Downloaded 10/04/21 04:32 AM UTC 3420 MONTHLY WEATHER REVIEW VOLUME 140 Tornadoes (Markowski Et Al

NOVEMBER 2012 L E E E T A L . 3419

The Bowdle, South Dakota, Cyclic Tornadic Supercell of 22 May 2010: Surface Analysis of Rear-Flank Downdraft Evolution and Multiple Internal Surges

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

CHRISTOPHER D. KARSTENS Iowa State University, Ames, Iowa

(Manuscript received 2 December 2011, in ﬁnal form 6 June 2012)

ABSTRACT

Mobile mesonet sampling in the hook echo/rear-flank downdraft (RFD) region of a tornadic supercell near Bowdle, South Dakota, provided the opportunity to examine RFD thermodynamic and kinematic attributes and evolution. Focused analysis of the fifth low-level mesocyclone cycle that produced two significant tornadoes including a violent tornado, revealed four RFD internal surge (RFDIS) events. RFDISs appeared to influence tornado development, intensity, and demise by altering the thermodynamic and kinematic character of the RFD region bounding the pretornadic and tornadic circulations. Significant tornadoes developed and matured when the RFD, modulated by internal surges, was kinematically strong, only weakly negatively buoyant, and very potentially buoyant. In contrast, the demise of the Bowdle tornado was concurrent with a much cooler RFDIS that replaced more buoyant and far more potentially buoyant RFD air near the tornado. This surge also likely contributed to a displacement of the tornado from the storm updraft. Development of the first tornado and rapid intensification of the Bowdle tornado occurred when an RFDIS boundary convergence zone interacted with the pretornadic and tornadic circulations, respectively. In the latter case, a strong vertical vortex sheet along an RFDIS boundary appeared to be a near-surface cyclonic vorticity source for the tornado. A downdraft closely bounding the right flank of the developing first tornado and intensifying Bowdle tornado provided some of the inflow to these circulations. For the Bowdle tornado, parcels were also streaming toward the tornado from its immediate east and northeast. A cyclonic–anticyclonic vortex couplet was observed during a portion of each significant tornado cycle.

1. Introduction and Wakimoto 2003), a considerable number of high- resolution kinematic datasets have been obtained from High-resolution time and space observations in and tornadic supercells. Given the difficulty involved in adjacent to the supercell hook echo/rear-flank down- positioning near-surface observing systems, far fewer draft (RFD) region (Markowski 2002) are crucial for datasets, either mobile mesonet (Straka et al. 1996) or understanding the processes involved in tornado de- StickNet (Weiss and Schroeder 2008), exist to describe velopment, maintenance, and decay. To understand why proximate tornado or tornadogenesis region thermo- some tornadoes are very intense, quite large, and/or long dynamic and kinematic characteristics. Only a few lived, kinematic and thermodynamic observations must mesonet studies provide a detailed description of the be obtained over a wide spectrum of tornadoes. With evolution of the RFD outflow in tornadic supercell cases the rapid evolution of mobile Doppler radar (Wurman (e.g., Lee et al. 2004; Hirth et al. 2008; Lee et al. 2010). et al. 1997; Bluestein and Pazmany 2000; Bluestein Furthermore, the number of documented mobile mesonet datasets of the RFD region during a violent tor- Corresponding author address: Dr. Bruce D. Lee, WindLogics nado [Fujita scale (F) and enhanced Fujita scale (EF) Inc., 201 NW 4th St., Grand Rapids, MN 55744. ($F4 or EF4)] is very small. To our knowledge these E-mail: [email protected] cases include the 8 June 1995 Allison and Wheeler, Texas,

DOI: 10.1175/MWR-D-11-00351.1

Ó 2012 American Meteorological Society Unauthenticated | Downloaded 10/04/21 04:32 AM UTC 3420 MONTHLY WEATHER REVIEW VOLUME 140 tornadoes (Markowski et al. 2002, hereafter MSR02); of these cases the RFDIS displayed substantial increases the 24 June 2003 Manchester, South Dakota, tornado in virtual potential temperature (uy) and ue from the (Lee et al. 2004); and perhaps the 2 June 1995 Friona and RFD air preceding it. In mesonet analysis from the Dimmit, Texas, tornadoes (MSR02). The study herein 29 May 2008 Tipton, Kansas, tornado, in addition to provides analysis of RFD outflow thermodynamic and again finding the RFDIS juxtaposed with the tornado, kinematic characteristics, internal gradients, and evolu- analysis revealed parcels with only very weak negative tion over a large portion of a cyclic tornadic supercell life buoyancy and considerable potential buoyancy moving span including near-tornado observations during a vio- out of the left flank of the surge (looking downstream) lent tornado. For economy, hereafter when we are re- and into the tornado (Lee et al. 2011). Recent radar ob- ferring to RFD outflow, we will simply use RFD. servations (e.g., Wurman et al. 2007; Marquis et al. 2008; Observational studies have shown that the RFD sur- Skinner et al. 2010; Wurman et al. 2010; Marquis et al. rounds or nearly surrounds the tornado (e.g., Brandes 2012) have also documented the RFDIS and RFDISB 1977, 1978; Lemon and Doswell 1979; MSR02), thus, the (sometimes referred to as a secondary RFD gust front), properties of the RFD within a few kilometers of the with a similar storm-relative position to mobile mesonet tornado are especially important across the tornado observations. Using dual-Doppler wind synthesis, Marquis life cycle. In particular, buoyancy characteristics of the et al. (2012) suggest that for some tornadoes, the RFDISB RFD may play an important role in the development, and RFDIS may play an important role in tornado main- intensification, maintenance, and diminution of near- tenance by influencing the convergence field surround- surface rotation (e.g., Leslie and Smith 1978; Davies-Jones ing the vortex and through baroclinic generation and and Brooks 1993; Adlerman et al. 1999; Davies-Jones tilting of horizontal vorticity. et al. 2001; Markowski 2002; Markowski et al. 2003, On 22 May 2010, a cyclic tornadic supercell produced 2008; Marquis et al. 2012). The analysis of mobile mes- a family of tornadoes near U.S. Highway 12 (HW 12) in onet measurements within the RFD by MSR02 and north-central South Dakota [(National Climatic Data Grzych et al. (2007, hereafter GLF07) revealed com- Center) NCDC 2010]. The largest and most intense of pelling evidence supporting the general conclusion that the tornadoes occurred near Bowdle, South Dakota, and tornadic and nontornadic supercells had differing RFD produced EF4 damage (hereafter called the Bowdle thermodynamic characteristics. Specifically, the results tornado). The Tactical Weather-Instrumented Sampl- of MSR02 (and supported by GLF07) showed that tor- ing in/near Tornadoes Experiment (TWISTEX) mobile nado likelihood, intensity, and longevity increase as the mesonet was able to take coordinated observations near-surface buoyancy, potential buoyancy, and equiv- within and very near the hook echo region of this storm alent potential temperature (ue) increase in the RFD, (hereafter called the Bowdle supercell) for an approxi- and as the convective inhibition (CIN) in the RFD de- mate 2.5-h period starting near the time it attained su- creases. percell characteristics (Browning 1964; Rotunno 1993). Research focus on the RFD has recently expanded Within this extended deployment, the notable aspects of into internal RFD kinematic and thermodynamic gra- the data sampling include the following: 1) a near-ideal dients, which sometimes manifest as an RFD internal sampling array across the tip of the hook echo just prior surge (RFDIS). An RFDIS represents a distinct accel- to and during development of a significant tornado im- eration of the outflow within a previously established mediately preceding the Bowdle tornado; 2) observa- RFD. A wind direction change often accompanies the tions within the RFD, much of the time at close range, surge. For the RFDISs analyzed herein, a wind speed covering nearly the entire life span of the Bowdle tor- 2 increase exceeding 13 m s 1 was observed in all cases nado; 3) at least five RFDISs, some of which were con- (the largest wind speed change in a surge was over current with tornado development, intensification, or 2 28 m s 1). The RFDIS boundary (RFDISB) delineates dissipation; and 4) sampling the RFD and RFD gust front the kinematic leading edge of the surge. Analysis of (RFDGF) through numerous low-level mesocyclone mobile mesonet data has associated the onset of the (LLM) cycles. RFDIS to tornadogenesis or tornado intensification (e.g., TWISTEX is an ongoing tornado research project Finley and Lee 2004; Lee et al. 2004; Finley and Lee with a complement of four mobile mesonet vehicles: 2008; Lee et al. 2010). A broad range of thermodynamic M1, M2, M3, and M4. One project team transports an characteristics in these surges have been found, but in array of quickly deployable probes (Karstens et al. 2010a) cases of tornadogenesis or tornado intensification, the designed to sample the tornado and near-tornado envi- surges have been only weakly negatively buoyant with ronment. The primary objective of the field portion of considerable potential buoyancy [i.e., sizeable parcel TWISTEX is to collect thermodynamic and kinematic convective available potential energy (CAPE)]. In some data with a mobile mesonet in the hook echo/RFD region

Unauthenticated | Downloaded 10/04/21 04:32 AM UTC NOVEMBER 2012 L E E E T A L . 3421

FIG. 1. Geopotential height field at 500 mb (m, solid lines) and FIG. 2. Surface synoptic analysis at 2300 UTC 22 May 2010 vector winds at 500 (black arrows) and at 850 mb (gray arrows) for with mean sea level pressure and subjective frontal positions. 2300 UTC 22 May 2010 (from RUC analysis). (bottom right) The Temperature (8F), dewpoint (8F), and wind speed (half barb 5 kt, 2 maximum wind vectors (m s 1) are shown. full barb 10 kt) are plotted in the station models. Isobars are shown every 2 mb. The gray dot locates the Bowdle supercell at 2300 UTC. of supercells, while also gathering thermodynamic and kinematic data with probes in or very near tornadoes. range. The cell that would eventually become the Bowdle supercell formed at approximately 2200 UTC within a 2. Synoptic and mesoscale environment short north–south-oriented line segment of cells near the Conditions over portions of South Dakota on the Missouri River north of the surface low. evening of 22 May 2010 were quite favorable for tor- To assess the deep-convective environment for nadic supercells. A southwest-flow upper-level pattern the Bowdle supercell (see Fig. 2 for cell location), was present over the northern plains as shown in the RUC 2300 UTC analysis along with mesonet surface 2300 UTC Rapid Update Cycle (RUC) model (Benjamin conditions were used to create a representative sound- et al. 2004) 500-mb analysis in Fig. 1. As seen in the ing and hodograph for western Edmunds County (Fig. 3) 850-mb vector winds in Fig. 1, a strong low-level jet in north-central South Dakota. This approach was stretched from the upper Midwest into the central plains. preferred over applying modifications to the Aberdeen, During the late afternoon a weak surface low developed South Dakota, 0000 UTC soundings given the sub- in central South Dakota within a trough of low pressure stantial mesoscale wind and thermodynamic gradients as shown in the 2300 UTC surface analysis in Fig. 2. in this area. Very large conditional instability was present 2 Southerly to south-southeasterly winds of 15–25 kt with lowest 50-mb mixed-parcel CAPE of 4840 J kg 1. 2 2 (1 kt 5 0.5144 m s 1) were present over southern and CAPE in the 0–3-km layer was 171 J kg 1, a value con- eastern South Dakota within the warm sector of this sidered favorable for tornadic supercells (Rasmussen 2003). 2 cyclone. Between 2200 and 0100 UTC the winds over CIN for the lowest 50-mb parcel ascent was 52 J kg 1. northern and northeast South Dakota, southeast North The virtual temperature correction was applied to the Dakota, and western Minnesota backed, likely in re- parcel temperature ascent curve as described in Doswell sponse to a north-northeastward moving area of di- and Rasmussen (1994) for the CAPE and CIN calcu- vergence that was associated with the upper-level jet lations. The lifting condensation level (LCL) was 575 m, stream (not shown). Of relevance to the evolution of the representing another significant tornado environment Bowdle supercell, winds north-northeast of the surface attribute (Rasmussen and Blanchard 1998; Thompson low and just north of the warm front (Fig. 2) were east- et al. 2003). southeasterly to easterly based on mobile mesonet ob- The low- and deep-layer shear for north-central and servations. Just prior to and during storm development, northeast South Dakota was conducive for storm rota- 2 mesonet-observed dewpoints in this area were around tion. Deep-layer 0–6-km vector shear of 32 m s 1 was 708F with dewpoint depressions roughly in the 68–98F more than sufficient to support supercells (Weisman

Unauthenticated | Downloaded 10/04/21 04:32 AM UTC 3422 MONTHLY WEATHER REVIEW VOLUME 140

times eliminating the need for a flux gate compass. Data were collected every 1 s. Instrument bias correction and quality control procedures were similar to those described by MSR02 and GLF07, and are detailed in Lee et al. (2011). In addition, thermodynamic observations were removed from stations M1 and M3 between 2321:30 and 2329:40 UTC because of suspected thermistor wetting that occurred in the wake of a semitrailer truck that lofted a substantial spray plume. A check was made to identify time periods where tailwinds were present at an intensity to cause a substantial reduction in the flow through the ‘‘J’’-shaped aspirated radiation shield housing the temperature and relative humidity sensors. The high- speedfansusedintheTWISTEXmesonetradiation shields (see Table A1), with volumetric output about 1.5 times the fans used in the original Straka et al. (1996) design, mitigated this problem somewhat. Tail- wind periods that likely reduced ventilation through the radiation shield were identified and indicated on the applicable graphics in section 4. These periods do FIG. 3. Skew T–logp thermodynamic diagram and hodograph for not represent invalid data, but rather, the data may a location near Bowdle, South Dakota, at 2300 UTC based on RUC reflect a reduced effective response time.1 Last, be- analysis. Temperature (heavy dark line), dewpoint (medium gray cause of a temporary barometer dropout, pressure for line), virtual temperature (dashed line), and 50-mb mixed layer M1 between 2330:00 and 2341:56 UTC was estimated parcel ascent path with virtual temperature correction (thick dark gray line) are shown. Altitudes (3100 m) and storm motion vector from nearby mesonet stations using time and distance for the Bowdle supercell shown on the hodograph. weighting. During this period the nearest station was within 0.5 km of M1 and the elevation difference between M1 and nearby mesonet stations was only a few meters. The M1 pressure errors from this estimation and Klemp 1982; Rasmussen and Blanchard 1998; process were assuredly less than 1 mb resulting in neg- Thompson et al. 2003). Storm-relative helicity (SRH) of ligible changes in the derived thermodynamic quantities 2 306 m2 s 2 for the 0–3-km layer was favorable for su- described below. percells (Rasmussen and Blanchard 1998; Thompson Derived thermodynamic variables were calculated in 2 et al. 2003) and SRH of 240 m2 s 2 for the 0–1-km layer addition to the variables measured directly by the mes- suggested enhanced potential for tornadic supercells onet. The equivalent potential temperature, ue, was (Rasmussen 2003; Thompson et al. 2003). The 0–1-km calculated as in MSR02 and GLF07 using the formula energy helicity index (Hart and Korotky 1991) of 7.3 and derivation of Bolton (1980). If moist entropy is ap- significant tornado parameter of 9.8 were very favorable proximately conserved for adiabatic processes, then ue indicators of a significant tornado environment as shown can be useful in parcel origin height analysis, assuming by Rasmussen (2003) and Thompson et al. (2003, 2004), a number of caveats outlined in section 4c. In surface respectively. parcel spatial comparisons, higher ue implies a rightward shift in the parcel path on a skew T–logp diagram, and thus, higher potential buoyancy. Since fluctuations in the 3. Data collection and methodology virtual potential temperature, uy (Glickman 2000), from A mobile mesonet (hereafter, mesonet) with station a base-state value are proportional to buoyancy, uy was design similar to that presented by Straka et al. (1996) calculated to assess RFD buoyancy characteristics. No was used to measure temperature, pressure, humidity, hydrometeors were assumed to be present for the uy and wind velocity in and near the supercell hook echo/ RFD region. (Table A1 provides a list of mesonet station instrumentation and accessories used by TWISTEX.) 1 Note that stagnation in the radiation shield only occurs for Field procedures were developed such that a global po- tailwind strength that roughly balances fan aspiration. Stronger sitioning system could be used for vehicle direction at all tailwinds ventilate the radiation shield through rear-to-front flow.

Unauthenticated | Downloaded 10/04/21 04:32 AM UTC NOVEMBER 2012 L E E E T A L . 3423 calculations. We had little confidence in estimating the conversion as described by MSR02 (for reference, liquid water mixing ratio (ql) from radar reflectivity KABR is located 97 km east of Bowdle). This process since 1) the lowest elevation of the Weather Surveil- put the mesonet data into the storm’s frame of refer- lance Radar-1988 Doppler (WSR-88D) radar beam ence. Time intervals of 5–6-min were used for most of from Aberdeen, South Dakota (KABR), was over 1 km the time–space conversion plots consistent with the ap- above ground level (AGL) for much of the sampling proximate time for a single WSR-88D volume scan; period; and 2) the rain intensity, when encountered by however, kinematic and thermodynamic structures near the teams, varied markedly on small space and time the hook echo are not in steady state for this length of scales. Fortunately, and due in large part to the Bowdle time, so the wings of plotted data carry less credence. supercell maintaining a classic structure (Moller et al. For example, in a typical 5-min time–space analysis, 1994) for most of the operations period, precipitation a feature captured on the wings of the analysis (2.5 min 2 exceeding a qualitative moderate intensity was only from the analysis center time) moving 10 m s 1 faster encountered by the teams during 1) a few brief periods than the assumed storm motion could be depicted of roughly 10–20 s where close-in teams encountered 1.5 km from its actual storm-relative position. Storm very narrow rain curtains, 2) an approximate 2-min pe- motion for time–space conversion was calculated from riod near the end of the Bowdle tornado at a point in the average motion of the mesocyclone centroid (using the RFD evolution where outflow already had large KABR 0.58 radial velocity) except during the Bowdle negative uy departures from the base state, and 3) during tornado. In the latter case, the calculated motion of this a 1–2-min period relatively late in the seventh LLM tornado, which at times encompassed a substantial cycle. Although the exclusion of ql in the formal calcu- portion of the LLM, was used. To remove some of the lation of uy results in less accurate values, only small very small time-scale fluctuations, 5-s averages were errors in uy are expected given the rainfall intensity ex- applied to the wind and thermodynamic data except perienced during all precipitating periods except the where noted. short episodes listed above. For example, had the radar reflectivity been 40 dBZ when teams were in moderate precipitation, an overestimate in uy of about 0.23 K 4. Observations and analysis would be incurred from neglecting the q estimated l a. Deployment overview from the parameterization of Rutledge and Hobbs (1984). In the few and brief periods of substantially The initial mesonet deployment targeted the hook echo/ heavier precipitation, larger overestimates of uy as RFD region of the developing Bowdle supercell along presented by Shabbott and Markowski (2006) would be U.S. Highway 83 in central Walworth County around expected. Teams observed falling hail in two brief in- 2235 UTC. The general mesonet deployment strategy on stances near and north-northeast of the neck of the this day was to establish a north–south array in front of the hook echo (Forbes 1978) along HW 12 (Fig. 4). In these RFDGF, let the RFDGF and RFD sweep over the mes- cases, only a few hail stones were observed falling over onet, transect the hook with a moving array as the mesonet a 1–2-min period. The stones were generally in the 2.5– repositioned to get back in front of the RFDGF, and 4.3-cm-diameter range. continue to repeat this process. This sampling strategy The CAPE and CIN for all data points were calculated was made possible by a good road network and relatively 2 using the sounding developed from RUC 2300 UTC low mean storm motion of 11 m s 1 toward the northeast analysis data (Fig. 3) and surface conditions provided by (early mature) or east-northeast to east (mature) as shown the mesonet. The CAPE and CIN are based on surface in Fig. 4. Of particular interest was the period between parcel ascent and include a virtual temperature correc- 2304–2348 UTC during which the supercell produced two tion. Surface elevation for the sounding was adjusted to substantial tornadoes, the second being the Bowdle EF4 be consistent with the elevation of the mesonet data. tornado(Fig.4).Datacollectioncontinueduptoabout To obtain the perturbation virtual potential temper- 0100 UTC, at which time, the Bowdle supercell transi- ature (u9y) and perturbation equivalent potential tem- tioned to a high-precipitation supercell structure (Moller perature (u9e), base states were determined from mesonet et al. 1994). In summary, the mesonet took nearly con- storm-inflow observations similar to GLF07, Hirth et al. tinuous observations in/near the Bowdle supercell hook (2008), and Lee et al. (2011). It is noteworthy that the echo/RFD region from about 2235–0100 UTC. difference in inflow uy (u ) at the beginning and end of e b. Spatial analysis the 2.5-h operations period was only 20.4 K (20.6 K). For portions of the analysis, data points were plotted Analysis of data collected over the fifth low-level relative to KABR radar data using time-to-space mesocyclone cycle (LLM5, Fig. 4) is the primary focus of

Unauthenticated | Downloaded 10/04/21 04:32 AM UTC 3424 MONTHLY WEATHER REVIEW VOLUME 140

FIG. 4. Bowdle supercell radar reflectivity (dBZ) at 0.58 tilt from KABR over a time period centered on deployments within LLM5 that cover the first and second tornadoes. Tornado paths are depicted with approximate width and red triangles show the location of the tornadoes at the radar update time. The bold dark gray lines denote primary mesonet deployment routes. Dashed white lines separate precipitation core regions between radar frames. Dashed rectangles correspond to display regions for Figs. 6, 11, and 14. Inset shows tracks of the first 7 LLMs with cycle number and approximate beginning and ending times. Brown and green lines in inset are reference highways and county boundaries, respectively. All times are in UTC. this subsection. LLM5 developed about 2304 UTC and several minutes as the mesonet sampled deeper into the was larger and much more intense than previous cycles RFD, uy and ue deficits were generally in the 1–2- and 6– based on visual and radar observations. Two significant 8-K ranges, respectively. Within the first several kilo- tornadoes formed during this cycle, the first of which meters west of the RFDGF, the ground-relative west to 2 developed at ;2313 UTC and lasted 6 min. This visually west-southwest winds were only in the 7–11 m s 1 substantial tornado, rated EF1 (NCDC 2010), stayed range. over open agricultural land, so there were few damage The mesonet established a north–south array in front indicators aside from downed power poles to estimate of the projected path of the tip of the hook echo as it tornado intensity. Teams positioned such that the hook evolved into a hammerhead shape between 2305 and echo/RFD would pass over the mesonet roughly 10 km 2309 UTC as shown in Fig. 6. During this time the LLM northeast of Lowry (Fig. 4). The mesonet sampled the intensified markedly based on the visibly observed early RFD with this cycle as the RFDGF crossed over rotation rate of the associated wall cloud (Fig. 7). Near- the teams between 2304–2305 UTC as shown in Fig. 5. ideal mesonet positioning in a north–south array cov- The air immediately behind the RFDGF, as sampled by ered most of the hook echo tip while a developing the three southern stations, was similar in thermody- (tornado cyclone) TC-scale (Agee et al. 1976) cyclonic namic character to the inflow air, but over the next circulation (;2 km wide) passed over the northern part

Unauthenticated | Downloaded 10/04/21 04:32 AM UTC NOVEMBER 2012 L E E E T A L . 3425 of the mesonet as shown in Fig. 6. The hammerhead [computed as suggested by Koch et al. (1983) for hook echo tip shape appeared to be associated with data distributions with a substantial degree of non- counter-rotating circulations manifest in the storm- uniformity]. The interpolation grid spacing was 250 m. relative velocity data. This vortex couplet is strikingly As shown in Fig. 6a, a well-defined divergence– similar to that shown in the numerical and observational convergence couplet exists with a convergent wind research of Straka et al. (2007) and Markowski et al. field in/near the TC and divergent wind field bounding (2008) in conjunction with their analysis of vortex arches this region to the south and southeast. Peak divergence 2 2 and related baroclinic generation and tilting of hori- and convergence in these areas were 6 3 10 3 s 1 and 2 2 zontal vorticity. Perhaps related to the vortex couplet, 1.5 3 10 2 s 1, respectively, limited in magnitude some- the middle portion of the mesonet detected a boundary what by the objective analysis interpolation grid. The with limited southerly extent just leading the couplet divergence field closely matches that documented to (Fig. 6). Ground-relative wind speeds increased about the right of the tornado track by Lee et al. (2011) for the 2 8ms 1 just behind this boundary; however, this was 29 May 2008 Tipton, Kansas, tornadic supercell. In the substantially less than that observed for the RFDISs Tipton case, parcels possessing only very weak negative featured herein. This boundary may represent the lead- buoyancy moved out of a bounding downdraft and into ing edge of a late-stage RFDIS. the right and front flank of the tornado. Similarly, in As shown by MSR02 and GLF07, tornado likelihood this analysis parcels with only weak negative buoyancy and intensity appear related to thermodynamic charac- (uy deficits of 1–2 K) are moving out of the divergence/ teristics of the RFD outflow. For the analysis centered downdraft region and being ingested into the right and on 2309:03 UTC (Fig. 6a), deficits in uy are small front flank of the TC. Parcels within this region of the throughout the hook echo tip, with values generally RFD had large potential buoyancy with CAPE in the 21 between 1 and 2.5 K. The magnitude of the uy de- 3300–3700 J kg range. partures, reflective of only weak negative buoyancy, are A noteworthy aspect of Fig. 6a is the north–south generally consistent with those associated with tornado convergence region aligned along an RFDISB that environments by MSR02 and GLF07. In contrast with connects up to the TC convergence area. This RFDISB the relative consistency of uy across the hook echo tip, convergence pattern and interaction with a convergent ue varied markedly. In the middle portion of Fig. 6b large TC is similar to that presented by Marquis et al. (2008, south–north u9e gradients were present across the hook 2012) for dual-Doppler analysis of the LLM and RFDISB echo tip, with the largest ue deficits along the north side of of the 30 April 2000 Crowell, Texas, tornadic supercell. the TC and in a region about 3 km south of the TC near Tornado formation herein occurred as the RFDISB an anticyclonic circulation in the wind field. Across the translated around the right side of the TC (see Fig. 8). middle portion of the array ue increased from east to Marquis et al. (2012) suggested a tornado maintenance west (i.e., farther into the RFD). For much of the hook connection created by boundary-augmented conver- echo tip, especially west of an approximate north– gence. The present case appears to extend this con- southlinebisectingthehooktip,ue deficits were near nection to tornado development. those found by MSR02 for RFDs associated with sig- The divergence analysis in Fig. 6a and u9e field in nificant tornadoes. Fig. 6b (see section 4c for details on ue-inferred parcel The mesonet array across the hook echo tip afforded origins), imply a complex vertical velocity pattern in the opportunity to analyze the divergence field. A two- the hook tip. The enhanced divergence region within the pass Barnes objective analysis scheme (Barnes 1964; RFD just south through east of the TC, suggests the Koch et al. 1983) was applied to the time–space con- possibility of localized downdraft forcing that may differ verted mesonet velocity components, confined by the in mechanism from the broader RFD. The location and mesonet station positions, to interpolate the data to scale of this feature is suggestive of an occlusion down- a two-dimensional grid for divergence analysis. To pre- draft similar to that shown in numerical simulations (e.g., vent overweighting the analysis with closely spaced Klemp and Rotunno 1983; Adlerman et al. 1999) and west–east points compared to the north–south data observations (e.g., Wakimoto and Cai 2000). Occlu- point spacing (dictated by the mesonet team spacing of sion downdrafts are driven by a dynamically induced roughly 1100 m), data were subsampled at 30-s intervals downward-directed vertical pressure gradient force asso- before the objective analysis. This process resulted ciated with the intensification of low-level vertical vor- in average east–west data spacing of approximately ticity. Outflow from this downdraft appears responsible 300 m. The average data spacing used in calculating the for the boundary just east of the divergence region in smoothing parameter in the weighting scheme and in Fig. 6. The hook echo tip is partially bisected by implied selecting the interpolation grid spacing was 600 m low-level updraft associated with RFDISB convergence

Unauthenticated | Downloaded 10/04/21 04:32 AM UTC 3426 MONTHLY WEATHER REVIEW VOLUME 140

FIG. 5. (a) uy perturbations (K) and (b) ue perturbations (K) for a 5-min period centered on 2304:40 UTC KABR 0.58 tilt reflectivity with time–space conversion applied. Ground-relative 2 2 2 winds depicted with half barb, full barb, and flag representing 2.5 m s 1,5 m s 1, and 25 m s 1, respectively. Stations with no staff had wind data removed in quality control. RFDGF is shown with a bold line. Periods of possible reduced thermodynamic sensor response time from tailwind-related decreased ventilation are indicated with a vertical line in the station model. Team arrangement for all mesonet–radar overlay figures, except Fig. 11, are from north to south: M4, M1, M3, and M2. that connects to the TC convergence/updraft area. A region roughly corresponds to a narrow rain curtain small-scale feature of interest in Fig. 6b is the very narrow rotating around the east and north side of the LLM. arc of larger ue deficits that run from east of the TC to Thetimescaleofthisfeatureresultsinthetime–space north-northwest of it. Video evidence reveals this depiction of u9e to locally break down as evidenced in

Unauthenticated | Downloaded 10/04/21 04:32 AM UTC NOVEMBER 2012 L E E E T A L . 3427

FIG. 6. (a) uy perturbations (K) and divergence and (b) ue perturbations (K) for a 5-min period centered on 2309:03 UTC KABR 0.58 tilt reﬂectivity. Winds are storm relative. Solid (outlined) lines are divergence (convergence) 2 2 for values 0.005 and 0.0025 s 1 (20.01 and 20.005 s 1). Largest divergence (convergence) isopleths are bold. The dashed bold line depicts an RFDISB and the thin dashed line designates a possible late-stage RFDISB. Extension to time–space analysis used to place RFDGF. Tornado track hatched. TC and anticyclonic circulations indicated. Other details are as in Fig. 5. See inset of Fig. 4 for storm-relative domain.

Unauthenticated | Downloaded 10/04/21 04:32 AM UTC 3428 MONTHLY WEATHER REVIEW VOLUME 140 the large ue gradient across adjacent points in the analysis. As will be addressed in section 4c, parcels along the arc appear to have a descended from a higher origin level than those in adjoining regions. Between 2310:40–2311:30 UTC, the RFDISB passed through the mesonet accompanied by an abrupt increase in wind speed and backing storm-relative direction (as indicated in Figs. 6 and 8). Associated with this RFDIS, just southwest of the TC, uy and ue increased by roughly 0.5–1.0 and 1.5–2.0 K, respectively (Fig. 8). While uy at the southernmost station increased by a similar amount, a much larger increase in ue of nearly 6 K was realized. In addition to parcels within the surge having only weak negative buoyancy, these parcels possessed large potential buoyancy with CAPE 2 increasing to around 4000 J kg 1. Tornado formation occurred ;2 min after RFDISB passage about 1.5 km east of the mesonet. Figure 7 shows this substantial tornado a few minutes later. Although the foray of M4 into the forward-flank reflectivity gradient (FFRG) was brief (Fig. 8), the comparatively cool thermodynamics along the FFRG stand in marked contrast to those observed within the FIG. 7. (a) Wall cloud at 2309:45 UTC as viewed from approximately 1 km to the south by M3, and (b) maturing tornado at RFD. The peak uy and ue deficits (24and213 K) tended u 2315:20 UTC as viewed from the west by M1. Both images are to be slightly to moderately larger for y, and much wide-angle perspectives. larger for ue, than typical for tornadic supercells based on the forward-flank downdraft analysis of Shabbott and Markowski (2006) and FFRG analysis of Skinner et al. (2011). Given the rearward FFRG sampling lo- (Figs. 9 and 10c). Most of the mesonet was embedded cation, perhaps smaller uy and ue deficits lie farther east. in RFD before tornado development. The Bowdle tornado developed at 2319 UTC roughly M4’s positioning along HW 12 before the tornado concurrent with the dissipation of its predecessor. crossed the road allowed RFD sampling within 2 km of The remnant circulation of the first tornado appeared the tornado 2–3 min after tornado formation. Figure 11 to merge with a TC-scale circulation associated with displays LLM-relative mesonet observations for a 6-min the Bowdle tornado. Tornado track and intensity charac- period centered on 2327 UTC. An RFDIS and associ- teristics, determined from damage survey information by ated boundary were detected along with an area of the National Weather Service and TWISTEX, along with marked divergence about 1 km to the south and south- extensive videography, are shown in Fig. 9 (see Karstens east of the tornado. Downdraft above this divergence et al. 2010b for details). The tornado formed 11 km west- region was evident in videography showing rapidly de- southwest of Bowdle and moved at an average speed of scending cloud tags. The location and scale of this fea- 2 about 8 m s 1 to the east-northeast over its 27-min life ture, similar to that observed just before the previous span. Peak damage path width of 1100 m and EF4 in- tornado, is suggestive of an occlusion downdraft. The tensity (NCDC 2010) occurred to the northwest and north RFDIS was kinematically strong with peak 1-s wind 2 of Bowdle. speeds of 40 m s 1 for both M1 and M4 south of the The mesonet quickly repositioned on HW 12 during tornado. Perhaps related to this surge, an anticyclonic the genesis and early development of the Bowdle tor- circulation juxtaposed to the east of the tornado be- nado (Figs. 4 and 9). The tornado began as a broad came evident in videography roughly 5–6 min after the circulation that quickly developed secondary vortices, RFDISB passed over M1 and M4. Similar to the pre- one of which became dominant (Fig. 10a). After a brief vious tornadic cycle, parcels with weak negative buoy- period of weakening, the tornado rapidly grew in width ancy were located near the tornado; however, farther and intensity (Fig. 10b) near the time it passed over rearward within the RFD, parcels were substantially HW 12 at 2324:20 UTC. Over the following 5–6 min the more negatively buoyant (u9y of 24to25 K) and had tornado became wide (nearly 1 km) and very intense much larger ue deficits (u9e of 212 to 215 K). Where

Unauthenticated | Downloaded 10/04/21 04:32 AM UTC NOVEMBER 2012 L E E E T A L . 3429

FIG. 8. (a) uy perturbations (K) and (b) ue perturbations (K) for a 6-min period centered on 2313:33 UTC KABR 0.58 tilt reﬂectivity. The tornado location at the analysis center time is indicated with a red triangle. RFDGF position is based on KABR radial velocity and consistency with prior time–space analysis. Other details are as in Fig. 6.

Unauthenticated | Downloaded 10/04/21 04:32 AM UTC 3430 MONTHLY WEATHER REVIEW VOLUME 140

FIG. 9. Bowdle tornado track, life cycle stage, and general implied intensity. observations exist, the storm-relative flow was not trans- porting the larger uy and ue deficit air toward the tornado. The positioning of M4 at locations along HW 12 as the tornado approached and then passed over the highway allowed the flow field bounding the tornado to be sampled. Figure 12 shows the tornado-relative position of M4 with parcel characteristics and tornado-relative winds for 2319:24–2329:37 UTC. This period corresponds to data collection roughly coincident with initial tornado development through a phase of rapid intensification and widening (Fig. 10). The central 6 min of this period correspond to an interval of M4 observations closely bounding the tornado that covers about a 2808 arc around the circulation. Two boundaries are depicted in Fig. 12, FIG. 10. Bowdle tornado as observed from the (a) north by M4 at both having kinematic and thermodynamic signatures, 2320:22 UTC (approximately 1.5 min after formation), (b) west by especially the southeastern RFDISB. The boundary north- M1 at 2325:30, and (c) south by M2 at 2330:37 UTC. Note that all northwest of the tornado may be the leading edge of images are wide-angle perspectives. a late-stage RFDIS that wrapped well around the LLM; however, the wind field change across and behind (east of) this boundary, modest relative to other surges pre- and east through northeast, air ranging from near-neutral sented herein, precludes a confident designation of the buoyancy to weak-negative buoyancy is quickly con- boundary being an RFDISB. For reference, KABR ra- verging toward the tornado. The buoyancy environment dial velocity at 2322:31 and 2327:00 UTC indicated a is generally consistent with the findings of MSR02 and portion of the RFDGF was located along an arc roughly GLF07. Larger negative buoyancy exists beyond about 3–4 km north through southeast of the approximate 2 km southwest of the tornado; however, the motion of tornado location (Fig. 11). Deficits in uy bounding the these parcels is not toward the tornado. tornado were small ranging from about 0.5 to 2.5 K Air bounding the tornado had somewhat higher ue (Fig. 12a). Notably, from east through northeast of the deficits than typically found in RFDs associated with tornado aligned along the tornado track, uy was close supercells producing significant tornadoes (,4K;MSR02). to storm inflow values. Somewhat larger, but still rel- As shown in Fig. 12b, from northwest through east- atively weak, negative buoyancy resides within about southeast, ue deficits were generally 4–6 K. These 2 km behind the southeastern RFDISB with u9y rang- values increased substantially within the southern RFDIS ing from 21to22.5 K. Considering the large radial with ue deficits exceeding 8 K south of the tornado. Still component of the tornado-relative flow to the south larger deficits (.12 K) existed several kilometers farther

Unauthenticated | Downloaded 10/04/21 04:32 AM UTC NOVEMBER 2012 L E E E T A L . 3431

FIG. 11. (a) uy perturbations (K) and (b) ue perturbations (K) for a 6-min period centered on 2327:00 UTC KABR 0.58 tilt reﬂectivity. In the hook echo tip region, M4 is closest to the tornado, followed by M1 and M2. Other details are as in Fig. 8. See inset of Fig. 4 for storm- relative domain. The location of the RFDGF was based on KABR radial velocity. west-southwest, but these parcels were not moving to- Large potential buoyancy existed in the near-tornado ward the tornado. Although relatively large, ue deﬁcits environment as evidenced by CAPE values exceeding 2 south of the tornado do not imply midlevel parcel origins 3000 J kg 1 nearly surrounding the tornado (Fig. 12c). 21 as will be discussed in section 4c. The large gradient in ue, It is notable that CAPE values exceeded 4000 J kg and to a lesser extent uy, underscore the thermodynamic along the projected tornado track. Even well back into heterogeneity found within the RFD of some supercells the cooler air 4 km southwest of the tornado, CAPE 2 (e.g., MSR02; Finley and Lee 2008; Hirth et al. 2008; Lee was still greater than 2000 J kg 1. CIN values were et al. 2010). Similar to uy, the warmest RFD ue values smallest along the projected tornado path with values 2 were found along the tornado track to the east-northeast. around 100 J kg 1 (Fig. 12d). Overall, the CIN values

Unauthenticated | Downloaded 10/04/21 04:32 AM UTC 3432 MONTHLY WEATHER REVIEW VOLUME 140

FIG. 12. Tornado-relative position of M4 with parcel characteristics for the period 2319:24–2329:37 UTC. (a) uy 21 21 perturbations (K), (b) ue perturbations (K), (c) CAPE (J kg ), and (d) CIN (J kg ). Tornado-relative winds are shown in (a)–(d). Wind speed depiction as in Fig. 5. Most station spacing is between 7–11 s. Dashed rings indicate approximate tornado width at 2320 UTC (smaller) and from 2326–2330 UTC (larger). The bold dashed line depicts an RFDISB and the thin dashed line designates a possible late-stage RFDISB. For reference, base-state CAPE and 2 CIN for (c) and (d) are 4890 and 57 J kg 1, respectively.

nearly surrounding the tornado were similar to those perspective, cyclonic vorticity appears to be moving into found for RFD associated with significant tornadoes the front flank of the tornado through a layer starting (MSR02). from near the surface. Additionally, the strong RFDISB Noteworthy aspects of the flow field shown in Fig. 12 convergence zone appears to intersect the tornado. Al- are the strong vertical vortex sheet and marked con- though we cannot determine how long this potentially vergence aligned along the RFDISB to the east of the significant boundary–tornado interaction lasted, Marquis tornado. The cyclonic vertical vorticity and convergence et al. (2012, see their Fig. 6) have recently shown very 2 calculated along the sheet are 0.04 and 0.05 s 1, re- similar RFDISB–tornado/tornado cyclone configurations spectively, with the caveat that these are rough estimates that lasted around 10 min. due to the limitations of the sampling density near the During the period between 2329 and 2342 UTC over RFDISB and of the time–space conversion. Importantly, which the Bowdle tornado reached its peak width and given the flow field configuration, from a tornado-relative intensity (Fig. 9), mesonet data were collected from

Unauthenticated | Downloaded 10/04/21 04:32 AM UTC NOVEMBER 2012 L E E E T A L . 3433

northwest at the northern stations and intensifying, with 2 peak 1-s speeds of 25–30 m s 1 over the entire mesonet. The surge had large negative buoyancy with uy drop-

ping 5–6 K at all stations. A dramatic drop in ue was observed with values falling about 15–18 K. Parcel potential buoyancy fell rapidly with CAPE dropping under 2 2 1000 J kg 1, while CIN rose to roughly 600 J kg 1. Notably, the onset of this cold surge coincided with the tornado contracting and ultimately dissipating near 2346 UTC. c. Parcel origins

A comparison was made between the RFD ue and the inflow sounding vertical profile of ue (Fig. 16) to determine the height AGL on the inflow sounding with

equivalent ue values to those sampled. If the assumption is made that moist entropy is approximately conserved, it is possible that the height from which the surface outﬂow parcels originated was near the corresponding

height on the inflow sounding with the same ue value. Caveats to the application of this assumption include influences of entrainment and acknowledgment that FIG. 13. Wide-angle perspectives of the Bowdle tornado as a parcel path may include large vertical excursions be- observed from the southeast by M1 at (top) 2337:40 and (bottom) fore arriving at the surface. A limitation on the analysis 2339:15 UTC. Distance between M1 and edge of tornado in bottom involved a low-level u vertical profile that made it dif- panel is approximately 1 km. e ficult to differentiate inflow sounding matching heights within the surface–950-m layer. southwest through east of the tornado. The tornado was Consistent with past tornado-associated RFD ther- wedge shaped over much of this time as shown in Fig. 13. modynamic studies of MSR02 and Lee et al. (2011), Starting at about 2337:30 UTC, a kinematically strong RFD parcel origin heights were not reflective of mid- and relatively ‘‘warm’’ RFDIS was sampled (Fig. 14). level air, but relatively low-level source regions. For The average increase in the 5-s winds for the mesonet 21 instance, when considering RFD in/near the hook echo was over 14 m s with peak 1-s winds in this surge of 21 tip in Figs. 6, 8, 11, 12, and 14, with some exceptions, the between 30–32 m s at the northernmost three sta- area largely contains parcels associated with the 1000– tions. Directly after RFDISB passage, uy and ue in- 1300-m layer. Even in some regions with relatively large creased by about 1 and 3–4.5 K, respectively, while 21 u deficits, matching inflow sounding heights were not CAPE rose to around 4300 J kg . Within this RFDIS e particularly high because of the vertical profile of ue that uy increased to within 1 K of storm inflow. Between falls off very quickly with height above 1 km (e.g., u9 of 2337–2340 UTC, the tornado contracted from a wedge e 217 K associated with 2 km AGL). The lowest parcel structure to a wide cylinder (Fig. 13) with violent motion origination heights of roughly 1 km were found within observed near the surface. Damage survey evidence the warm RFDIS shown in Fig. 14. In contrast, parcels was consistent with the visual impressions (EF4, Fig. 9). associated with the largest heights of around 2100 m The timing of this contraction and intensification were found in the cold RFDIS shown in Fig. 15. Other appeared associated with this intense and relatively locations of higher origination heights were found on warm RFDIS. the southern periphery of the hook echo tip in Fig. 6 Another RFDISB passed over the mesonet at about (1700 m), within an arc of lower u aligned with a narrow 2342 UTC with this surge having a drastically different e 2 rain curtain east and north of the TC in Fig. 6 (1600 m), thermodynamic character as shown in Fig. 15. The along the FFRG in Fig. 8 (1900 m), and along the hook surge was accompanied by winds veering to the echo neck west of the tornado in Fig. 11 (1900 m).

It is plausible in RFD areas of modest ue deﬁcit that 2 2 Because of the tornado and LLM slowing to 2–3 m s 1, the parcels originated in storm inﬂow below 1 km, rose some spatial distribution of observations in the time–space format is distance in the storm updraft, and then were forced to compressed. descend by a downward-directed dynamic pressure

Unauthenticated | Downloaded 10/04/21 04:32 AM UTC 3434 MONTHLY WEATHER REVIEW VOLUME 140

FIG. 14. (a) uy perturbations (K) and (b) ue perturbations (K) for a 5-min period centered on 2339:31 UTC KABR 0.58 tilt reflectivity. Data are separated by 20 s. Other details are as in Fig. 8. See inset of Fig. 4 for storm-relative domain. gradient force while undergoing some mixing with air portions of the strong RFD, especially when considering along the trajectory. This process could lead to the the vertical profile of dewpoint depression in the lowest observed lower ue values compared to storm inflow. 2 km of the sounding (Fig. 3). Observed precipitation rates make hydrometeor drag d. Contrasting RFD characteristics between and a less likely primary downdraft forcing mechanism. Small within LLM cycles uy deficits, largely reflective of only small temperature differences from inflow values, reduce the possibility that The RFD heterogeneity over the duration of LLM5 is evaporative cooling is primarily driving the ‘‘warmer’’ a striking feature of that cycle (section 4b); however, the

Unauthenticated | Downloaded 10/04/21 04:32 AM UTC NOVEMBER 2012 L E E E T A L . 3435

FIG. 15. (a) uy perturbations (K) and (b) ue perturbations (K) for a 5-min period centered on 2343:54 UTC KABR 0.58 tilt reflectivity. Data are separated by 20 s. Other details are as in Fig. 8. Nested inset shows area depicted in main graphic. Because of the difference be- 2 tween the slow LLM reference speed (2–3 m s 1) and much-faster RFDISB speed (con- firmed by KABR radial velocity), the time–space conversion depicts the RFDISB as being more north–south oriented than its actual configuration (northern portion of the boundary located farther east.) extended data collection also allows the comparison of information and thermodynamic and kinematic data RFD characteristics of different LLM cycles (see inset analyses are presented in Fig. 17 and Table 1. Eight of Fig. 4). To facilitate the comparison of RFD char- cases of RFD and RFDIS analysis are examined com- acteristics between LLM cycles and to more readily prising several LLM cycles spanning a roughly 1.5-h present intra-RFD heterogeneity, sampling region period. In an effort to mitigate instrument equilibration

Unauthenticated | Downloaded 10/04/21 04:32 AM UTC 3436 MONTHLY WEATHER REVIEW VOLUME 140

mesonet data were used 3 km rearward of an RFDGF or RFDISB to better isolate readings to the feature being examined. The initial deployment on the Bowdle supercell during its first LLM cycle (case 1) shared very similar RFD buoyancy and potential buoyancy characteristics to that observed for case 2 (RFD), and to slightly lesser extent, cases 3 (RFDIS) and 5 (RFDIS) of LLM5, with small uy deficits and very large CAPE (Table 1). A prominent difference between nontornadic case 1 and all LLM5 cases, especially those associated with significant tornadoes, was in the observed kinematic strength. Per- haps this is best seen in the maximum 20-s mean wind

speeds (Max Wspd20s) with the peak speed in case 1 of 2 only 8.9 m s 1 compared with peak speeds exceeding 2 20 m s 1 for all tornadic LLM5 cases. Although stronger than case 1, RFD wind speeds associated with weakly tornadic LLM7 comprising cases 7 and 8 were substantially lower than the tornadic portions of LLM5. For context, at approximately 0004 UTC, tornado development was observed roughly 3 km north- northwestofGretna(Fig.4).Visibilityofthetornado u FIG. 16. Vertical proﬁle of e (K) for the inﬂow sounding. was lost at 0009:40 UTC as the wide, multiple-vortex circulation of relatively weak intensity became shrou- dedinprecipitation.WebelievethiscouldbetheEF0 effects on the analysis and to better represent airmass tornado reported in Storm Data (NCDC 2010) with properties of the RFD or RFDIS being studied, mesonet a dissipation time of 0012 UTC 6.5 km northeast of thermodynamic data were not used within a 1-min period Gretna, albeit with a start time 6 min earlier than re- after crossing an RFDGF or RFDISB. Additionally, no ported. The non-RFDIS part of the RFD of LLM7 (case

FIG. 17. Mesonet storm-relative sampling regions (gray) for RFD and RFDIS thermodynamic and kinematic characteristics presented in Table 1. The sampling regions depicted reflect mesonet data coverage (area within 1 km of a mesonet observation) and are limited in east– west dimension by the RFDGF (bold line), RFDISB (dashed bold lines), or a 3-km distance from boundary restriction (see text for details). The thin line corresponds to KABR 35-dBZ radar reflectivity at 0.58 tilt and outlines the Bowdle supercell hook echo at eight reference times (UTC). The filled circle indicates the approximate mesocyclone centroid indicated from 0.58 tilt KABR Doppler radial velocity.

Unauthenticated | Downloaded 10/04/21 04:32 AM UTC NOVEMBER 2012 L E E E T A L . 3437

TABLE 1. Thermodynamic and kinematic characteristics for eight cases of RFD or RFDIS sampling depicted in Fig. 17. Reference times are in UTC. Tornado presence/intensity designations associated with observing period for each case are no tornado (N) and the enhanced Fujita (EF) scale rating (only a single value is listed for the Bowdle tornado). Values of u9y and u9e are in K, values of parcel origin height (Zo) 21 are in m, and values of CAPE and CIN are in J kg . Ground-relative maximum 5-s mean wind speed (Max Wspd5s) and maximum 20-s 21 mean wind speed (Max Wspd20s) are in m s . Internal RFD or RFDIS data collected within 1 min of a boundary crossing were not used in the thermodynamic max/min analysis (see text for details). The arrows indicate whether the core variable in that row set is generally decreasing (Y), increasing ([), or remaining largely unchanged (4) in a direction orthogonal to the boundary and farther into the air mass [e.g., for case 6 the Y after the 22.0 in the Max u9y row indicates that uy is decreasing (uy deﬁcits increasing) farther into the air mass from the leading edge].

Case 1 Case 2 Case 3 Case 4 Case 5 Case 6 Case 7 Case 8 Reference time 2239:41 2304:40 2313:33 2327:00 2339:39 2343:54 0004:34 0008:25 LLM cycle LLM1 LLM5 LLM5 LLM5 LLM5 LLM5 LLM7 LLM7 Region RFD RFD RFDIS RFDIS RFDIS RFDIS RFD RFDIS Tornado N N EF1 EF4 EF4 EF4 EF0 EF0 Max u91y 0.7 4 10.7 Y 21.3 [ 21.4 Y 20.8 [ 22.0 Y 21.8 4 22.3 Y Min u92y 0.7 22.1 22.1 23.6 21.9 26.8 22.3 23.9 Max u91e 2.0 Y 10.1 Y 23.7 [ 27.2 Y 21.2 [ 26.4 Y 24.0 Y 27.0 Y Min u92e 7.4 210.2 26.0 211.8 25.1 221.8 28.9 213.3 Max Zo 1388 [ 1705 [ 1263 Y 1841 [ 1213 Y 2128 [ 1596 [ 1887 [ Min Zo ,950 ,950 1136 1359 981 1280 1150 1340 Max CAPE 5280 Y 4900 Y 4170 [ 3550 Y 4640 [ 3690 Y 4110 Y 3550 Y Min CAPE 3520 2970 3750 2660 3910 910 3210 2390 Max CIN 140 [ 206 [ 170 Y 292 [ 161 Y 622 [ 184 [ 293 [ Min CIN 41 56 129 162 100 154 130 159

Max Wspd5s 10.1 14.5 21.8 36.2 29.2 28.5 8.6 18.9 Max Wspd20s 8.9 13.5 20.3 35.0 27.4 25.8 7.9 16.1

2 7) had relatively weak winds very similar in strength to (case 2 vs case 6) occurring for LLM5 of 3990 J kg 1. case 1 (Table 1). Although a distinct increase to Consistent with the large variance in CAPE for LLM5, 21 21 16.1 m s in the Max Wspd20s was observed in the CIN ranged from 56 to 622 J Kg . Because of the ver- RFDIS of LLM7 (case 8), the wind speed strength was tical profile of ue in the inflow sounding (Fig. 16), even substantially below the tornadic portions of LLM5 and large ue deficits documented for LLM5 did not yield the duration of the stronger winds in the RFDIS was remarkably large differences in inferred parcel origin quite short (;2 min). Case 7 had buoyancy, potential height when comparing case 2 (sub-950 m) to case 6 buoyancy, and ue values similar to that seen for other (2128 m). Another facet of internal RFD heterogeneity is cases (e.g., case 3). In contrast, the RFDIS of this cycle seen in the general change in the core variables in a di- (case 8) was associated with markedly falling values rection orthogonal to a feature’s leading edge boundary of these quantities and rising inferred parcel origin and farther into the air mass. For example, as shown height and CIN. At comparative distances within a few in Table 1, out of the 8 RFD/RFDIS case samplings, kilometers of the tornado, this RFDIS was generally uy (ue) stayed the same in 2 (0), decreased in 4 (6), and cooler than the surges observed in LLM5 except the rose in 2 (2). final surge. A final aspect to intra-LLM cycle RFD variability can Large intra-LLM cycle RFD thermodynamic and ki- be seen in the large changes in wind speeds. Increased nematic variability is an obvious feature of this dataset peak wind speeds within the RFD associated with as shown previously and quantified in Table 1. Consid- RFDISs can be readily seen for both LLM5 and LLM7 ering, for instance, parcel buoyancy in LLM5, u9y ranges in Table 1. Of interest is the relationship between peak from 10.7 K in the early RFD sampling to 26.8 K (case RFD winds and tornado presence and intensity. When

2 vs case 6). The maximum–minimum spreads in u9e for comparing Max Wspd20s across LLM cycles, ranging 2 2 all three featured LLM cycles were substantial, with from 8.9 m s 1 in nontornadic case 1, to 16.1 m s 1 in 2 values of 9.4, 21.9, and 9.3 K for LLM1, LLM5, and weakly tornadic case 7, to 35 m s 1 in tornadic (EF4) LLM7, respectively. Large maximum–minimum spreads case 4, the results are strongly suggestive that higher in parcel potential buoyancy, consistent with the con- RFD wind speeds are related to tornado presence and siderable range in ue deﬁcit, was seen for these LLM intensity in the Bowdle storm. Even within LLM5 that cycles, with a remarkable intracycle CAPE difference produced two tornadoes, by a large margin, the highest

Unauthenticated | Downloaded 10/04/21 04:32 AM UTC 3438 MONTHLY WEATHER REVIEW VOLUME 140

Max Wspd20s values were associated with the EF4 tornado.

5. Summary and discussion Extended-duration mesonet sampling in the hook echo/RFD region of the Bowdle supercell provided the opportunity to examine RFD thermodynamic and kinematic attributes and evolution. Special attention was given to characterizing numerous RFDISs and re- lating the surges to tornado development, or to changes in tornadointensityandsize.FocusedanalysisofLLM5 that produced two significant tornadoes including the Bowdle EF4 tornado revealed four RFDISs. Part of the mesonet array was within 1 km of the tornadoes or their developing circulations during key deployments, FIG. 18. Schematic illustration of hook echo tip region features lending confidence that the detected RFDISs and as- during the rapid intensification stage of the Bowdle tornado. sociated RFDISBs were interacting with the tornadic Subjective tornado-relative streamlines with thermodynamic fields or pretornadic circulations. (see legend) corresponding to the approximate streamlined regions. The thin black line approximates a 35-dBZ reflectivity The first RFDIS of LLM5 had a marked kinematic contour. The circled T represents the tornado. signal and was accompanied by increased parcel buoyancy, potential buoyancy, and ue. Tornado formation was concurrent with the translation of the RFDISB An RFDIS coupled with a convergent, near-neutrally around the right flank of a TC-scale circulation. The buoyant and very potentially buoyant RFD flow field convergence pattern produced by the interacting appeared to play an important role in the rapid in- RFDISB and convergent TC is similar to that found in tensification stage of the Bowdle tornado. As RFD air dual-Doppler wind synthesis by Marquis et al. (2012) was converging on the tornado from the east and for cases where RFDISB-augmented convergence near northeast, a kinematically strong RFDIS with larger, but a tornado was potentially important for tornado main- still relatively weak, negative buoyancy interacted with tenance. For the first tornado, it appears the RFDISB the tornado and the ‘‘warm’’ inflow. With convergent played a role in tornado development, and likely, early flow within this surge bounding the tornado to the south, maintenance. A well-defined cyclonic–anticyclonic vor- convergent flow in place to the east and northeast, tex couplet had quickly developed before the RFDIS and a strong area of convergence and cyclonic vertical moved into, or formed within, the southern portion of vorticity along the RFDISB that appeared to intersect the hook echo tip. The development of this vortex the front flank of the tornado, a favorable setup was in couplet may have been related to outflow and an ac- place for inward transport of angular momentum and companying boundary, perhaps associated with a late- vertical vorticity stretching near the tornado. The loca- stage RFDIS, just leading this feature (Fig. 6). The tion and scale of the downdraft associated with the presence of counter-rotating vortices is suggestive of RFDIS was suggestive of an occlusion downdraft. An baroclinic vorticity generation associated with an RFD anticyclonic component to a counter-rotating vortex ‘‘cool’’ pool and subsequent tilting of the vortex lines couplet was observed a short time after the RFDISB by an updraft (vortex line arches discussed by Straka passed the mesonet. et al. 2007; Markowski et al. 2008). An augmented The Bowdle tornado primary intensification stage, as convergence zone from the RFDISB–TC interaction illustrated in Fig. 18, may represent an optimal situation that acted on the cyclonic vertical vorticity pool may combining the following: 1) RFD thermodynamics with have represented the final step in the tornadogenesis just enough negative buoyancy for baroclinic vorticity process. Of note, prior to surge arrival, near-surface generation, but not so much as to be detrimental (i.e., the parcels with only weak negative buoyancy and con- ‘‘Goldilocks’’ scenario as mentioned by Markowski et al. siderable potential buoyancy were streaming into the 2008), in this case the baroclinity was provided by an TC from a downdraft of scale and location suggestive of RFDIS; 2) an augmented tornado/TC convergence zone an occlusion downdraft along its right flank similar to due to the RFDIS and RFDISB interactions with the the Tipton, Kansas, tornado of 29 May 2008 (Lee et al. vortex; 3) near-neutrally buoyant and very potentially 2011). buoyant air converging on the left flank of the tornado;

Unauthenticated | Downloaded 10/04/21 04:32 AM UTC NOVEMBER 2012 L E E E T A L . 3439 and 4) a strong vertical vortex sheet aligned along an There appeared to be a relationship between the RFD RFDISB oriented such that cyclonic vorticity moves into kinematic strength and the occurrence of significant the front flank of the tornado through a layer starting tornadoes. LLM5 had far greater RFD winds than other from near the surface. LLMs sampled, while also producing the only two sig- The final two RFDISs of LLM5 were kinematically nificant tornadoes during the sampling period. This re- strong, but had vastly different thermodynamic charac- lationship held within LLM5, as the strongest of the ter. The first of these occurred concurrent with what RFD winds in this cycle, associated with surges, were appears to be a second tornado intensification period sampled during the Bowdle tornado. The kinematically that included EF4 damage. In this warm surge, uy, ue, and strong RFDISs, excluding the last RFDIS of this cycle, CAPE increased within about 1 km of the tornado. We were coupled with air possessing only weak negative speculate that the added convergence from this surge, buoyancy and large potential buoyancy. This coupled acting on the already intense circulation pool of the very RFDIS kinematic and thermodynamic signal is similar large tornado, fostered tornado contraction and inten- to that analyzed for other cases of strong or violent sification through vortex stretching. In the short term, the tornadoes (e.g., Lee et al. 2004; Finley and Lee 2008; Lee influx of near-neutrally buoyant air with very large po- et al. 2010). Last, just as the largest internal RFD kine- tential buoyancy may have aided tornado intensification matic variability was embodied in the surges, so too was via enhanced vortex stretching. The last RFDIS, having the largest thermodynamic variability with differing large negative buoyancy and CIN, coincided with the surges exhibiting vastly different thermodynamic char- tornado contracting and ultimately dissipating. Although acteristics (Table 1). mesonet observations are not available on the left flank of Analysis of numerous hook echo/RFD datasets cov- the tornado during the dissipation stage, the close prox- ering conditions supporting a variety of tornado events imity of observations to the tornado on its right flank will be required to understand the following: 1) the role lead us to conclude that tornado demise was due to the and frequency RFDISs play in tornado development, strong cold pool accompanying the final RFDIS replacing intensification and demise, 2) whether the RFDIS is more buoyant and far more potentially buoyant RFD a manifestation of an intrinsic change in the LLM/TC air bounding the tornado. Additionally, this strong surge strength or structure that might also have a role in tor- appears to have influenced the alignment of the tornado nado development or evolution, 3) the prevalence of with the storm midlevel updraft. Dowell and Bluestein strong RFDISB vertical vortex sheets and how often (2002) and Marquis et al. (2012) have shown that the they align to provide a source of cyclonic vertical vor- misalignment of tornado and storm updraft is related to ticity for the tornado, and 4) if the combination of strong tornado demise. Shortly after surge passage over the RFD winds, relatively weak negative buoyancy, and mesonet, the tornado was displaced westward with re- substantial potential buoyancy bounding or partially spect to the hook echo tip and midlevel updraft (inferred bounding the tornado is a necessary condition for the from the supercell weak echo region), toward the neck of development, intensification, and maintenance of strong the hook. Based on KABR radial velocity with support- tornadoes. ing mesonet data, the core of the RFDIS passed roughly 1–1.5 km south of the tornado, initially placing the tor- Acknowledgments. Tim Samaras is recognized for his nado within the northern periphery of the surge. Ex- tireless support of TWISTEX and its research and ed- pansion of the RFDIS south through east of the tornado, ucation mission. We are grateful to all the TWISTEX consistent with a surge-associated divergent flow field, volunteers, especially technical and team leaders Matt was evident in the radial velocity display. This outflow Grzych, Tony Laubach, and Carl Young. Jon Merage expansion likely accounts for the late northward turn of and Sean Mullins are also recognized for generously the tornado and end to its eastward movement (Fig. 9). providing video and mesonet station data used for

The vast majority of RFD parcels had comparative ue data comparison. We thank Andy Gabrielson and Skip values to those found on the inflow sounding between Talbot for sharing video of the event. Leland Treichel, 1 and 1.3 km. For some areas of the RFD, the parcel Edmunds County Emergency Manager, was very helpful origin height was likely lower considering some mixing in facilitating damage survey activities. We are very with lower ue air during vertical excursions. It was difficult appreciative of the thoughtful manuscript reviews of to find midlevel air in the RFD observations over the full Mr. Patrick Skinner and two anonymous reviewers. Par- 2.5-h sample. The highest parcel origin heights between tial support for author Karstens was provided by NOAA 1.9 and 2.1 km were found within the final RFDIS of Grants NA06OAR4600230, NA08OAR4600887, and LLM5, sometimes near the neck of the hook, and in a NA09OAR4600222. WindLogics Inc. provided support short-duration sampling within the FFRG region. for this publication.

Unauthenticated | Downloaded 10/04/21 04:32 AM UTC 3440 MONTHLY WEATHER REVIEW VOLUME 140

APPENDIX Mobile Mesonet Instrumentation and Accessories

TABLE A1. Instrumentation and accessories on the TWISTEX mobile mesonet.

Instrument/Accessories Model Output or comment Accuracy 2 2 Wind monitor RM Young 05103V Speed (0–100 m s 1) 60.3 m s 1 or 1% of reading Direction (08–3608) 638 Temperature/RH RM Young 41382 Temperature (2508 to 1508C) 60.38C RH (0%–100%) 61% Pressure Vaisala PTB220B Pressure (500–1100 mb) 60.25 mb Pressure port Quad Disk Probe From All Weather, Inc. GPS Delorme LT-40 Time, lat, lon Fan Delta FFB0912SH Super Radiation shield aspiration (90 CFM) High Speed Camcorder Sony HDR-XR500V Windshield mounted 1920 3 1080 HD

REFERENCES supercell during project ANSWERS 2003. Preprints, 22nd Conf. on Severe Local Storms, Hyannis, MA, Amer. Meteor. Adlerman, E. J., K. K. Droegemeier, and R. Davies-Jones, 1999: Soc., P11.3. [Available online at http://ams.confex.com/ams/ A numerical simulation of cyclic mesocyclogenesis. J. Atmos. 11aram22sls/techprogram/paper_82005.htm.] Sci., 56, 2045–2069. ——, and ——, 2008: Mobile mesonet observations of an intense Agee, E. M., J. T. Snow, and P. R. Clare, 1976: Multiple vortex RFD and multiple RFD gust fronts in the May 23 Quinter, features in the tornado cyclone and the occurrence of tornado Kansas tornadic supercell during TWISTEX 2008. Preprints, families. Mon. Wea. Rev., 104, 552–563. 24th Conf. on Severe Local Storms, Savannah, GA, Amer. Barnes, S. L., 1964: A technique for maximizing detail in numerical Meteor. Soc., P3.18. [Available online at http://ams.confex.com/ weather map analysis. J. Appl. Meteor., 3, 396–409. ams/24SLS/techprogram/paper_142133.htm.] Benjamin, S. G., and Coauthors, 2004: An hourly assimilation- Forbes, G. S., 1978: Three scales of motions associated with tor- forecast cycle: The RUC. Mon. Wea. Rev., 132, 495–518. nadoes. U.S. Nuclear Regulatory Commission, Contract Rep. Bluestein, H. B., and A. L. Pazmany, 2000: Observations of tor- NUREG/CR-0363, 359 pp. [NTIS PB-288291.] nadoes and other convective phenomena with a mobile, 3-mm Glickman, T., Ed., 2000: Glossary of Meteorology. 2nd ed. Amer. wavelength, Doppler radar: The Spring 1999 Field Experi- Meteor. Soc., 855 pp. ment. Bull. Amer. Meteor. Soc., 81, 2939–2951. Grzych, M. L., B. D. Lee, and C. A. Finley, 2007: Thermodynamic ——, and R. M. Wakimoto, 2003: Mobile radar observations of analysis of supercell rear-flank downdrafts from Project severe convective storms. Radar and Atmospheric Science: ANSWERS. Mon. Wea. Rev., 135, 240–246. A Collection of Essays in Honor of David Atlas, Meteor. Hart, J. A., and W. Korotky, 1991: The SHARP workstation Monogr., No. 52, Amer. Meteor. Soc., 105–136. v1.50 users guide. NOAA/National Weather Service, 30 pp. Bolton, D., 1980: The computation of equivalent potential tem- [Available from NWS Eastern Region Headquarters, 630 perature. Mon. Wea. Rev., 108, 1046–1053. Johnson Ave., Bohemia, NY 11716.] Brandes, E. A., 1977: Gust front evolution and tornado genesis as Hirth, B. D., J. L. Schroeder, and C. C. Weiss, 2008: Surface viewed by Doppler radar. J. Appl. Meteor., 16, 333–338. analysis of the rear-flank downdraft outflow in two tornadic ——, 1978: Mesocyclone evolution and tornadogenesis: Some ob- supercells. Mon. Wea. Rev., 136, 2344–2363. servations. Mon. Wea. Rev., 106, 995–1011. Karstens, C. D., T. M. Samaras, B. D. Lee, W. A. Gallus, and C. A. Browning, K. A., 1964: Airflow and precipitation trajectories Finley, 2010a: Near-ground pressure and wind measurements within severe local storms which travel to the right of the in tornadoes. Mon. Wea. Rev., 138, 2570–2588. winds. J. Atmos. Sci., 21, 634–639. ——, ——, W. A. Gallus Jr., C. A. Finley, and B. D. Lee, 2010b: Davies-Jones, R. P., and H. E. Brooks, 1993: Mesocyclogenesis Analysis of near-surface wind flow in close proximity to tor- from a theoretical perspective. The Tornado: Its Structure, nadoes. Preprints, 25th Conf. on Severe Local Storms, Denver, Dynamics, Prediction, and Hazards, Geophys. Monogr., Vol. 79, CO, Amer. Meteor. Soc., P10.12. [Available online at http:// Amer. Geophys. Union, 105–114. ams.confex.com/ams/pdfpapers/176188.pdf.] ——, R. J. Trapp, and H. B. Bluestein, 2001: Tornadoes and tor- Klemp, J. B., and R. Rotunno, 1983: A study of the tornadic region nadic storms. Severe Convective Storms, Meteor. Monogr., within a supercell thunderstorm. J. Atmos. Sci., 40, 359–377. No. 28, Amer. Meteor. Soc., 167–221. Koch, S. E., M. DesJardins, and P. J. Kocin, 1983: An interactive Doswell, C. A., III, and E. N. Rasmussen, 1994: The effect of ne- Barnes objective map analysis scheme for use with satellite glecting the virtual temperature correction on CAPE calcu- and conventional data. J. Climate Appl. Meteor., 22, 1487– lations. Wea. Forecasting, 9, 625–629. 1503. Dowell, D. C., and H. B. Bluestein, 2002: The 8 June 1995 McLean, Lee, B. D., C. A. Finley, and P. Skinner, 2004: Thermodynamic and Texas, storm. Part II: Cyclic tornado formation, maintenance, kinematic analysis of multiple RFD surges for the 24 June 2003 and dissipation. Mon. Wea. Rev., 130, 2649–2670. Manchester, SD cyclic tornadic supercell during Project Finley, C. A., and B. D. Lee, 2004: High resolution mobile mesonet ANSWERS 2003. Preprints, 22nd Conf. on Severe Local observations of RFD surges in the June 9 Basset, Nebraska Storms, Hyannis, MA, Amer. Meteor. Soc., P11.2. [Available

Unauthenticated | Downloaded 10/04/21 04:32 AM UTC NOVEMBER 2012 L E E E T A L . 3441

online at http://ams.confex.com/ams/11aram22sls/techprogram/ in midlatitude cyclones. XII: A diagnostic modeling study of paper_82000.htm.] precipitation development in narrow cold-frontal rainbands. ——, ——, C. D. Karstens, and T. M. Samaras, 2010: Surface ob- J. Atmos. Sci., 41, 2949–2972. servations of the rear-flank downdraft evolution associated Shabbott, C. J., and P. M. Markowski, 2006: Surface in situ ob- with the Aurora, NE tornado of 17 June 2009. Preprints, 25th servations within the outflow of forward-flank downdrafts of Conf. on Severe Local Storms, Denver, CO, Amer. Meteor. supercell thunderstorms. Mon. Wea. Rev., 134, 1422–1441. Soc., P8.27. [Available online at http://ams.confex.com/ams/ Skinner, P. S., C. C. Weiss, A. E. Reinhart, W. S. Gunter, J. L. pdfpapers/176133.pdf.] Schroeder, and J. Guynes, 2010: TTUKa mobile Doppler ra- ——, ——, and T. M. Samaras, 2011: Surface analysis near and dar observations of near-surface circulations in VORTEX2. within the Tipton, Kansas, tornado on 29 May 2008. Mon. Preprints, 25th Conf. on Severe Local Storms, Denver, CO, Wea. Rev., 139, 370–386. Amer. Meteor. Soc., P15.3. [Available online at http://ams. Lemon, L. R., and C. A. Doswell III, 1979: Severe thunderstorm confex.com/ams/pdfpapers/176236.pdf.] evolution and mesocyclone structure as related to tornado- ——, ——, J. L. Schroeder, L. J. Wicker, and M. I. Biggerstaff, genesis. Mon. Wea. Rev., 107, 1184–1197. 2011: Observations of the surface boundary structure within Leslie, L. M., and R. K. Smith, 1978: The effect of vertical stability the 23 May, 2007 Perryton, Texas, supercell. Mon. Wea. Rev., on tornadogenesis. J. Atmos. Sci., 35, 1281–1288. 139, 3730–3749. Markowski, P. M., 2002: Hook echoes and rear flank downdrafts: A Straka, J. M., E. N. Rasmussen, and S. E. Fredrickson, 1996: A review. Mon. Wea. Rev., 130, 852–876. mobile mesonet for finescale meteorological observations. ——, J. M. Straka, and E. N. Rasmussen, 2002: Direct surface J. Atmos. Oceanic Technol., 13, 921–936. thermodynamic observations within the rear-flank downdrafts ——, ——, R. P. Davies-Jones, and P. M. Markowski, 2007: An of nontornadic and tornadic supercells. Mon. Wea. Rev., 130, observational and idealized numerical examination of low- 1692–1721. level counter-rotating vortices toward the rear flank of su- ——, ——, and ——, 2003: Tornadogenesis resulting from the percells. Electron. J. Severe Storms Meteor., 2 (8), 1–22. transport of circulation by a downdraft: Idealized numerical Thompson, R. L., R. Edwards, J. A. Hart, K. L. Elmore, and simulations. J. Atmos. Sci., 60, 795–823. P. Markowski, 2003: Close proximity soundings within su- ——, E. Rasmussen, J. Straka, R. Davies-Jones, Y. Richardson, percell environments obtained from the Rapid Update Cycle. and R. J. Trapp, 2008: Vortex lines within low-level mesocy- Wea. Forecasting, 18, 1243–1261. clones obtained from pseudo-dual-Doppler radar observa- ——, ——, and C. M. Mead, 2004: An update to the supercell tions. Mon. Wea. Rev., 136, 3513–3535. composite and significant tornado parameters. Preprints, 22nd Marquis, J. M., Y. Richardson, J. Wurman, and P. Markowski, Conf. on Severe Local Storms, Hyannis, MA, Amer. Meteor. 2008: Single- and dual-Doppler analysis of a tornadic vor- Soc., P8.1. [Available online at https://ams.confex.com/ams/ tex and surrounding storm-scale flow in the Crowell, 11aram22sls/techprogram/paper_82100.htm.] Texas, supercell of 30 April 2000. Mon. Wea. Rev., 136, Wakimoto, R. M., and H. Cai, 2000: Analysis of a nontornadic 5017–5043. storm during VORTEX 95. Mon. Wea. Rev., 128, 565–592. ——, ——, P. Markowski, D. Dowell, and J. Wurman, 2012: Weisman, M. L., and J. B. Klemp, 1982: The dependence of nu- Tornado maintenance investigated with high-resolution dual- merically simulated convective storms on vertical wind shear Doppler and EnKF analysis. Mon. Wea. Rev., 140, 3–27. and buoyancy. Mon. Wea. Rev., 110, 504–520. Moller, A. R., C. A. Doswell III, M. P. Foster, and G. R. Woodall, Weiss, C. C., and J. L. Schroeder, 2008: The 2007 and 2008 1994: The operational recognition of supercell thunder- MOBILE Experiment: Developing and testing of the TTU storms environments and storm structures. Wea. Forecasting, StickNet platforms. Preprints, 24th Conf. on Severe Local 9, 327–347. Storms, Savannah, GA, Amer. Meteor. Soc., 5.1. [Available NCDC, 2010: Storm Data. Vol. 52, No. 5, 668 pp. [Available from online at http://ams.confex.com/ams/pdfpapers/141842.pdf.] National Climatic Data Center, 151 Patton Ave., Asheville, Wurman, J., J. M. Straka, E. N. Rasmussen, M. Randall, and NC 28801.] A. Zahrai, 1997: Design and deployment of a portable, pencil- Rasmussen, E. N., 2003: Refined supercell and tornado forecast beam, pulsed, 3-cm Doppler radar. J. Atmos. Oceanic Tech- parameters. Wea. Forecasting, 18, 530–535. nol., 14, 1502–1512. ——, and D. O. Blanchard, 1998: A baseline climatology of ——, Y. Richardson, C. Alexander, S. Weygandt, and P. F. Zhang, sounding-derived supercell and tornado forecast parameters. 2007: Dual-Doppler and single-Doppler analysis of a tornadic Wea. Forecasting, 13, 1148–1164. storm undergoing mergers and repeated tornadogenesis. Mon. Rotunno, R., 1993: Supercell thunderstorm modeling and theory. Wea. Rev., 135, 736–758. The Tornado: Its Structure, Dynamics, Prediction, and Haz- ——, K. Kosiba, P. Markowski, Y. Richardson, D. Dowell, and ards, Geophys. Monogr., Vol. 79, Amer. Geophys. Union, P. Robinson, 2010: Finescale single- and dual-Doppler anal- 57–74. ysis of tornado intensification, maintenance, and dissipation Rutledge, S. A., and P. V. Hobbs, 1984: The mesoscale and mi- in the Orleans, Nebraska, supercell. Mon. Wea. Rev., 138, croscale structure and organization of clouds and precipitation 4439–4455.

Unauthenticated | Downloaded 10/04/21 04:32 AM UTC