or FC probe produced estimates of updraft location for correction period. Three additional probes (01A, 02B, StickNet and mobile mesonet probes, set endpoints to and 19A) incurred large, consistent biases in wind di- mobile mesonet transects, and collected storm-inflow rection due to the flux-gate compass becoming dislodged data to be used as a base state in calculations of ther- and rotating with respect to the anemometer position. modynamic perturbations. These probes exhibited similar wind direction biases d. Quality assurance with respect to surrounding probes during the Perryton case, as well as all later deployments and in the post- 1) STICKNET season mass test. Therefore, wind direction values are StickNet data collected during MOBILE-07 were corrected using the bias detected in the postseason mass initially reduced to 1 Hz for each probe to ensure con- test despite being several times over the limit described in sistency. Data spikes and dropouts were objectively re- Markowski et al. (2002). A complete listing of StickNet moved by quality assurance software, and additional bias corrections utilized for the Perryton case is presented dubious observations were subjectively flagged and re- in Table 3. moved from the record. Further, due to the large network 2) MOBILE MESONET of probes available, several occurrences of instrument malfunction were identified and removed from the data The quality assurance of mobile mesonet data fol- record (Table 2). Time series and radar overlay plots lowed precedents set by Markowski et al. (2002), were created using a 5-s moving average of quality- Markowski (2002b), Shabbott and Markowski (2006), controlled observations. Grzych et al. (2007), and Hirth et al. (2008). Biases were The calculation of StickNet biases is challenging as determined using data collected as the probes traveled there are very limited periods of quiescent data avail- as a caravan through quiescent conditions prior to storm able for each probe as present in most mobile mesonet initiation and were removed according to the limits de- datasets (Grzych et al. 2007; Hirth et al. 2008). Instead, scribed by Markowski et al. (2002). Additional bias instrument biases in StickNet probes were determined limits of 0.02 hPa were applied to observations of vapor by intercomparing 60-min periods of data from mass pressure and saturation vapor pressure to ensure the tests in quiescent conditions performed prior to the start accurate calculation of derived thermodynamic vari- and at the conclusion of MOBILE-07. This method has ables (Table 4). Kinematic values were flagged and the disadvantage of being unable to capture biases that discarded for periods when the magnitude of the vehicle developed over the course of the field project or dy- acceleration vector exceeded 1 m s22 (Markowski 2002b; namic biases dependent on environmental conditions. Grzych et al. 2007). However, the majority of the StickNet biases detected in Thermodynamic data from the mobile mesonet were MOBILE-07 were consistent between the two mass intercompared with StickNet data during periods when tests, lending confidence to the validity of the bias values the two observing platforms were within 500 m of one utilized. Due to the proximity in time to the Perryton another. It was found that the spatial bias introduced by deployment, biases in this study are determined using the response time of the thermodynamic instrumen- the mass test occurring prior to MOBILE-07 on 15 May tation affects the position and magnitude of thermody- 2007. Correction and removal of biased observations namic gradients. Using an approximate time constant of follows that of Markowski et al. (2002) with the exception 1 min for slow temperature and relative humidity values

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TABLE 3. StickNet bias corrections for 23 May 2007. Definitions as TABLE 4. Mobile mesonet bias corrections for 23 May 2007. in Table 1. Saturated vapor Probe ID T (8C) RH (%) Pressure (hPa) WD (8)WS(ms21) Probe ID Fast T (8C) Slow T (8C) pressure (hPa) 01A 10.88 233.09 FC 10.34 20.066 02B 20.81 10.94 128.21 T4 10.34 10.030 03A 114.45 06B 110.74 07A 10.67 21.81 where r is density and a is equal to 0.1 3 the radar re- 10A 10.31 flectivity factor (in dBZ). Errors are introduced into ur 13A 20.72 15A 10.22 calculations if a reflectivity gradient between the lowest 16B 20.25 available radar elevation angle and the surface is present 17A 10.24 and, also, if ice is present within the sample volume. As 19A 10.39 256.10 the lowest available SMART-R reflectivity data are 21A 10.29 210.22 approximately 490 m above the surface and hail was 22B 20.58 23A 10.25 10.31 detected by multiple type B StickNet probes, as well as reported by the northernmost mobile mesonet probe (J. Merchant 2007, personal communication) within the

FFRG, it is stressed that values of ur presented herein represent a best available estimate. Furthermore, to ac- (Straka et al. 1996), a mobile mesonet probe moving at count for the effects of storm propagation between radar 21 approximately 13.5 m s will travel ;800 m before fully scans, ur is calculated over time-to-space-converted responding to the new environment. The differing storm- StickNet data using SMART-R reﬂectivity data coin- relative motion of the StickNet and mobile mesonet ob- cident with the beginning of the time-to-space conver- serving platforms may act to either stretch or compress sion analysis period. There will be additional error in the thermodynamic gradients where probes of both platform latter portions of the analysis period due to storm evo- types interact (Skinner et al. 2010). To mitigate the in- lution, though it is noted that this error will be limited as consistencies between StickNet and the mobile mesonet the FFRG was exiting the ﬁnescale StickNet array dur- analysis arising from this effect, observations from two ing the analysis period. mobile mesonet probes traversing thermodynamic gra- Pseudoequivalent potential temperature uep, which dients (T2 and T3) have been neglected in this study. will be referred to as equivalent potential temperature

The two remaining probes (FC and T4) remained out- ue herein, is calculated according to the derivation of side of the FFRG in a region of relatively homogeneous Bolton (1980).1 As noted by Markowski (2002b) and thermodynamic conditions and are retained. Shabbott and Markowski (2006), use of ue as a tracer of parcel origin is problematic due to diabatic effects e. Derived variables within the supercell environment and it is best viewed as Virtual potential temperature including the effects of the height on an inflow sounding at which a parcel originated. the condensate mixing ratio qc, also known as the density potential temperature ur (Emanuel 1994), is calculated as in Shabbott and Markowski (2006) using f. Base state approximation To produce analyses of perturbations to thermody- ur 5 u(1 1 0:61qy 2 qc), (1) namic variables, it is necessary to define a base state for air parcels within the inflow of the storm. For MOBILE-07, where u represents potential temperature and qy is the the base state is calculated using averages of all probes water vapor mixing ratio. Calculations of virtual po- residing outside of storm outflow and precipitation. The tential temperature uv neglect the effects of qc in (1). presence of precipitation is detected directly using the Shared Mobile Atmospheric Research and Teaching rainfall sensors on type B StickNet probes; otherwise, it Radar (SMART-R) (Biggerstaff et al. 2005) radar re- was determined as areas where the reflectivity factor is

ﬂectivity is used to estimate qc according to Hane and greater than 25 dBZ, as in Shabbott and Markowski Ray (1985):

1 1 10a 4/7 Bolton’s (1980) Eq. (43) is used for calculation of equivalent 5 potential temperature and Eq. (21) is used for condensation qc 4 , (2) r 1:73 3 10 temperature.

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(2006). Based on these criteria, the base state is calcu- where w represents the weight function; t and n are the lated from an average of the FC and T4 mobile mesonet distances from the current grid point to the data point probes over a 10-min period as the low-level mesocy- being considered tangential and normal to storm mo- clone crossed the instrument array. tion, respectively; and kt (kn) represents a ‘‘smoothing parameter’’ tangential (normal) to storm motion as de- g. Analysis techniques fined by Koch et al. (1983), If a steady-state feature of interest is propagating 2Dn 2 along a constant vector, point observations of the fea- k 5 5:052 , (4) p ture may be expanded in the horizontal plane using time-to-space conversion (Taylor 1938). The Taylor with Dn representing the mean spacing between obser- hypothesis is difficult to apply to supercells as they often vations. A Dn of 0.7 km (1.4 km) is utilized for data evolve on rapid time scales, making the steady-state collected tangential (normal) to the Perryton storm. assumption problematic. Past studies (Markowski et al. 2002; Markowski 2002b) have assumed a steady state for small time periods (approximately 5 min) to limit the 3. 23 May 2007, Perryton, Texas, supercell impacts of storm evolution on the analysis; however, this study will use a 10-min steady-state assumption similar a. Storm evolution to Hirth et al. (2008). Although cyclic mesocyclone gen- On 23 May 2007, Project MOBILE-07 targeted a warm esis (Burgess et al. 1982) was observed as the Perryton front extending from roughly Amarillo, Texas (KAMA), supercell crossed the instrument array, the time-to-space- northeastward through northwestern Oklahoma and converted values encompass a much broader scale than into south-central Kansas. The air mass behind the front the mesocyclone itself in order to assess the impacts of was conditionally favorable for surface-based convec- forward-flank inflow air properties on storm morphology tion with greater than 1000 J kg21 of mixed-layer con- and evolution. Therefore, errors associated with the vective available potential energy (CAPE) as per the steady-state assumption will minimally impact the anal- 0000 UTC KAMA sounding (Fig. 3). It is likely that ysis and conclusions. higher values of CAPE were present in the environment A second difficulty in performing time-to-space con- of the Perryton supercell as the KAMA sounding was version is specifying the motion of the feature of interest. located approximately 140 km southwest of the StickNet Past studies have performed tornado-relative (Hirth et al. deployment and exhibited surface dewpoint values roughly 2008) or storm-relative time-to-space conversions based 2 K lower than the base state of the Perryton storm. on the highest observed reflectivity factor (Shabbott and Southeasterly surface winds at 7–10 m s21 under the Markowski 2006). Storm motion of the Perryton supercell southern portion of a 500-hPa velocity maximum of ap- is estimated using the motion of the intersection point of proximately 20 m s21 resulted in favorable conditions for the RFGF fine line in the SMART-R data with the supercell development, with 0–6-km shear greater than FFRG, as this was the most reliably identifiable feature 25 m s21 in the KAMA sounding and clockwise curva- available with cyclic mesocyclogenesis ongoing during ture in the low levels of the hodograph (Fig. 3). the analysis period. All time-to-space-converted data Convection initiation occurred shortly after 2100 UTC presented will be referred to as relative to the RFGF to the southwest of Perryton (KPYX). This convection, intersection with the FFRG as the boundary may move hereafter referred to as ‘‘storm A,’’ rapidly became independently of other storm features. supercellular and produced a brief tornado at 2131 UTC Time-to-space-converted observations are gridded in rural areas to the northwest of Miami, Texas (NCDC onto a two-dimensional plane and interpolated using a 2007). MOBILE-07 targeted storm A for deployment Barnes objective analysis scheme (Barnes 1964). Due to shortly after initiation; however, due to the lack of an the time-to-space-converted values having much higher acceptable road network, the deployment began well spatial resolution tangential to storm motion, every hun- ahead of the storm along Highway 83 to the southeast of dredth time-to-space-converted observation is included KPYX (Figs. 4a and 4b). Shortly after the initial de- in the objective analysis and an anisotropic Barnes ployment began, additional convection initiated along weighting function is employed (Trapp and Doswell the right, rear flank of storm A. ‘‘Storm B’’ quickly be- 2000): came the dominant cell and the deployment was shifted to the southeast after five probes were placed in the path of 2t2 n2 w 5 exp 2 , (3) storm A (Figs. 4c–f). A total of 20 StickNet probes and the kt kn four-probe mobile mesonet were aligned from northwest

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FIG. 3. Skew T–logp diagram and hodograph from KAMA at 0000 UTC 24 May 2007. Boldface crosses at ground level represent the base-state temperature and dewpoint of the Perryton supercell between 2245 and 2255 UTC. to southeast along an approximately 35-km stretch of vertical vorticity associated with the new mesocyclone Highway 83 as storm B approached (Figs. 5 and 6). began to decrease as the storm began a more northward Additional convection continued to develop along the propagation (Figs. 6c and 6d). Storm B merged with the southwestern flank of storm B and it evolved into an HP trailing convection shortly after 2300 UTC; however, it supercell embedded in a broken line of convection as it produced two additional brief tornadoes between 2330 crossed the instrument array. Storm Data (NCDC 2007) and 2345 UTC when it was embedded in the line. reported that storm B produced a brief tornado rated as As storm B approached the instrument array, the EF0 on the enhanced Fujita scale between 2239 and majority of the probes, including the entire finescale 2240 UTC less than 5 km to the south of the north- array, sampled conditions along the FFRG. The probes westernmost three StickNet probes. This tornado is evident then sampled the RFD of the storm as the low-level in SMART-R 0.758 elevation radial velocity data with mesocyclone passed to the north of the finescale array. greater than 50 m s21 gate-to-gate shear sampled ap- Three probes originally deployed for storm A remained proximately 490 m above the surface at 35-km distance to the north of the mesocyclone and sampled the RFD from the radar (Fig. 5b). Analysis of SMART-R radial trailing the center of circulation. velocity data shows storm B underwent cyclic mesocyclo- b. Time series analysis genesis over the 20 min following tornado dissipation as it crossed the StickNet array. As the storm impinged on the Time series of the thermodynamic and kinematic finescale array around 2245 UTC, the parent mesocyclone observations of each probe were analyzed. For brevity, of the tornado had weakened considerably and the RFGF five probes spanning the breadth of the instrument array had moved well ahead of it (Figs. 5c and 5d). By 2250 UTC (Fig. 7) are presented here. inbound velocities within the RFD and outbound veloci- The two northwesternmost probes (14B and 16B) ties within the forward flank had accelerated and a broad begin the data record in residual RFD outflow from mesocyclone signature was present over the western por- storm A. Probe 14B remains in this outflow to the north tions of the finescale array (Figs. 6a and 6b). By 2255 UTC, of the precipitation core of storm B throughout the

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FIG. 4. KAMA 0.58-elevation scan reﬂectivity (dBZ) for (a) 2110, (b) 2131, (c) 2152, (d) 2213, (e) 2230, and (f) 2251 UTC. StickNet probes deployed at the time of each scan are indicated by dark circles. period and experiences light and variable winds ac- observed during the wind shift, bringing probe 16B into companied by strong thermodynamic deﬁcits (Fig. 8a). an environment more similar to probe 19A to the The data record of probe 16B (Fig. 8b) begins in a sim- southeast than probe 14B to the northwest. Though this ilar environment to 14B, although the probe experiences boundary passage is nearly coincident with tornado- higher wind speeds within the precipitation core of genesis approximately 5 km to the south-southwest of storm B. Over the period from 2236 to 2240 UTC, the probe 16B, it is unclear if the boundary extended wind direction observed by 16B veers to nearly due southward to the immediate environment of the tornado. easterly and a gradual acceleration is observed after During the 10 min following tornado dissipation from

2240 UTC. A subtle increase in uv (ue) of 1 (3) K is 2240 to 2250 UTC, probes 16B (Fig. 8b) and 19A

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FIG. 5. StickNet and mobile mesonet observations overlaid with the SMART-R 0.758-elevation scan (a),(c) re- ﬂectivity (dBZ) and (b),(d) radial velocity (m s21) at (a),(b) 2239 and (c),(d) 2245 UTC. The station plot includes virtual potential temperature (K, bottom left) and equivalent potential temperature (K, top left); full wind barbs represent 5 m s21. Subjectively analyzed gust front positions, mesocyclone position, and tornado position are denoted by solid lines, M, and T, respectively. Range rings every 5 km are included with the SMART-R location approximately 10 km east of the bottom-right corner of each image.

(Fig. 9a) experienced steadily accelerating, nearly east- This downdraft is first observed by the StickNet array erly winds as cyclic mesocyclogenesis occurred to the within the occlusion of the initial RFGF as analyzed in south. Strong uv (ue) deficits of approximately 5 (8) K SMART-R data (Fig. 6b), suggesting that this boundary were observed during this period, which coupled with represents a secondary RFGF (Finley and Lee 2004; Lee evidence of a kinematic shift to the east of the probes et al. 2004; Marquis et al. 2008; Karstens et al. 2010; Lee in the SMART-R data (Fig. 5d), suggest the probes lie et al. 2011). While uv and ue deficits within the RFD surge within the RFD of storm B to the north-northwest of the in the immediate vicinity of the tornado are unavailable, developing low-level mesocyclone. A secondary bound- the steep decline in thermodynamic values associated ary passed over both probes between 2248 and 2251 UTC with the passage of the secondary RFD boundary sug- and was characterized by a rapid acceleration of wind gests that changes in thermodynamic character are pos- speed to values greater than 20 m s21 (Figs. 8b and 9a), sible between RFD surges in a single storm, similar to the with divergence developing between the two probes findings of Finley and Lee (2004), Lee et al. (2004), Finley

(Figs. 6a and 6b), and a sharp increase in both uv and ue and Lee (2008), and Finley et al. (2010). deficits to values greater than 7 and 14 K, respectively. During the analysis period, the FFRG moved slowly The divergence between probes 16B and 19A and sim- northward across probes within the finescale array as the ilarly large thermodynamic deficits observed suggest southern portion of the RFD impinged upon them from that the probes are sampling air parcels within a down- the southwest. Probes 22B (Fig. 9b) and 15A (Fig. 10) draft originating to the north and west of the mesocyclone. are representative of all probes within the FFRG and

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FIG. 6. As in Fig. 5, but at (a),(b) 2250 and (c),(d) 2254 UTC. exhibit little change in thermodynamic character, with c. Time-to-space conversion and objective analysis a deficit of 1–2 K in virtual potential temperature and a modest surplus of 1–3 K in equivalent potential tem- To better visualize the boundaries observed in the perature compared to the base state. Kinematically, Perryton supercell, time-to-space conversion of StickNet probes within the FFRG experienced similar values to the and mobile mesonet data is conducted over a 10-min probes within storm inflow with east-southeasterly winds period centered on the time of low-level mesocyclone between 5 and 10 m s21, though a subtle backing of the passage across the instrument array at 2250 UTC winds is observed moving northward relative to the low- (Figs. 11–14). level mesocyclone. The passage of the RFGF is marked The passage of the RFGF is apparent in the objective by a veering of the wind to south-southwesterly approx- analysis as a sharp gradient in both the virtual and imately 3 min prior to any response in the thermody- equivalent potential temperature trailing the initial wind namic values. Approximately 2–3 min after the initial shift. Winds along the RFGF veer to south-southwesterly, then gradually back within the RFD to southeasterlies kinematic response, a nearly linear decrease in uv coupled u as the mesocyclone moves north and decreases in in- with a steep decline in e is noted. There is subtle evidence 2 of the passage of the secondary RFGF in probe 22B and tensity toward the end of the analysis period. Addi- other probes along the western edge of the finescale array tionally, a secondary RFGF is likely present in the at 2254 UTC, suggesting the possibility of the RFD surge observation gap between probes to the northwest of the propagating cyclonically around the low-level mesocyclone. The wind speeds observed with RFGF passage are 2 It is noted that the mesocyclone center analyzed in Figs. 11–14 was weaker in probes located in the eastern portion of the determined from SMART-R data at 2245 UTC. Therefore, wind finescale array, which is supported by a decrease in the observations from later in the analysis period will correspond to a radial velocities in the SMART-R data (Figs. 6c and 6d). mesocyclone position farther north and east than the denoted position.

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FIG. 7. Positions of StickNet probes displayed in Figs. 8–10. Positions are overlaid on SMART-R 0.758-elevation scan reflectivity (dBZ) data at (a) 2239 and (b) 2250 UTC. Subjectively analyzed gust front positions, mesocyclone position, and tornado position are denoted by solid lines, M, and T, respectively. Range rings every 5 km are included with SMART-R location approximately 20 km southeast of the bottom-right corner of each image. mesocyclone center and the finescale array to the south- moves northward with respect to the mesocyclone. In east of the mesocyclone based on analysis of probes 16B contrast to observations within the RFD, opposing per- and 19A (Figs. 5, 6, 8b, and 9a). turbations of uv and ue are present across the FFRG. The broad region of FFRG encompassed by the Small deficits of virtual potential temperature are present finescale array displays similar kinematic values to the within the forward flank, resulting in a southeastward- inflow, but with a gradual backing of the wind as one directed buoyancy gradient. The buoyancy gradient is

FIG. 8. From top to bottom, plots of wind speed, wind direction, uv perturbation, and ue perturbation for (a) probe 14B and (b) probe 16B. Arrows indicate the subjective analysis of boundary passage.

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FIG. 9. As in Fig. 8, but for probes (a) 19A and (b) 22B. more pronounced and more southward directed when 2002b; Shabbott and Markowski 2006), it would neces- density potential temperature is considered (Fig. 13). sitate parcel origins below ground in the Perryton su- Analysis of the equivalent potential temperature reveals percell. As this scenario is obviously not possible, other positive perturbations of the ue values within the FFRG sources for the origin of the positive ue perturbation with maximum positive perturbations coincident with values must be considered. One possibility is heteroge- higher radar reflectivities in the SMART-R data. neity within the inflow resulting in a varying base state. Slight heterogeneity was observed between the two probes utilized to produce the base state for the Perryton 4. Discussion supercell. However, differences between base states calculated from the two individual inflow probes differ a. The FFRG boundary by less than 0.5 K and a gradient of ue is evident between The primary boundary observed within the forward probes within the FFRG and the inflow probes (Fig. 12). flank of the Perryton supercell occurred across the Additionally, three of the other four cases analyzed in FFRG and was largely thermodynamic in nature (Figs. MOBILE-07 show similar positive perturbations of equiv- 11–13). The boundary was characterized by small defi- alent potential temperature across the FFRG with the cits in virtual and density potential temperature coupled remaining case exhibiting values similar to those ob- with positive perturbations in equivalent potential tem- served by the inflow probe (Table 5). A separate com- perature. Little change in the kinematic variables was plication in the Perryton supercell is that, due to the observed, with only a slight backing of the wind occurring east–west orientation of the instrument array, it is uncer- across the FFRG coincident with the thermodynamic tain if the positive perturbations of ue extend southward perturbations. from the leading edge of the precipitation shield in re- The observation of equivalent potential temperature gions west of the inflow probes. Separate deployments in values higher than the base state is an important finding MOBILE-07 conducted along north–south roads on 22 May from Project MOBILE-07 and requires closer inspec- 2007 near Hill City, Kansas, and 31 May 2007 near Hough, tion. If values of ue are considered as heights on an inflow Oklahoma (Table 5), exhibit positive ue perturbations sounding at which air parcels originate (Markowski northward from the southernmost extent of the FFRG.

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FIG. 10. (a) As in Fig. 8, but for probe 15A. (b) From top to bottom, plots of temperature, uv, dewpoint temperature, and ue for probe 15A.

These observations, combined with the ue gradient ob- As equivalent potential temperature is strongly cou- served across the eastern edge of the FFRG in the pled to water vapor mixing ratio values, while virtual Perryton supercell (Fig. 12), suggest the possibility that potential temperature is primarily dependent on tem- the positive perturbations in ue are limited to within the perature values (Fig. 10b), an increase in water vapor FFRG of the Perryton storm as well. There is evidence mixing ratio coupled with a decrease in temperature of a slight decrease in ue of approximately 1 K in probes could produce opposing perturbations of uv and ue. initially within the FFRG of the Perryton supercell as Analysis of water vapor mixing ratio perturbations within the precipitation shield exits the instrument array to the Perryton supercell shows a broad swath of positive the north, but positive perturbations in ue are retained perturbations aligned roughly with positive perturba- through the duration of the deployment. As the identi- tions of ue (Fig. 14), suggesting that a flux of water va- fication of any base state is subjective, the magnitude por is responsible for the positive perturbations in ue. of the ue perturbations with respect to true storm inflow The origin of this flux is unclear; however, one po- is not fully known and the perturbations are best in- tential source would be a flux of water vapor from terpreted as being relative to that outside of the pre- a precipitation-moistened surface. Despite uncertainty cipitation region of the supercell. as to the origin of the positive perturbations in qy,the Though additional convection trailed the Perryton lack of a distinct wind shift across the FFRG coupled supercell (Fig. 4), no other convection existed ahead of the with small thermodynamic perturbations compared to storm and it remained south of the warm front throughout those observed within the RFD suggest that air parcels its life cycle, likely precluding the possibility of higher-ue air within the FFRG are better characterized as modified being lifted over a denser air mass before descending in the storm inflow rather than outflow from the FFD. This downdrafts of the Perryton supercell (Hirth et al. 2008). result is kinematically similar to dual-Doppler studies

However, this does not preclude the possibility of higher-ue by Beck et al. (2006) and Frame et al. (2009) as well air existing aloft in the environment of the Perryton su- as several simulations produced by Beck and Weiss percell, possibly as a result of anvil shading (Markowski (2008), which resulted in modest deﬁcits of uv with et al. 1998), and descending in the forward ﬂank. littlechangeinue.

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21 FIG. 11. Time-to-space-converted wind observations (m s ), selected observations of virtual potential temperature perturbations (K), and gridded uv perturbations (K) for 2245–2255 UTC 23 May 2007. Wind observations are averaged over 10 s and plotted every 30 s with observations of uv perturbations displayed with white text. Selected uv deﬁcits are contoured with solid lines. Here, M denotes the mesocyclone position with the boldface dashed line denoting the position of the primary RFGF. Wind observations lying outside of objectively analyzed data occur with probes lacking thermodynamic observations. Plots are overlaid on 0.758 SMART-R reﬂectivity data (dBZ) from 2245 UTC.

Parcel trajectories calculated in dual-Doppler and Several numerical modeling studies have identified numerical modeling studies have identified near-surface the tilting and subsequent stretching of baroclinically air parcels along the FFRG as a source region for air generated streamwise horizontal vorticity along strong entering the low-level mesocyclone (Klemp and Rotunno buoyancy gradients within the forward flank as an 1983; Rotunno and Klemp 1985; Davies-Jones and Brooks important source of vertical vorticity in the low-level 1993; Wicker and Wilhelmson 1995; Adlerman et al. 1999; mesocyclone (Klemp and Rotunno 1983; Rotunno and Dowell and Bluestein 2002b; Beck et al. 2006; Beck Klemp 1985; Wicker and Wilhelmson 1995; Adlerman and Weiss 2008). However, parcel trajectories are et al. 1999). Contrasting these findings, Shabbott and not available for the Perryton supercell, so it is stressed Markowski (2006) found that storms producing strong that the potential impacts of the FFRG boundary on deficits of uv and ur within the forward flank, which re- low-level mesocyclogenesis and maintenance are sults in greater potential for baroclinic generation of dependent on the ability of the low-level mesocyclone vorticity, produced weaker low-level mesocyclones and to ingest air parcels with a long residence time along were less likely to be tornadic than storms with smaller the boundary, which likely varies considerably among uv and ur deficits. The association of small thermodynamic supercells and even over the life cycle of a single perturbations within the forward flank with stronger storm. low-level mesocyclones is attributed to air parcels with

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FIG. 12. As in Fig. 11, but for equivalent potential temperature perturbation (K). greater potential buoyancy resulting in greater vertical (1983), Dowell and Bluestein (2002a), and Shabbott and accelerations within the low-level updraft, enhancing Markowski (2006), significant baroclinically generated the tilting of streamwise horizontal vorticity into the streamwise vorticity can be generated along weak buoy- vertical and subsequent stretching. Though storms with ancy gradients given a long parcel residence time. smaller thermodynamic deficits and greater potential Small deficits of uv were observed within the FFRG of buoyancy within the forward flank have been found to the Perryton supercell (Fig. 11), but were oriented nearly produce stronger low-level mesocyclones, the finding does perpendicular to wind observations, which would limit not preclude baroclinically generated streamwise hori- the potential residence time of air parcels within the zontal vorticity from being an important source for low- baroclinic vorticity generation region. However, deficits level mesocyclogenesis. As noted by Klemp and Rotunno of ur within the FFRG are stronger than the uv deficits

TABLE 5. Maximum positive perturbation values of ue observed within the FFRG for each of ﬁve cases in MOBILE-07 as well as perturbation value of uv at time of max ue.

Date Location Tornadic/nontornadic u9e u9v Probe ID 22 May Hill City, KS Tornadic 1.775 22.891 24B 23 May (case 1) Perryton, TX Tornadic 3.546 20.189 18B 23 May (case 2) Canadian, TX Nontornadic 0.607 20.771 T4 31 May (case 1) Keyes, OK Nontornadic 1.770 21.750 23A 31 May (case 2) Hough, OK Tornadic 2.517 0.194 T2

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FIG. 13. As in Fig. 11, but for density potential temperature perturbation (K).

and are oriented more favorably for longer parcel resi- positive perturbations in ue would likely also be charac- dence times along the buoyancy gradient (Fig. 13). Given terized by lower values of convective inhibition (CIN), favorable parcel trajectories, air parcels traveling along a lower lifted condensation level (LCL), and a lower LFC the FFRG of the Perryton supercell would have the than for the base-state storm inflow, each of which has potential to generate substantial streamwise horizontal been associated with an increasing likelihood of torna- vorticity for ingestion by the low-level updraft. dogenesis (Rasmussen and Blanchard 1998; McCaul and Equivalent potential temperature is commonly used Weisman 2001; Markowski 2002b; Davies 2004). as a parcel tracer because it is conserved for both dry- and The presence of negative perturbations in uv and ur moist-adiabatic processes. Therefore, an increase in sur- coupled with positive perturbations in ue across the face ue would result in a rightward shift in the parcel path FFRG in the Perryton supercell suggest the possibility of surface air on a skew T–logp diagram. Consequently, that a supercell can ingest air parcels containing both despite air parcels within the FFRG being more dense substantial amounts of baroclinically generated stream- than inflow parcels near the surface due to lower uv, the wise horizontal vorticity and greater potential buoyancy positive perturbation in ue represents an increase in than parcels residing in the unmodified inflow environ- potential buoyancy due to the rightward shift of the ment. It is once again stressed that thermodynamic per- parcel path. If air parcels traveling through the FFRG of turbations along the FFRG only offer potential impacts the Perryton storm were forced to rise to their level of on low-level mesocyclogenesis and maintenance. Parcel free convection (LFC) by convergence under the low- trajectories, residence times along buoyancy gradients, level updraft, they would have greater CAPE than depths of the thermodynamic perturbations, and contri- parcels originating elsewhere in the storm inflow. The butions from air parcels originating in the RFD likely all

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21 FIG. 14. As in Fig. 11, but for water vapor mixing ratio perturbation (g kg ). play a role in low-level mesocyclogenesis and can vary can be tilted into cyclonic vertical vorticity by the among storms and even the life cycle of a single supercell. updraft and contains air with greater potential buoyancy and smaller convective inhibition than inflow b. Conclusions and future work parcels residing outside of the FFRG that can increase Surface observations collected within the 23 May 2007 vertical accelerations and enhance stretching of cy- Perryton, Texas, supercell have been presented. The clonic vertical vorticity within the updraft. following conclusions are presented following our d Supercells may exhibit a multiple RFGF structure analysis of 20 StickNet probes, two mobile mesonet with surges exhibiting distinct kinematic and thermo- probes, and SMART-R radar data. dynamic characteristics from the broader-scale RFD. To fully address the impacts the FFRG has on d Air parcels residing within the FFRG of the Perryton supercell evolution, much further research is needed. supercell appear to be representative of storm inflow Primarily, dense observations of the FFRG need to be modified through latent chilling and a flux of water integrated with dual-Doppler generated parcel trajec- vapor. tories or assimilated into three-dimensional numeric d A thermodynamic boundary consisting of negative cloud models to identify which air parcels, if any, within

perturbations of uv and ur and positive perturbations the FFRG are ingested by the low-level mesocyclone. of ue can develop across the FFRG of supercells. This Additionally, a three-dimensional analysis provided by boundary is a potential source of low-level baroclini- dual-Doppler analyses and/or storm-scale numerical cally generated streamwise horizontal vorticity that models will allow for a full vorticity budget of the low-level

Unauthenticated | Downloaded 10/05/21 06:23 AM UTC 3748 MONTHLY WEATHER REVIEW VOLUME 139 mesocyclone to be developed to quantify the impacts of Burgess, D. W., V. T. Wood, and R. A. Brown, 1982: Mesocyclone the FFRG boundary. A case study of a single storm has evolution statistics. Preprints, 12th Conf. on Severe Local been presented in this paper. To identify the prevalence Storms, San Antonio, TX, Amer. Meteor. Soc., 422–424. Davies, J. M., 2004: Estimations of CIN and LFC associated with tor- of opposing perturbations of uv/ur and ue within super- nadic and nontornadic supercells. Wea. Forecasting, 19, 714–726. cells, many additional cases from nontornadic, weakly Davies-Jones, R. P., and H. E. Brooks, 1993: Mesocyclogenesis tornadic, and strongly tornadic supercells are needed to from a theoretical perspective. The Tornado: Its Structure, perform a statistical analysis of the presence and char- Dynamics, Prediction, and Hazards, Geophys. Monogr., Vol. acter of the FFRG boundary. Finally, only near-surface 79, Amer. Geophys. Union, 104–114. Dowell, D. C., and H. B. Bluestein, 1997: The Arcadia, Oklahoma, observations of the FFRG are available in this study. storm of 17 May 1981: Analysis of a supercell during torna- Observations obtained by radiosondes, unmanned ae- dogenesis. Mon. Wea. Rev., 125, 2562–2582. rial systems, or thermodynamic retrieval of temporally ——, and ——, 2002a: The 8 June 1995 McLean, Texas, storm. Part resolute dual-Doppler analyses would be beneficial in I: Observations of cyclic tornadogenesis. Mon. Wea. Rev., 130, diagnosing the character of the FFRG aloft. It is noted 2626–2648. ——, and ——, 2002b: The 8 June 1995 McLean, Texas, storm. Part that datasets collected during the second Verification of II: Cyclic tornado formation, maintenance, and dissipation. the Origin of Rotation in Tornadoes Experiment Mon. Wea. Rev., 130, 2649–2670. (VORTEX2) have the potential to address all of the Emanuel, K. A., 1994: Atmospheric Convection. Oxford University items listed above. Press, 580 pp. Finley, C. A., and B. D. Lee, 2004: High resolution mobile mesonet observations of RFD surges in the June 9 Basset, Nebraska Acknowledgments. We are grateful for the dedication supercell during Project ANSWERS 2003. Preprints, 22nd and enthusiasm displayed by the student participants Conf. on Severe Local Storms, Hyannis, MA, Amer. Meteor. from TTU and the University of Michigan during Soc., P11.3. [Available online at http://ams.confex.com/ams/ MOBILE-07. Terra Thompson is acknowledged for pdfpapers/82005.pdf.] assistance with the procurement and processing of ——, and ——, 2008: Mobile mesonet observations of an intense SMART-R data. Brian Hirth, Dr. Ian Giammanco, and RFD and multiple RFD gust fronts in the May 23 Quinter, Kansas tornadic supercell during TWISTEX 2008. Preprints, Jaret Rogers are recognized for their assistance with 24nd Conf. on Severe Local Storms, Savannah, GA, Amer. objective analysis software, StickNet instrumentation, Meteor. Soc., P3.18. [Available online at http://ams.confex. and deployment strategy, respectively. Dr. George Bryan, com/ams/pdfpapers/142133.pdf.] Dr. Bruce Lee, and one anonymous reviewer are thanked ——, ——, M. Grzych, C. D. Karstens, and T. M. Samaras, 2010: for their thoughtful reviews of this paper. This study was Mobile mesonet observations of the rear-flank downdraft evolution associated with a violent tornado near Bowdle, SD supported by National Science Foundation Grant ATM on 22 May 2010. Preprints, 25th Conf. on Severe Local Storms, 0800542. M. Biggerstaff, L. Wicker, and the SMART Denver, CO, Amer. Meteor. Soc., 8A.2. [Available online at radars were supported by National Science Foundation http://ams.confex.com/ams/pdfpapers/176132.pdf.] Grants ATM-0619715 and 0802717, and M. 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