Composite VORTEX2 Supercell Environments from Near-Storm Soundings

508 MONTHLY WEATHER REVIEW VOLUME 142

MATTHEW D. PARKER Department of Marine, Earth, and Atmospheric Sciences, North Carolina State University, Raleigh, North Carolina

(Manuscript received 23 May 2013, in ﬁnal form 29 August 2013)

ABSTRACT

Three-dimensional composite analyses using 134 soundings from the second Verification of the Origins of Rotation in Tornadoes Experiment (VORTEX2) reveal the nature of near-storm variability in the environments of supercell thunderstorms. Based upon the full analysis, it appears that vertical wind shear increases as one approaches a supercell within the inflow sector, providing favorable conditions for supercell maintenance (and possibly tornado formation) despite small amounts of low-level cooling near the storm. The seven analyzed tornadic supercells have a composite environment that is clearly more impressive (in terms of widely used metrics) than that of the five analyzed nontornadic supercells, including more convective available potential energy (CAPE), more vertical wind shear, higher boundary layer relative humidity, and lower tropospheric horizontal vorticity that is more streamwise in the near-storm inflow. The widely used supercell composite parameter (SCP) and significant tornado parameter (STP) summarize these differences well. Comparison of composite environments from early versus late in supercells’ lifetimes reveals only subtle signs of storm-induced environmental modification, but potentially important changes associated with the evening transition toward a cooler and moister boundary layer with enhanced low-level vertical shear. Finally, although this study focused primarily on the composite inflow environment, it is intriguing that the outflows sampled by VORTEX2 soundings were surprisingly shallow (generally #500 m deep) and retained consid- 2 erable CAPE (generally $1000 J kg 1). The numerous VORTEX2 near-storm soundings provide an un- precedented observational view of supercell–environment interactions, and the analyses are ripe for use in a variety of future studies.

1. Introduction ‘‘relationships between supercell storms and their environments’’.1 Supercell thunderstorms have considerable societal What few prior observations we have suggest that impact through their propensity to produce tornadoes as there is likely a large degree of spatial and temporal well as significant severe hail, winds, and heavy pre- variability associated with the ‘‘environment’’ near su- cipitation. Based upon a wide variety of studies, it has percells. For example, Markowski et al. (1998b) used a increasingly become clear that the lower-tropospheric network of soundings from the first VORTEX field cam- profiles of temperature, humidity, and winds are im- paign (in 1994–95) to show that storm-relative helicity portant to supercells’ formation, maintenance, and tor- varies regionally (lengths ;100 km and intervals ,3h)on nado production [e.g., the climatologies of Rasmussen many tornado outbreak days, especially in the vicinity of and Blanchard (1998), Markowski et al. (2003), and preexisting mesoscale boundaries. This is troublesome Thompson et al. (2003, 2012)]. Accurate characteriza- given that many supercell process studies use numerical tion of these important fields is challenging because models with homogeneous initial conditions represented actual measurements are rarely made near active su- by a single preconvective sounding [e.g., as reviewed by percells, particularly above the surface. Toward this end, Letkewicz et al. (2013)]. In addition, even within such one of the key objectives of the second Verification of idealized models, substantial near-storm environmental the Origins of Rotation in Tornadoes Experiment modifications may be attributable to local, storm-induced (VORTEX2; Wurman et al. 2012) was to understand

Corresponding author address: Matthew D. Parker, North Car- 1 The quoted text appeared in the VORTEX2 Scientiﬁc Program olina State University, Campus Box 8208, Raleigh, NC 27615-8208. Overview that was submitted to the National Science Foundation E-mail: [email protected] in 2006.

DOI: 10.1175/MWR-D-13-00167.1

Ó 2014 American Meteorological Society Unauthenticated | Downloaded 09/28/21 02:18 AM UTC FEBRUARY 2014 P A R K E R 509 perturbations. For example, Brooks et al. (1994) used a tornado-producing storms, so no comparison between simulation to demonstrate that near-storm values of CAPE tornadic and nontornadic storms was possible. and helicity might vary by as much as a factor of 2 across The aim of the present work is to substantiate the spans of ,10 km. The true nature of such supercell-induced spatial and temporal patterns of environmental vari- near-storm variability (especially that linked to temper- ability and storm-induced modification through analysis ature and humidity changes above the surface) has not of numerous sets of contemporaneous near-supercell yet been fully constrained by observations. soundings from VORTEX2. Further details about the This observational gap exists largely because it is quite VORTEX2 sounding attributes and data processing are rare for multiple near-storm upper air soundings to be reviewed in section 2, after which the resulting com- launched simultaneously from different storm-relative posites are presented and interpreted in section 3. The positions. As reviewed by Potvin et al. (2010), many paper concludes with some ideas for extending this work historical studies of storm environments have selected and a summary in section 4. one proximity sounding for each case, and then performed statistical analysis of those soundings without concern for each sounding’s unique distance from the 2. Methods storm or time separation from key events during the a. VORTEX2 sounding operations and storm’s lifetime (e.g., tornado formation). Such a sim- characteristics plification is the understandable result of the rather sparse operational sounding network (standard sound- Supercell sampling during VORTEX2 was unique ings are made only at 0000 and 1200 UTC at roughly 75 (compared to the studies reviewed in section 1) because locations in the contiguous United States). four nearly synchronous sounding measurements were Analysis soundings from models such as the Rapid regularly made from the near inflow (;30–40 km from Update Cycle (RUC; Benjamin et al. 2004) have been the storm’s updraft), distant inflow (;70–100 km from invaluable in filling these routine observational gaps, the storm’s updraft), and forward and rear flanks of and have been the basis for establishing a number of active supercells (e.g., Fig. 1; see also Fig. 12 of Wurman prominent climatologies for convective storm ingredients et al. 2012). The sounding units were fully mobile, and (e.g., Markowski et al. 2003; Thompson et al. 2003, 2007). the sampling strategy was ‘‘storm-following’’ rather than As useful as these analysis soundings have proven, they tethered to particular locations. The pattern of four are probably not reliable for assessing near-storm vari- contemporaneous soundings was repeated at an ;45– ability, which would rely heavily on the model’s pa- 60-min interval for actively targeted storms. This basic rameterized representation of the storms. Furthermore, near-storm sampling approach was undertaken for more Coniglio (2012) compared RUC analyses and 1-h forecasts than 20 supercell cases during VORTEX2 (see, e.g., to preconvective soundings from VORTEX2 and found Wurman et al. 2012, their Fig. 3 and Table 3). substantial model errors (‘‘large relative to their potential All of the VORTEX2 mobile soundings were made impact on convective evolution’’) even at the analysis time. with Vaisala RS92 radiosondes. Before launching, each Potvin et al. (2010) used what is probably the most sonde’s measurements of temperature, humidity, pres- elegant approach to assessing near-storm variability with sure, and GPS location were checked against portable conventional observations, combining approximately instruments at the launch site. After the field campaign, 1200 proximity soundings from the vicinity of significant all of the soundings were quality-controlled by the Na- [enhanced Fujita scale 2 (EF2) or stronger] tornadoes tional Center for Atmospheric Research (NCAR) Earth and binning them as a function of distance and time from Observing Laboratory (EOL); the details of these quality the storms. From this, they found that soundings close to control procedures are explained in a ‘‘readme’’ docu- tornadic storms (less than 1 h and less than 40-km sep- ment that is available from the EOL VORTEX2 data aration) on average exhibited parameters less favorable archive (http://data.eol.ucar.edu/master_list/?project5 for tornadoes than those from somewhat farther away. VORTEX2; the documentation is also available from They inferred that the soundings from close to the storm the author), and they largely mirror those reported by were unrepresentative of the far-field environment ow- Loehrer et al. (1996, 1998). The author performed addi- ing to what they deemed to be ‘‘convective feedbacks.’’ tional subjective quality assessment by reviewing skew However, Potvin et al. (2010) were still limited by having T–logp diagrams and sonde trajectories for each sounding only one sounding per storm, and they did not attempt in the composite. Soundings that encountered other nearby to account for differences in azimuth (only distance) storms or had unexplained erratic profiles were discarded. from the storms in their dataset. In addition, the To represent near-storm variability as cleanly as pos- Potvin et al. (2010) dataset included only significant sible, all soundings in the analysis were from periods of

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FIG. 1. Plan view of averaged base scan radar reﬂectivity (shaded), storm-relative sounding launch points (circles), and storm-relative sounding trajectories for the 12 supercells included in the present analyses. All axis labels are distances (km) from the supercell’s updraft position. In this ﬁgure (only) the x and y coordinate axes have not been aligned with respect to the deep-layer vertical wind shear (which varies among cases).

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TABLE 1. Summary of the cases and soundings used in this study. Only the number of quality soundings made during storm maturity (i.e., only those used for the actual composites) are reported in the fourth column; no preconvective or postdemise soundings are included. The case date in the left-hand column is the calendar date in LST, whereas the sounding times in the right-hand column are launch times in UTC (operations often continued into the next day UTC). (The VORTEX2 ﬁeld catalog mentioned in the footnotes is available online at http://catalog.eol.ucar.edu/vortex2_2010/.)

Supercell Location Tornadic? No. of soundings Sounding times 6/5/2009 Southeast WY/NE Panhandle Yes 16 2143–0105 UTC 6/6/2009 Central NE No 8 2213–2345 UTC 6/11/2009 Southeast CO No 13 2329–0216 UTC 5/12/2010 Western OK Yes 14 2230–0139 UTC 5/17/2010 Southeast NM No 10 2044–2315 UTC 5/18/2010 TX Panhandle Yes 18 2152–0147 UTC 5/19/2010 Western/central OK Yes 13 2057–0039 UTC 5/25/2010 Western KS Yes 8 2326–0110 UTC 5/26/2010 Eastern CO No* 12 2217–0021 UTC 6/7/2010 NE Panhandle Yes 7 2305–0040 UTC 6/10/2010: S1 Eastern CO No** 7 2342–0042 UTC 6/10/2010: S2 Eastern CO Yes 8 0136–0230 UTC

* The 26 May 2010 supercell was tornadic prior to the armada’s arrival (with the latest tornado reported around 2110 UTC) but did not produce any tornadoes during the active sampling period (the first near-storm sounding was launched at 2217 UTC). This storm was placed in the nontornadic category because it was described in the mission scientist’s summary report as a ‘‘high-based storm... [that] failed to have significant cloud base rotation during the data collection period’’ (the quoted text is from the VORTEX2 2010 field catalog). The main conclusions of the study did not change when the composites were remade with this case in the tornadic category. ** Two distinct supercells were sampled on 10 June 2010 (here labeled simply S1 and S2), the first of which was nontornadic and the second of which was tornadic. Even though there was a tornadic supercell later on within the same general region, the first storm was placed in the nontornadic category because rather large gradients in environmental parameters were observed on this day (Y. Richardson 2013, personal communication), and because mission summaries described the first storm as having ‘‘little low-level rotation’’ and ‘‘never [having] appeared to pose a significant tornado threat’’ (the quoted text is from the VORTEX2 2010 field catalog). active VORTEX2 supercell sampling (not from before Upon close inspection, three of these days were deemed storm initiation or after storm demise). Given the nomi- to be unsuitable for inclusion in a composite analysis 2 nal ascent rate of roughly 5 m s 1 for soundings during (e.g., because of widespread intervening convection or VORTEX2, sondes could travel horizontal distances sparse sampling on a poor road network). This left greater than 50 km during the time that it took them to 11 days, containing 12 well-sampled supercells. After ascend to the tropopause (e.g., Fig. 1). Therefore, whereas having removed any dubious soundings (section 2a), for most past studies have treated soundings as if they were the final analysis an individual storm was required to measurements of the local vertical column, the present have at least six remaining nearby soundings so that study exploited the true storm-relative positions of each 1-s meaningful perturbations from an averaged base state measurement from each sounding (this process is explained could be computed. These selection and quality control in section 2c). Using these true storm-relative positions in steps yielded a total of 134 near-supercell soundings. As the analysis adds considerable realism to the horizontal summarized in Table 1, of the final 12 supercells, 7 were structures of the final fields, much as demonstrated for tornadic (with a total of 84 usable soundings) and 5 were a VORTEX2 squall line by Bryan and Parker (2010). nontornadic (with a total of 50 usable soundings). The preponderance of tornadic soundings is attributable to b. Selection of cases for the composites the fact that such storms tended to be sampled for longer From among all VORTEX2 cases, there were 14 days periods of time during VORTEX2. with at least eight near-storm soundings (i.e., two com- plete launches from the set of four soundings units) c. Use of adjusted spatial coordinates 2 made near an active, VORTEX2-targeted supercell. To account for differences in surface elevation among soundings (both on a given day, and from case to case), the vertical coordinate transformation of Gal-Chen and 2 Notably, this threshold eliminated several impressive VORTEX2 Somerville (1975) was used: cases from consideration (e.g., 9 June 2009, 10 May 2010, and 13 June 2010), but a faithful mapping of near-storm perturbations relies on z 2 z 5 sfc having a representative mean base state and multiple sounding z* H 2 , (1) times from each sector of a mature storm. H zsfc

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FIG. 2. Trajectories for the 134 soundings used in this study, projected onto three Cartesian planes; (lower left) x*–y* plan view plot; (upper left) x*–z* cross-sectional plot; (lower right) y*–z* cross-sectional plot. As explained in the text, all of the positions are storm relative (with each storm updraft position relocated to x* 5 0 km, y* 5 0 km) and all of the trajectories have been rotated to align the x* axis with each reference sounding’s 0–6-km bulk vertical shear vector. The averaged base scan radar reflectivity is shaded on the plan view chart, using one base scan image for each sounding (recentered and rotated into x*, y* space just as the soundings are). wherein all heights on the right-hand side are above common horizontal coordinate system (x*, y*) as fol- mean sea level (MSL). For this study, H was taken to be lows. Updraft-mesocyclone locations were recorded for 12 km (roughly the MSL height of the tropopause) and all target storms based upon manual tracking of the zsfc was the altitude at which the particular sounding was (bounded) weak echo region, hook echo, and rotational launched. This coordinate has the property of being very velocity signatures in every Weather Surveillance Radar- nearly equal to height above ground level (AGL) ap- 1988 Doppler (WSR-88D) level-II volume scan from proaching the ground, and very nearly equal to height during active VORTEX2 sampling. The updraft- MSL approaching altitude H. Such an approach is sim- mesocyclone locations were used to compute storm- ilar to that endorsed by Trier et al. (2000), who noted relative positions for each 1-s sounding record, with (x 5 0, that the transformation accounts for wide variations in y 5 0) for the updraft position at each time. Finally, each surface elevation while approximately preserving in- individual sounding’s positions and wind vectors were ro- tegrated quantities such as convective available poten- tated about the updraft location so that the final x* co- tial energy (CAPE) and convective inhibition (CIN). ordinate axis was aligned with the 0–6-km vector wind The main reason for using a height-based (instead of difference3 from that storm’s ‘‘reference sounding’’ (an pressure based) terrain-following coordinate was that each average of distant inflow soundings, as explained in the sonde’s GPS altitude at launch was checked against a more reliable instrument than was the pressure. Also, use of z* allows for a cleaner separation of terrain elevation effects 3 The 0–6-km vector wind difference (sometimes called the 0–6-km from synoptic and mesoscale pressure fluctuations. bulk vertical shear vector) was chosen from among several alternatives To account for different storm orientations on dif- simply because it most closely aligned the radar reflectivity fields and ferent days, all soundings were then converted into a sonde trajectories (e.g., in the combined view shown in Fig. 2).

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FIG. 3. Theoretical response function for the vertical and horizontal Barnes analyses. For the vertical analysis, the response refers to the vertical wavelengths shown on the bottom axis (m). For the horizontal analysis, the response curve refers to the horizontal wavelengths shown on the top axis (km). The red, green, and blue lines highlight the wavelengths at which the theoretical response is 0.9, 0.5, and 0.1 respectively. next subsection). The contemporaneous WSR-88D base r2 (x, y, z) w x y z 5 2 m scan data from each sounding launch were also con- m( , , ) exp k , (3) verted into (x*, y*) coordinates to produce corresponding reflectivity imagery for reference. The effects of wherein rm(x, y, z) is the distance between the location reorienting the sounding and radar data can be surmised of datum m and the analysis grid point (x, y, z), and k is by comparing the individual cases in Fig. 1 versus the a parameter defining the filtering scale of the analysis combined population of 134 soundings in Fig. 2. scheme. Inspection of (3) reveals that the choice of k defines a ‘‘radius of influence’’ R at which wm has fallen 2 pffiffiffi d. Objective analysis off to a value of e 1: R 5 k. For the VORTEX2 soundings, the vertical and hori- The soundings were interpolated to a common grid zontal data spacings differed greatly, such that a singular using the Barnes (1973) analysis technique [following value of R (i.e., an isotropic analysis) was not useful. the formalism of Koch et al. (1983)]. Traditionally, the Instead, distinct vertical and horizontal influence radii analyzed values of some variable c(x, y, z) are calculated (Ry and R ) were needed. Pauley and Wu (1990) rec- as a weighted sum of the M total input data via h ommended that R be set to roughly 4/3Dn, where Dn is M the data spacing. Because the nominal rate of ascent for å c 21 mwm(x, y, z) the VORTEX2 soundings was roughly 5 m s , the 5 c(x, y, z) 5 m 1 , (2) vertical data spacing of the 1-s records was generally M å close to 5 m. It is less straightforward to define the hor- wm(x, y, z) m51 izontal data spacing because the soundings were un- evenly spaced (e.g., Figs. 1 and 2). A fairly intuitive c D wherein each m is a single datum (here, a 1-s sounding approximation is the ‘‘random data spacing’’ nr sug- measurement), and wm is that datum’s corresponding gested by Koch et al. [1983, their Eq. (12)], which rep- weighting. In turn, the weighting function is given by resents ‘‘what the average data spacing would be inside

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FIG. 4. Plan view (x*–y*) plots for all 134 supercell soundings, valid at (a) the surface (z* 5 0 km), (b) z* 5 1 km, 5 5 u0 (c) z* 3 km, and (d) z* 6 km. The perturbation equivalent potential temperature e is shaded (K), with the base state value of ue for each level reported in the titles. Perturbation wind vectors are plotted using the scale vectors as u0 shown (all vectors are rotated into the x*–y* coordinates). The weighted standard deviation in e (K) at each grid cell is contoured in white (the calculation is explained in a footnote to the main text). All axis labels are distances (km) from the supercell updraft position. The 30- and 50-dBZ radar reflectivity levels from Fig. 2 are shown as thick white contours for reference. a square data area A if M observations were uniformly were subjectively determined (through many iterations) distributed across the area.’’ For the entire set of 134 to be those that provided the highest effective resolution soundings, the computed horizontal Dnr 5 11.8 km. without producing obvious localized maxima or minima Following the recommendations of Pauley and Wu due to individual sounding data records. To prevent ex- (1990), this suggests that the data (if well behaved) could trapolation across large data gaps (and outside of the tolerate Barnes analysis settings of Ry ’ 7 m and Rh ’ edges of the well-sampled area), the final values for both 16 km. Pauley and Wu (1990) noted that, in practice, the vertical and horizontal analyses were masked at any larger radii than this are acceptable, but will be associ- grid point where there were fewer than two input data ated with ‘‘an analysis which will appear too smooth’’ for within a distance of R, or fewer than five input data within many applications. However, in the present study a a distance of 2.5R. Details unique to the vertical and moderate amount of smoothing was ultimately necessary horizontal analysis procedures are reviewed in the up- to seamlessly combine each unique case and sounding coming subsections. 4 trajectory. The final values of Ry 5 50 m and Rh 5 20 km 1) VERTICAL ANALYSIS As a first step, individual sounding data were analyzed 4 Many studies also use a second ‘‘correction pass’’ of the Barnes to common z* levels with a vertical spacing of 50 m using scheme to refine the analysis values (e.g., Koch et al. 1983). In the present study it was found (through trial and error) that there was a vertical Barnes procedure as described above. The no obvious benefit to a second pass given the somewhat high degree top of the vertical grid was z* 5 12 km, because most of smoothness required to blend the different cases. VORTEX2 soundings were cut off at or below that

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FIG. 5. Plan view (x*–y*) plots for all 134 supercell soundings: (a) number of soundings within the 20-km radius of inﬂuence at the 2 2 surface level (shaded); (b) CAPE of the surface parcel (shaded, J kg 1), CIN of the surface parcel (contoured, J kg 1), and storm-relative 2 2 surface wind vectors (m s 1, scaled as shown); (c) 0–3-km storm-relative helicity (m2 s 2, shaded), magnitude of the 0–6-km bulk wind 2 2 difference (m s 1, contoured), and perturbation surface wind vectors (m s 1, scaled as shown); and (d) height of the LCL of the surface 2 2 parcel (m, shaded), 0–1-km storm-relative helicity (m2 s 2, contoured), and 0–1-km vector wind difference (m s 1, scaled as shown). All vectors are rotated into the x*–y* coordinates. All vertical integrals and differences are calculated in a 1D column at the individual x*, y* grid point. All axis labels are distances (km) from the supercell updraft position. The 30- and 50-dBZ radar reﬂectivity levels from Fig. 2 are shown as thick black contours for reference.

5 altitude. The 50-m radius of influence Ry is large relative averaging all available ‘‘distant inflow’’ soundings from to the original data spacing (as reviewed above) but was that case. Every sounding was then converted into per- necessary to filter out noisiness in the data associated turbations in potential temperature u, mixing ratio qy, with swinging of the sonde (after launch and in regions and wind components by subtracting the individual of turbulence). The theoretical Barnes response in sounding’s values from the reference sounding for that height (Fig. 3, using the bottom x axis) reveals that supercell. The preceding vertical analysis procedures vertical wavelengths below roughly 100 m should be strongly muted in the analysis. The surface (z* 5 0m) point was treated uniquely by halving Ry, which yields a more localized analysis in order to compensate for the 5 An average of all ‘‘distant inflow’’ soundings was chosen over fact that the interpolation at the surface point is one- a single preconvective sounding so that the diurnal cycle in temperature would not dominate the calculated perturbations in longer- sided (since there are no data below the ground). After lived storms. Interested readers can look ahead to Fig. 13 to see the completing the vertical analysis for each sounding, diurnal distribution of the soundings used to calculate the reference a reference sounding for each supercell was created by state for each case.

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the ﬁnal blended composite perturbation ﬁelds were com- pleted, the mean base state sounding (a straight average of the reference soundings from each supercell) was then added back as needed to compute total values of variables.

3) ADDITIONAL CALCULATIONS Quantities that involve either vertical integration (such as CAPE, CIN, and storm-relative helicity) or vertical derivatives (such as vertical wind shear) were computed using the final analysis fields in 1D local columns at each grid point. For each column the CAPE integration was stopped at 12 km (the top of the gridded analysis) even if the lifted parcel had some remaining positive buoyancy. Vertical velocities were computed kinematically6 by FIG. 6. Plan view (x*–y*) plot of surface virtual temperature 0 integrating the anelastic continuity equation. Therefore, perturbation Ty (shaded, K) and storm-relative surface wind vec- 2 tors (m s 1, scaled as shown) for all 134 supercell soundings. The because of the smoothness of the analyzed horizontal locations of the gridpoint profiles in Figs. 7 and 9 are indicated with wind fields, the vertical velocities are characteristically 2 black letters (D 5 distant inflow, N 5 near inflow, F 5 forward mesoscale (on the order of 1 m s 1), not convective (on 5 2 flank, R rear flank) and the positions of the two cross sections in the order of 10 m s 1), in magnitude. Fig. 8 are indicated with dashed white line segments. Vectors are rotated into the x*–y* coordinates. All axis labels are distances (km) from the supercell updraft position. The 30- and 50-dBZ radar reflectivity levels from Fig. 2 are shown as thick black contours 3. Results for reference. a. Full supercell composite were applied to each sounding individually prior to the 1) GENERAL CHARACTER OF THE COMPOSITE horizontal analysis. The end-product of the procedures outlined in section 2 2) HORIZONTAL ANALYSIS is a detailed picture of the environment of VORTEX2 After each sounding had been converted into per- supercells; the perturbation equivalent potential tem- u0 turbation form in the x*, y*, z* coordinate system, the perature e field serves to highlight the general character final 2D composite perturbation fields at each level of the analysis (Fig. 4). The choice of Rh leads to a (every 50 m in z*) were created using a horizontal somewhat smooth analysis, but it is clear from the spatial Barnes procedure as described above. The horizontal distribution of soundings (e.g., Fig. 2) that there is a grid had a spacing of 5 km in x* and y*. The 20-km radius great deal of information available for the analysis of influence R closely approximates 4/3Dn , as reviewed within the inflow sector (to the southeast of the storm) at h r ; above, and the associated theoretical Barnes response ranges of less than 75 km. This corresponds with the 7 (Fig. 3, using the top x axis) reveals that horizontal zone where the standard deviations in the variables u0 wavelengths below roughly 40 km should be strongly (exemplified for e in Fig. 4, contours) are typically the muted in the analysis. During an earlier version of the composite analysis it was discovered that the present value of Rh , while suitable for combining and smoothing 6 Although sondes’ GPS rates of ascent were also analyzed, it is the different cases, caused low-level perturbations from unclear how to account for the variations among soundings that the outflow sector soundings to bleed into the inflow occur solely due to the differing amounts of helium used in each sector of the composite supercell. To overcome this balloon. Intense convective updrafts and downdrafts were obvious problem, the analysis grid points from the inflow sector within the GPS vertical velocity data, but there is low confidence . , (and visible noise) in the much weaker vertical velocities found (defined as x* 0 km and y* 0 km) were overwritten outside of storms. Launch-by-launch helium usage was not pre- by values from an analysis in which only inflow sector cisely documented during VORTEX2, so no simple correction sondes were used (but all other settings were identical). could be applied. The full analysis and inflow sector analysis were then 7 The standard deviation is typically computed using the simple blended together by use of a horizontal, nine-point average of squared deviations from the mean (and then taking the square root of this average). In the present study, given the uneven smoother. This procedure resulted in sharper gradients distribution of nearby soundings, the standard deviation is computed near the outflow boundaries, with minimal contamina- using a weighted average of squared deviations from the mean, with tion of the inflow sector by the outflow soundings. Once weights determined from the horizontal Barnes analysis.

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FIG. 7. (left) Skew T–logp thermodynamic diagram and (right) hodograph for gridpoint profiles at the distant inflow (‘‘D’’ in Fig. 6, here colored red) and near inflow (‘‘N’’ in Fig. 6, here colored blue) locations within the full analysis for all 134 supercell soundings. On the hodographs, the surface, 1-, 3-, and 6-km data points are denoted with symbols as shown, with the observed mean storm motion plotted with ‘‘M’’ (the storm motion is the same for both hodographs). For reference, the vertical wind profile associated with the near inflow hodograph is plotted atop the 2 2 2 skew-T diagram (half barb 5 2.5 m s 1, barb 5 5ms 1, flag 5 25 m s 1). smallest; in other words, there is strong similarity among a supercell. Notably, however, the vertical vorticity cases within the inflow sector. present in the analysis is an order of magnitude smaller The standard deviations become larger toward the than what is typically observed in radar studies of su- northern and western edges of the analysis (Fig. 4) be- percells (due to the diffusiveness of the analysis and the cause of both the much wider variation among cases in fact that few soundings actually directly sampled the these areas and the smaller number of soundings con- updraft and mesocyclone). straining the analysis there (e.g., Fig. 2). In particular, it 2) DISCUSSION OF SPECIFIC INGREDIENTS is important to remember that any background mesoscale variability in the larger-scale environment (i.e., not Because it is not possible to show every variable on what is induced by the storm itself) will also appear in every level, the remainder of the article emphasizes the analysis; in other words, the largest standard de- a number of common parameters derived from the raw u0 viations associated with the negative e in the domain’s analysis data (e.g., CAPE, CIN, measures of vertical northwest corner are at least partly attributable to the wind shear; Fig. 5). The prevailing distant inflow envi- existence of various synoptic and/or mesoscale bound- ronment for the 12-supercell composite has substantial 2 aries in some of the cases. Even so, a number of realistic CAPE and modest CIN (roughly 2150 and 225 J kg 1, near-storm features clearly appear. respectively; Fig. 5b). As one follows the storm-relative u0 As one would expect, negative e values occur in the surface winds toward the updraft from the southeast outflow sectors at the surface and z* 5 1 km (Figs. 4a,b) (Fig. 5b), a slight increase in CIN occurs (toward values u0 2 21 whereas in the midlevels a maximum in e appears in the of roughly 65 J kg ). As shown by composite surface general location of the updraft (Fig. 4c) and tends to- virtual temperature perturbations (Fig. 6), parcels ward the downshear side of the updraft farther aloft flowing toward the storm in the inflow sector would (Fig. 4d). Wind perturbations at the surface (Fig. 4a) are characteristically cool by roughly 0.5–1.0 K on their way in line with what would be expected in the outflow sec- to the updraft. However, this low-level cooling is weak tors, and become increasingly rotational in the vicinity enough and shallow enough so as to almost escape de- of the updraft aloft (Figs. 4b–d), as one might expect for tection in composite gridpoint soundings (Fig. 7) and

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0 FIG. 8. Vertical cross sections of virtual temperature perturbation Ty (shaded, K), equivalent potential tempera- 21 ture ue (contoured, K), and storm-relative winds lying in the plane of the cross section (m s , scaled as shown). The cross sections extend from the inflow sector through the (a) rear and (b) forward flank outflow boundary. The orientations of the cross sections are shown in Fig. 6. The vertical velocity component was computed kinematically (as explained in section 2e) and is here multiplied by 40 so that it is visible (i.e., the scale vector length is equivalent to 2 1ms 1 in the vertical). vertical cross sections (Fig. 8). Associated with the effect was noted in the supercell simulation of Ziegler shallow cooling, there are also small decreases in CAPE et al. (2010), who attributed an inflow layer of strato- 2 (from roughly 2150 to 2100 J kg 1, not crossing a shading cumuli to the lifting that resulted from flow blocking interval in Fig. 5b) and lifting condensation level (LCL) by the storm’s cold pool. Very few other effects of the height (from roughly 1050 to 900 m; Fig. 5d) for the in- nearby storm are evident in the vertical temperature flow surface parcels. profile, although there is a signal of moistening and The location of this very shallow cooling in the inflow slight warming throughout the middle and upper tro- sector could be attributable to shading by either the posphere (Fig. 7) that is most likely attributable to net parent storm or inflow clouds. Although there are no condensation in and near the targeted supercell. direct measurements of cloud cover available within The prevailing deep-layer vertical wind shear [repre- the composite dataset to corroborate this, anvil shading sented by the 0–6-km bulk wind difference (BWD)] is effects have been previously shown to influence near- rather uniform in space (Fig. 5c; also see the hodograph storm inflow air (e.g., Markowski et al. 1998a; Frame and in Fig. 7), and it is everywhere above the baseline of 2 Markowski 2010), and the depth of influence can be 18–20 m s 1 that is commonly thought to be necessary quite shallow (e.g., Bryan and Parker 2010; their Fig. 8). for supercell organization (e.g., Rasmussen and Blanchard Alternatively, the ascending motion and upward-sloping 1998; Thompson et al. 2003). Interestingly, however, the ue surfaces shown in Fig. 8a (for x* . 0 km) and Fig. 8b lowest 1 km of the near inflow wind profile (hodograph) (for y* . 0 km), as well as the local enhancement in ue is dramatically different from the far field (Fig. 7), seen at z* 5 1 km in Fig. 4b, are at least consistent with showing substantial backing and increases in speed. This the notion that inflow cloudiness (i.e., ‘‘feeder bands’’) is almost certainly an effect of the supercell upon its en- could have developed in some of the cases. Such an vironment via lowered near-surface pressures (associated

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FIG. 9. As in Fig. 7, except for the gridpoint profiles at the distant inflow (‘‘D’’ in Fig. 6, here colored red), rear flank (‘‘R’’ in Fig. 6, here colored blue), and forward flank (‘‘F’’ in Fig. 6, here colored green) locations within the full analysis for all 134 supercell soundings. The plotted storm motion is the same for all three hodographs. The reference vertical wind profile is for the rear flank hodograph.

with latent heat release and dynamic pressure effects, e.g., An increasing body of work suggests that outflows are Klemp and Rotunno 1983; Weisman and Klemp 1984). important sources of horizontal vorticity (which can be The net effect is a dramatic increase in the 0–1- and subsequently reoriented into vertical vorticity) for both 0–3-km storm-relative helicity (SRH) values compared to supercells and tornadoes. Although these outflows have the far field (Figs. 5c,d), an effect that has also been re- been thoroughly sampled at the surface by mobile in- ported in high-resolution supercell simulations (e.g., struments (e.g., Markowski et al. 2002; Grzych et al. Fig. 7 of Brooks et al. 1994). In addition, although the 2007; Wurman et al. 2012), observations aloft in these 0–1-km BWD displayed on the hodograph in Fig. 7 is parts of storms have generally been lacking. As might be almost the same for the near and distant inflow profiles, expected, CAPE decreases and the magnitude of CIN Fig. 5d shows a general tendency for increasing length of increases in the outflow sectors to the north and west of the 0–1-km bulk vertical shear vectors as one approaches the composite storm (Fig. 5b), although it is noteworthy the updraft from the southeast. that the surface parcels within the outflow still possess 2 In summary, the prevailing far-field environment of nonzero CAPE (exceeding 1000 J kg 1 over a broad the composite VORTEX2 supercell already is favorable area). This is important because supercell updrafts reg- for the development and maintenance of supercell ularly reingest evaporatively cooled outflow air (this thunderstorms (large CAPE, 0–6-km BWD, and 0–3-km explains the existence of wall clouds, for example; SRH; e.g., Rasmussen and Blanchard 1998; Thompson Rotunno and Klemp 1985). There is also some evidence et al. 2003), and it also has ingredients that are widely that the potential buoyancy (i.e., CAPE) of this outflow regarded as indicating enhanced tornado probabilities air may be correlated to the likelihood of tornadogenesis (rather low LCL heights and substantial 0–1-km SRH (e.g., Markowski et al. 2002; Grzych et al. 2007). and 0–1-km BWD; e.g., Markowski et al. 2002, 2003; Both vertical cross sections (Fig. 8) and point sound- Thompson et al. 2003). Contrasts between the tornadic ings (Fig. 9) in the composite’s outflow sectors reveal and nontornadic storms are presented in section 3b. The that the layer of low-level cooling is quite shallow (500 m most obvious difference between the distant and near- deep or less), with temperature deficits of only 1–3 K inflow environments was a dramatic increase in lower relative to the ambient environment (at least until one is tropospheric vertical wind shear. well to the northwest of the storm; Figs. 6 and 8a). In

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FIG. 10. As in Figs. 5a and 5b, but for the (a),(b) 84 tornadic supercell soundings and (c),(d) 50 nontornadic soundings. contrast, many classically configured model simulations Notwithstanding the preceding caveats, the VORTEX2 of supercells produce outflows .1 km deep with ;10-K supercell cold pools appeared to be surprisingly shallow surface temperature deficits8 [e.g., as demonstrated by and weak in the particular locations where outflow Morrison and Milbrandt (2011)]. It is likely that the soundings were launched. composite underestimates what occurs in nature be- A final interesting point about the composite super- cause very few VORTEX2 soundings were actually cell’s outflow sectors is that the vertical wind profiles launched in the coldest parts of the cold pools (because (hodographs) there retain roughly the same shape as the of the hindrance of strong downdrafts and winds, heavy prevailing inflow environment (Fig. 9), such that the precipitation, and lightning). Such sampling issues are values of 0–1-km BWD, 0–1-km SRH, and 0–3-km SRH a limitation given that single-storm measurements of are not substantially different from those in the inflow supercell cold pools have shown considerable hetero- sector (Figs. 5c,d). The orientation of the bulk shear geneity in outflow temperature (e.g., Markowski et al. vectors in the forward and rear flanks (Figs. 5d and 9) 2002) and depth (e.g., Ziegler 2013). Even so, a number suggests that ambient vortex lines should thread through of other studies have indeed found weak temperature the storm from the environment with relatively consis- deficits in the forward flanks of supercells (Shabbott and tent orientations (again, with the caveat that the input Markowski 2006; Beck and Weiss 2013), and a few fea- soundings were generally not made in the strongest parts tures of the forward and rear flank do seem to be well of the observed storms’ outflows). This is potentially captured in the composite soundings (warming in the important because the way in which vertical vorticity middle troposphere and substantial moistening through- emanates from downdrafts may be quite sensitive to the out most of the middle and upper troposphere; Fig. 9). low-level wind profile in the outflow sector (e.g., Parker and Dahl 2013). b. Tornadic versus nontornadic composites 8 Such strong simulated cold pools may partly reflect un- certainties in the microphysical parameterizations used by many To assess how the near-storm environments varied models (a topic that has been widely addressed of late; e.g., Snook between tornadic and nontornadic supercells, the sound- and Xue 2008; Dawson et al. 2010; Morrison and Milbrandt 2011). ings were divided using the designations of tornadic versus

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FIG. 11. As in Figs. 5c and 5d, but for the (a),(b) 84 tornadic supercell soundings and (c),(d) 50 nontornadic soundings.

nontornadic as reported in Table 1, and the composite In assessing differences in low-level shear, an inter- procedure was then rerun. Because the nontornadic esting contrast is that the distant inflow values of 0–1- sample size was smaller (as discussed in section 2), the and 0–3-km SRH are much lower in the nontornadic overall coverage of useful information was more limited cases, but they increase considerably near the supercell (cf. Figs. 10a,c). Even so, comparisons between the sub- (Fig. 11). In contrast, the values in the tornadic supercell sets, and between the tornadic cases and the full com- composite vary much less across the inflow sector. One posite, are informative. speculative interpretation is that, although the near- Almost uniformly across the board, the far-field en- storm values of vertical wind shear in nontornado cases vironmental parameters were more impressive in the were within 10%–20% of those in the tornadic cases, tornadic composite (Figs. 10 and 11), including higher these values were primarily storm-generated (as they 2 CAPE (roughly 2200 vs 1800 J kg 1, with values of CIN were not present in the distant inflow environment). The that were comparable), higher 0–6-km BWD (roughly local leftward turning of the 0–1-km bulk shear vectors 2 27 vs 24 m s 1), and lower LCL heights (roughly 900 vs in the vicinity of the nontornadic supercell’s forward 1200 m). Gridpoint soundings (Fig. 12) make it apparent flank (Fig. 11d) would be consistent with enhanced that the differences in CAPE and LCL height are almost baroclinic generation of a westward-pointing horizontal entirely attributable to lower values of boundary layer vorticity component associated with comparatively mixing ratio in the nontornadic cases. This is one of the cooler air to the north (unfortunately, the baroclinity in most glaring differences between the VORTEX2 tor- the composites is not sufficiently well resolved to cal- nado and nontornado cases in this study, and it echoes culate a realistic rate). In turn, this enhanced cooling the findings of Markowski et al. (2002) and Thompson would be consistent with the dryer environmental et al. (2003). boundary layer on nontornado days (Fig. 12). Indeed,

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FIG. 12. As in Fig. 7, but for the gridpoint profiles at the distant inflow position (equivalent to ‘‘D’’ in Fig. 6) in the tornadic supercell analysis (here colored green), the near inflow position (equivalent to ‘‘N’’ in Fig. 6) for the tornadic supercell analysis (here colored red), and the near inflow position (again, equivalent to ‘‘N’’ in Fig. 6) for the nontornadic supercell analysis (here colored blue). The mean storm motion for the tornadic storms is plotted in red and for the nontornadic storms is plotted in blue on the hodograph. The reference vertical wind profile is for the nontornadic near inflow hodograph. such enhanced forward flank cooling has been previously by the fact that the tornadic supercells appeared to move found to be anticorrelated with tornado occurrence more strongly to the right of the hodograph9 (Fig. 12), (Shabbott and Markowski 2006). Ultimately, how- which entails stronger low-level storm-relative winds ever, these are the kinds of sensitivities that can only be and thus an increased flux of streamwise vorticity into properly examined with hypothesis tests in a numerical the updrafts of the tornadic storms. Even though the model. direct linkage to tornadoes is not yet fully understood, The curious pairing of similar near-storm 0–3-km it has long been established that increased import of SRH with comparatively lower 0–1 km SRH in the streamwise vorticity enhances the low-level vertical nontornadic cases also points to a rather different ho- vorticity in the updraft (e.g., Davies-Jones 1984). In turn, dograph shape between the two groupings (Fig. 12). The the enhanced low-level vertical vorticity may promote 3-km winds are quite similar between the tornadic and stronger dynamic lifting in the part of the storm where nontornadic storms, however the winds in the lowest tornadogenesis typically occurs (Markowski and 1–2 km are more strongly backed in the nontornadic Richardson 2014). cases (Fig. 12; cf. Figs. 10b,d). As noted above, the near- c. Early-in-life versus late-in-life composites storm 0–1-km bulk shear vector for the nontornadic cases therefore points well to the left of that for the To this point, the composites discussed in section 3 tornadic cases (Fig. 12; cf. Figs. 11b,d). For the tornadic have incorporated soundings from the full span of in- near-inflow point, the 0–1-km storm-relative winds dividual supercells’ lifetimes (e.g., Table 1). To assess are nearly orthogonal to the 0–1-km bulk shear vector how the near-storm environments varied over the course (implying that the horizontal vorticity is primarily of supercell lifetimes, the soundings were divided into streamwise), whereas for the nontornadic near-inflow early-in-life and late-in-life subsets, and the composite point, the 0–1-km mean storm-relative winds are more nearly parallel to the 0–1-km bulk shear vector (implying a substantial crosswise component of vorticity). These 9 It is not readily apparent from the composites what would ex- differences in shear orientation are then compounded plain this more deviant rightward motion in the tornadic cases.

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FIG. 13. Histogram of sounding launch times (shown with respect to time of local sunset) for each storm in this study. The 68 soundings included in the early-in-life subset are plotted in red. The 66 soundings included in the late-in-life subset are plotted in blue. Soundings used to compute the reference sounding for each storm are denoted by a closed circle. All other soundings are denoted by a 3 symbol. For illustrative purposes, the time of the median launch (for all soundings) relative to sunset is plotted with a green line. procedure was then rerun. As shown in Fig. 13, the total Although the 0–6-km BWD is almost identical in the soundings for each day were divided in time as evenly as early and late-in-life composites (cf. Figs. 15a,c), there is possible without subdividing any set of near-simultaneous some subtle veering of the flow above z* 5 6 km (com- launches. This approach yielded 68 early-in-life soundings paring the hodographs in Fig. 16), which may reflect and 66 late-in-life soundings. As described in section 2, increasing upper-level divergence over time from the all of the analyzed soundings were launched during storm to the immediate northwest. More importantly, sampling of an active supercell, so the time separation the winds below roughly 3 km AGL have increased in between the early-in-life and late-in-life groups ranges speed in the late-in-life analysis (Fig. 16). It is possible by case from roughly 1 to 3 h (Fig. 13). that this partly represents an accumulation over time of The differences between the early-in-life and late-in- the near-storm accelerations that were evident in the full life soundings seem to be primarily consistent with the composite’s near-storm profile in Fig. 7. However, given fact that VORTEX2 sampling was undertaken in the the noticeable low-level cooling and stabilization dis- late afternoon and early evening (Table 1 and Fig. 13). cussed above, it is also presumed that the lower tropo- The majority of the early-in-life soundings were launched spheric winds accelerated over time in response to .2 h prior to sunset, and the majority of the late-in-life declining turbulent mixing during the boundary layer’s soundingswerelaunchedwithin2hofsunset.Notsur- evening transition [i.e., the classic Blackadar (1957) prisingly, then, the late-in-life soundings had less CAPE, mechanism, also discussed in the context of supercells by a higher magnitude of CIN, and lower LCL heights (cf. Maddox (1993)]. Although it is impossible to cleanly Figs. 14b,d and 15b,d). Gridpoint soundings (Fig. 16) separate the two effects in the present dataset, the end make it clear that this effect is largely due to low-level results are clear: dramatic increases in 0–1-km and 0– cooling and moistening as the evening transition of the 3-km SRH, along with modest increases in the 0–1-km boundary layer begins (note also the very shallow stable BWD (Fig. 15; see also the hodograph in Fig. 16). layer at the surface in the late-in-life sounding). Direct At the suggestion of a reviewer, the early and late thermodynamic modifications of the near-inflow envi- composites were remade by dividing the soundings evenly ronment by the storm are harder to identify apart from based on time relative to local sunset (i.e., separating them some subtle warming and moistening evident above at the green line drawn in Fig. 13). Conceptually, this is less 400 hPa in the later sounding (Fig. 16). than ideal, because several cases are placed almost entirely

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FIG. 14. As in Figs. 5a and 5b, but for the (a),(b) 68 early-in-life and (c),(d) 66 late-in-life supercell soundings. into one category or the other (meaning that funda- composite parameter (SCP) and the significant tornado mental differences between cases are projected onto the parameter (STP). These parameters10 represent com- purported diurnal signals). Even so, the end results were binations of thermodynamic and vertical wind shear between-group differences with the same signs but even ingredients that either best separate supercells from larger magnitudes than in the early-in-life versus late-in- nonsupercells (in the case of SCP) or best separate life comparisons. This lends confidence that the temporal storms producing significant tornadoes (EF2 or greater) changes described above are predominantly diurnal in from nontornadic supercells (in the case of STP). nature. Because all of the storms in the composite were supercells, it is probably not surprising that the SCP is d. Composites of operational forecasting parameters well above the threshold of 1 for all four of the supercell So far, many contrasts have been drawn between the subsets (contours in Fig. 17). Even so, the values are distant and near inflow locations, between tornadic and highest for the tornadic supercells (and the large values nontornadic supercells, and between the early and late- in-life time periods. Since many of these differences are offsetting in some sense (i.e., unfavorable low-level 10 In the present work, SCP and STP are calculated using the cooling is paired with a favorable increase in vertical Thompson et al. (2004, 2007) updates to the original Thompson wind shear), it is difficult to assess overall whether the et al. (2003) formulas, with the exception that, because the present environmental variations have a net impact on the data extend only to 12 km, the surface–6-km bulk wind difference is questions of supercell maintenance and tornado pro- used in place of the effective bulk wind difference (which approximates the bulk vertical shear over one-half of the storm duction. As a proxy, it can be helpful to refer to the work depth). For the purposes of visualizing spatial variability and sim- of Thompson et al. (2003, 2004, 2007), who introduced ple comparison among various storm subsets (e.g., tornadic versus and refined two skillful operational indices: the supercell nontornadic), the effect of this substitution is negligible.

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FIG. 15. As in Figs. 5c and 5d, but for the (a),(b) 68 early-in-life and (c),(d) 66 late-in-life supercell soundings. span a much greater fraction of the inflow sector). Fit- relative humidity are proportionally greater than the tingly, STP in the distant inflow for the tornado cases increases in stability. This may be a point that has op- is also well above the threshold of 1, whereas STP in erational utility, as it implies that marginal environ- the distant inflow for the nontornado cases is below 1 ments may become more favorable over time even as (Figs. 17a,b). It is interesting that, in all four subset the boundary layer stabilizes. However, given the very composites in Fig. 17, the STP increases noticeably as nonlinear interplay between environmental wind pro- one travels toward the storm. Assuming that the near- files and updraft intensity (e.g., Weisman and Rotunno storm enhancements in VORTEX2 storms are common, 2000), these are again the kinds of sensitivities that are it is possible that the most discriminatory STP threshold best examined with a numerical model. It may well be would actually be a value .1 in the near-inflow region. that the statistical skill of simple multiplicative param- Even so, the substantial differences in distant inflow eters like SCP and STP does not directly correspond to values (roughly 3.1 in the tornado composite versus the actual changes in supercell processes and storm– 0.8 in the nontornadic composite at x* 5 60 km, y* 5 environment interactions. 240 km) are probably the most representative of those used in climatology studies and by operational fore- casters (since the near-storm variability captured in the 4. Conclusions present composites is not routinely sampled). a. Future work Just as there is a dependence of the SCP and STP on distance from the storm, there are also modest increases Section 3 advanced a number of speculative hypoth- in both parameters later in the supercells’ lifetimes eses to explain the measured environmental heteroge- (Figs. 17c,d). Apparently, in both cases (closer proximity neity as well as differences between tornadic versus or later time), increases in vertical wind shear and nontornadic supercells and early versus late-in-life

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FIG. 16. As in Fig. 7, but for the gridpoint profiles at the near inflow position (equivalent to ‘‘N’’ in Fig. 6) for the early-in-life supercell analysis (here colored red) and the late-in-life supercell analysis (here colored blue). The mean storm motions for the two samples are plotted in their corresponding colors. The reference vertical wind profile is for the late-in-life hodograph. supercell environments. Unfortunately, this composite variations in the base state among storm days, down- is only a starting point: true physical attribution is difficult wind drifting of the sondes, and launch-by-launch dif- using a dataset such as this. One could use composite ferences in surface elevation. The dense sounding gridpoint soundings from the present subgroupings coverage in the near-storm environment provides an un- (tornadic vs nontornadic, early-in-life vs late-in-life) to precedented observational view of supercell–environment study sensitivities of simulated storms to the observed interactions. environmental differences. One could also potentially The composite analyses make it clear that the use of use a somewhat coarser analysis (i.e., with larger Rh)as a single sounding to represent the ‘‘environment’’ of a ‘‘synoptic’’ initial condition within which an idealized a case is potentially risky: supercell and tornado in- simulated supercell is triggered [much as Coniglio and gredients (in terms of most widely used metrics) appear Stensrud (2001) did for derechos]. This might be one to improve as one approaches the storm from the distant avenue toward increasingly realistic incorporation of inflow region, and also as the storm’s lifetime proceeds environmental heterogeneity in idealized supercell stud- through the boundary layer transition of late afternoon ies, a trend that has already begun (e.g., Richardson et al. and early evening. In both cases (moving closer to the 2007; Ziegler et al. 2010; Letkewicz et al. 2013). The storm or later in the storm’s lifetime) the trade-off seems long-range goal of such modeling work would be to to be modest low-level cooling that is offset by increases understand the governing physical processes in super- in the vertical wind shear. It is not clear how much cells and tornadoes with due consideration of influences physical similarity is involved in this apparent parallel from the near-storm environment. The value of the pres- between the environmental changes linked to proximity ent VORTEX2 composite supercell environments is in and those linked to time. providing better observational constraints for model initial From the perspective of forecasting indices, the com- conditions and better quality assurance for the subsequent posite environment of the tornadic supercells is clearly model output of such studies. more impressive than that of the nontornadic supercells in the far field, with considerably higher values of b. Summary the operational supercell composite parameter (SCP) This study combined 134 soundings from VORTEX2 and significant tornado parameter (STP). An inter- supercells into a 3D composite analysis, accounting for esting result is that the near-storm environment of the

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FIG. 17. Plan view (x*–y*) plots of the significant tornado parameter (STP, shaded) and supercell composite parameter (SCP, contoured) for (a) the 84 tornadic supercell soundings, (b) the 50 nontornadic supercell soundings, (c) the 68 early-in-life supercell soundings, and (d) the 66 late-in-life supercell soundings. The STP and SCP are computed as explained in the text using a 1D column at the individual x*, y* grid point. All axis labels are distances (km) from the supercell updraft position. The 30- and 50-dBZ radar reflectivity levels from Fig. 2 are shown as thick black contours for reference. nontornadic supercells is substantially enhanced com- VORTEX2 sondes were generally not launched in the pared to the far field (presumably by the storm itself), main downdraft and heavy precipitation zones, these with parameter values approaching those of the tornadic findings are a noteworthy benchmark; almost all his- cases. The observed near-storm backing and lengthen- torical measurements of supercell outflows have been ing of the nontornadic wind profile may ironically be made at the surface only. related to the seemingly unfavorable boundary layer The present results motivate a handful of questions dryness of the nontornadic cases: a speculative inter- about how supercells modify their surroundings, and pretation is that low-level evaporative cooling accounts how the differences between tornadic and nontornadic for baroclinic reorientation of the low-level shear vec- supercell environments result in internal changes to the tors in the nontornadic near-storm hodographs. The storms themselves. Unfortunately, the answers to such ‘‘enhanced’’ near-storm nontornadic hodographs also questions are beyond the reach of this simple dataset, and have increasingly crosswise horizontal vorticity, which is will likely require hypothesis-driven numerical modeling presumably less favorable for low-level updraft rotation. experiments. Hopefully, the VORTEX2 sounding com- Finally, it is intriguing to see that the composite posites provide an updated, robust observational con- supercell cold pools are quite shallow (perhaps having straint upon what constitutes ‘‘realism’’ in such models. depths of only 500 m), at least based on the locations where VORTEX2 soundings were launched. In addi- Acknowledgments. Funding for this research was pro- tion, appreciable surface-based outflow CAPE (exceeding vided by the National Science Foundation under Grants 2 1000 J kg 1 for all supercell subsets in this study) appears ATM-0758509 and AGS-1156123. The author grate- to remain within close range of the updraft. Even though fully acknowledges the assistance of Brice Coffer and cold pools are known to be spatially heterogeneous, and Chris MacIntosh, who tracked and recorded the updraft

Unauthenticated | Downloaded 09/28/21 02:18 AM UTC 528 MONTHLY WEATHER REVIEW VOLUME 142 positions for most of the storms shown in Table 1. The Letkewicz, C. E., A. J. French, and M. D. Parker, 2013: Base-state author thanks the Convective Storms Group at North substitution: An idealized modeling technique for approxi- Carolina State University for comments on this paper, mating environmental variability. Mon. Wea. Rev., 141, 3062– 3086. as well as the numerous conference and workshop par- Loehrer, S. M., T. A. Edmands, and J. A. Moore, 1996: TOGA ticipants who discussed earlier versions of this work. COARE upper-air sounding data archive: Development and Finally, the formal reviews by Adam Houston, Russ quality control procedures. Bull. Amer. Meteor. Soc., 77, 2651– Schumacher, and Conrad Ziegler helped to refine many 2671. aspects of the presentation. ——, S. F. Williams, and J. A. Moore, 1998: Results from UCAR/ JOSS quality control of atmospheric soundings from field projects. Preprints, 10th Symp. on Meteorological Obser- REFERENCES vations and Instrumentation, Phoenix, AZ, Amer. Meteor. Soc., 1–6. Barnes, S. L., 1973: Mesoscale objective analysis using weighted Maddox, R. A., 1993: Diurnal low-level wind oscillation and storm- time-series observations. NOAA Tech. Memo. ERL NSSL-62, relative helicity. The Tornado: Its Structure, Dynamics, Predic- 60 pp. [Available from National Severe Storms Laboratory, tion, and Hazards, Geophys. Monogr., Vol. 79, Amer. Geophys. Norman, OK 73069.] Union, 591–598. Beck, J., and C. Weiss, 2013: An assessment of low-level baroclinity Markowski, P. M., and Y. P. Richardson, 2014: The influence of and vorticity within a simulated supercell. Mon. Wea. Rev., environmental low-level shear and cold pools on tornado- 141, 649–669. genesis: Insights from idealized simulations. J. Atmos. Sci., in Benjamin, S. G., G. A. Grell, J. M. Brown, T. G. Smirnova, and press. R. Bleck, 2004: Mesoscale weather prediction with the RUC ——, E. N. Rasmussen, J. M. Straka, and D. C. Dowell, 1998a: hybrid isentropic–terrain-following coordinate model. Mon. Observations of low-level baroclinity generated by anvil Wea. Rev., 132, 473–494. shadows. Mon. Wea. Rev., 126, 2942–2958. Blackadar, A. K., 1957: Boundary layer wind maxima and their ——, J. M. Straka, E. N. Rasmussen, and D. O. Blanchard, 1998b: significance for the growth of nocturnal inversions. Bull. Variability of storm-relative helicity during VORTEX. Mon. Amer. Meteor. Soc., 38, 282–290. Wea. Rev., 126, 2959–2971. Brooks, H. E., C. A. Doswell III, and J. Cooper, 1994: On the en- ——, ——, and ——, 2002: Direct surface thermodynamic obser- vironments of tornadic and nontornadic mesocyclones. Wea. vations within the rear-flank downdrafts of nontornadic and Forecasting, 9, 606–618. tornadic supercells. Mon. Wea. Rev., 130, 1692–1721. Bryan, G. H., and M. D. Parker, 2010: Observations of a squall ——, C. Hannon, J. Frame, E. Lancaster, A. Pietrycha, R. Edwards, line and its near environment using high-frequency rawin- and R. L. Thompson, 2003: Characteristics of vertical wind sonde launches during VORTEX2. Mon. Wea. Rev., 138, profiles near supercells obtained from the rapid update cycle. 4076–4097. Coniglio, M. C., 2012: Verification of RUC 0–1-h forecasts and SPC Wea. Forecasting, 18, 1262–1272. mesoscale analyses using VORTEX2 soundings. Wea. Fore- Morrison, H., and J. Milbrandt, 2011: Comparison of two-moment casting, 27, 667–683. bulk microphysics schemes in idealized supercell thunder- ——, and D. J. Stensrud, 2001: Simulation of a progressive derecho storm simulations. Mon. Wea. Rev., 139, 1103–1130. using composite initial conditions. Mon. Wea. Rev., 129, 1593– Parker, M. D., and J. M. L. Dahl, 2013: Toy model simulations of 1616. surface vorticity generation in downdrafts. Extended Ab- Davies-Jones, R., 1984: Streamwise vorticity: The origin of updraft stracts, 15th Conf. on Mesoscale Processes, Portland, OR, rotation in supercell storms. J. Atmos. Sci., 41, 2991–3006. Amer. Meteor. Soc., 12.3. [Available online at https://ams. Dawson, D. T., M. Xue, J. A. Milbrandt, and M. K. Yau, 2010: confex.com/ams/15MESO/webprogram/Paper227679.html.] Comparison of evaporation and cold pool development be- Pauley, P. M., and X. Wu, 1990: The theoretical, discrete, and ac- tween single-moment and multimoment bulk microphysics tual response of the Barnes objective analysis scheme for one- schemes in idealized simulations of tornadic thunderstorms. and two-dimensional fields. Mon. Wea. Rev., 118, 1145–1164. Mon. Wea. Rev., 138, 1152–1171. Potvin, C. K., K. L. Elmore, and S. J. Weiss, 2010: Assessing the Frame, J., and P. Markowski, 2010: Numerical simulations of ra- impacts of proximity sounding criteria on the climatology of diative cooling beneath the anvils of supercell thunderstorms. significant tornado environments. Wea. Forecasting, 25, 921– Mon. Wea. Rev., 138, 3024–3047. 930. Gal-Chen, T., and R. Somerville, 1975: On the use of a coordinate Rasmussen, E. N., and D. O. Blanchard, 1998: A baseline clima- transformation for the solution of the Navier–Stokes equa- tology of sounding-derived supercell and tornado forecast tions. J. Comput. Phys., 17, 209–228. parameters. Wea. Forecasting, 13, 1148–1164. Grzych, M. L., B. D. Lee, and C. A. Finley, 2007: Thermody- Richardson, Y. P., K. K. Droegemeier, and R. P. Davies-Jones, namic analysis of supercell rear-flank downdrafts from Project 2007: The influence of horizontal environmental variability on ANSWERS. Mon. Wea. Rev., 135, 240–246. numerically simulated convective storms. Part I: Variations in Klemp, J. B., and R. Rotunno, 1983: A study of the tornadic re- vertical shear. Mon. Wea. Rev., 135, 3429–3455. gion within a supercell thunderstorm. J. Atmos. Sci., 40, 359– Rotunno, R., and J. Klemp, 1985: On the rotation and propagation 377. of simulated supercell thunderstorms. J. Atmos. Sci., 42, 271– Koch, S. E., M. DesJardins, and P. J. Kocin, 1983: An interactive 292. Barnes objective map analysis scheme for use with satellite Shabbott, C. J., and P. M. Markowski, 2006: Surface in situ ob- and conventional data. J. Climate Appl. Meteor., 22, 1487– servations within the outflow of forward-flank downdrafts of 1503. supercell thunderstorms. Mon. Wea. Rev., 134, 1422–1441.

Unauthenticated | Downloaded 09/28/21 02:18 AM UTC FEBRUARY 2014 P A R K E R 529

Snook, N., and M. Xue, 2008: Effects of microphysical drop size from the central United States during the 1998 warm season. distribution on tornadogenesis in supercell thunderstorms. Mon. Wea. Rev., 128, 3376–3395. Geophys. Res. Lett., 35, L24803, doi:10.1029/2008GL035866. Weisman, M. L., and J. B. Klemp, 1984: The structure and classi- Thompson, R. L., R. Edwards, J. A. Hart, K. L. Elmore, and P. M. fication of numerically simulated convective storms in direc- Markowski, 2003: Close proximity soundings within supercell tionally varying wind shears. Mon. Wea. Rev., 112, 2479–2498. environments obtained from the Rapid Update Cycle. Wea. ——, and R. Rotunno, 2000: The use of vertical wind shear versus Forecasting, 18, 1243–1261. helicity in interpreting supercell dynamics. J. Atmos. Sci., 57, ——, ——, and C. M. Mead, 2004: An update to the supercell 1452–1472. composite and significant tornado parameters. Preprints, 22nd Wurman, J., D. Dowell, Y. Richardson, P. Markowski, E. Rasmussen, Conf. on Severe Local Storms, Hyannis, MA, Amer. Meteor. D. Burgess, L. Wicker, and H. B. Bluestein, 2012: The Sec- Soc., P8.1. [Available online at http://ams.confex.com/ams/ ond Verification of the Origins of Rotation in Tornadoes pdfpapers/82100.pdf.] Experiment: VORTEX2. Bull. Amer. Meteor. Soc., 93, 1147– ——, C. M. Mead, and R. Edwards, 2007: Effective storm-relative helicity and bulk shear in supercell thunderstorm environ- 1170. ments. Wea. Forecasting, 22, 102–115. Ziegler, C. L., 2013: A diabatic Lagrangian technique for the ——, B. T. Smith, J. S. Grams, A. R. Dean, and C. Broyles, 2012: analysis of convective storms. Part II: Application to a radar- Convective modes for significant severe thunderstorms in the observed storm. J. Atmos. Oceanic Technol., 30, 2266–2280. contiguous United States. Part II: Supercell and QLCS tor- ——, E. R. Mansell, J. M. Straka, D. R. MacGorman, and D. W. nado environments. Wea. Forecasting, 27, 1136–1154. Burgess, 2010: The impact of spatial variations of low-level Trier, S. B., C. A. Davis, and J. D. Tuttle, 2000: Long-lived meso- stability on the lifecycle of a simulated supercell storm. Mon. convective vortices and their environment. Part I: Observations Wea. Rev., 138, 1738–1766.

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