508 MONTHLY WEATHER REVIEW VOLUME 142 Composite VORTEX2 Supercell Environments from Near-Storm Soundings 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 envi- ronments of supercell thunderstorms. Based upon the full analysis, it appears that vertical wind shear in- creases 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 per- formed 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,