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Downloaded 09/25/21 04:09 PM UTC OCTOBER 1997 DOWELL and BLUESTEIN 2563 Hand, the Source of Low-Level Rotation Is More Contro- Versial 2562 MONTHLY WEATHER REVIEW VOLUME 125 The Arcadia, Oklahoma, Storm of 17 May 1981: Analysis of a Supercell during Tornadogenesis DAVID C. DOWELL AND HOWARD B. BLUESTEIN School of Meteorology, University of Oklahoma, Norman, Oklahoma (Manuscript received 29 October 1996, in ®nal form 10 February 1997) ABSTRACT On 17 May 1981, an extensive dataset was collected for a supercell thunderstorm that produced an F2 tornado near Arcadia in central Oklahoma. Coordinated dual-Doppler scans of the storm by 10-cm research radars were collected at approximately 5-min intervals from 30 min before the tornado touched down until 15 min after the tornado had dissipated. The Arcadia storm was also well sampled by a 444-m-tall instrumented tower. The low- level in¯ow, updraft, mesocyclone, and rear precipitation core of the supercell all passed across the tower. A comparison of the instrumented tower measurements with a dual-Doppler synthesis reveals that the latter qualitatively resolved the low-level ¯ow. However, the magnitudes of the low-level horizontal winds and updraft speed were underestimated. In addition, the vertical shear of the horizontal wind in the lowest kilometer was unresolved in the Doppler winds. In the storm environment, horizontal vorticity was strong (;1.5 3 1022 s21) and approximately streamwise over the depth of the instrumented tower. Just upstream (northeast) of the updraft, the magnitude of horizontal vorticity was nearly twice this value and had likely been enhanced by baroclinic generation of horizontal vorticity and/or stretching of horizontal vorticity. Tilting of the resulting horizontal vorticity into the vertical produced the pretornadic low-level mesocyclone. Low-level mesocyclone in¯ow was primarily from the east, but during the tornadic stage, parcels approaching from the north and west were also drawn into the circulation. The tornado formed southeast of the mesocyclone center and near the tip of the re¯ectivity hook echo while low-level mesocyclone vorticity was increasing. Tornadogenesis occurred near the nose of the rear downdraft within a region of horizontal shear between southeasterly in¯ow into the storm and westerly out¯ow from the rear downdraft. Pressure retrievals suggest the rear downdraft south of the mesocyclone center was associated with a downward-directed perturbation pressure gradient force. The tornado and the parent storm dissipated as out¯ow surged eastward ahead of the updraft. This case study is the ®rst to include a comparison of independent measurements of the wind ®eld in and near the low-level mesocyclone of a supercell. The wind analysis is also complemented by the instrumented tower thermodynamic measurements. 1. Introduction TEX) in the southern and central plains of the United States (Rasmussen et al. 1994). Radiosondes, mobile Recent progress in numerical simulations and ad- mesonet instrument packages, portable Doppler radar, vances in observational capabilities have sparked re- airborne Doppler radar, and other implements were all newed interest in veri®cation of the details of how a directed toward the broad goal of understanding the for- supercell thunderstorm (Browning 1964) produces a tor- mation of tornadoes. nado. Computer simulations of supercells are becoming Tornadogenesis relies on horizontal convergence more and more successful at resolving tornadolike vor- within the boundary layer to amplify vertical vorticity tices (Wicker and Wilhelmson 1995; Grasso and Cotton to magnitudes characteristic of tornadoes (Ward 1972; 1995). With the nearly nationwide coverage provided Lewellen 1993). In some cases, thunderstorms concen- by the WSR-88D network, tornadic storms are being trate vertical vorticity already present in the storm en- sampled relatively frequently by Doppler radar (Guer- vironment (Wilson 1986; Brady and Szoke 1989; Wak- rero and Read 1993; Magsig and Burgess 1996). During imoto and Wilson 1989). In contrast, supercell thun- 1994±95, scientists collaborated on the Veri®cation of derstorms appear to act upon low-level vertical vorticity the Origins of Rotation in Tornadoes Experiment (VOR- produced by the storm itself (Barnes 1970; Brandes 1984b; Rotunno and Klemp 1985). Tilting in the storm updraft of horizontal vorticity associated with the environmental vertical wind shear Corresponding author address: David C. Dowell, School of Me- teorology, University of Oklahoma, 100 East Boyd, Room 1310, Nor- is generally believed to be the source of rotation for the man, OK 73019-0628. midlevel mesocyclone of a supercell (Barnes 1970; E-mail: [email protected] Brandes 1984b; Rotunno and Klemp 1985). On the other q1997 American Meteorological Society Unauthenticated | Downloaded 09/25/21 04:09 PM UTC OCTOBER 1997 DOWELL AND BLUESTEIN 2563 hand, the source of low-level rotation is more contro- versial. Numerical simulations by Rotunno and Klemp (1985) suggest that the development of low-level ro- tation awaits the presence of evaporatively cooled air near the surface. Once the storm's cold pool is estab- lished, horizontal vorticity produced baroclinically along the cool air boundary upstream of the updraft is tilted into the vertical, and the vertical vorticity is am- pli®ed by stretching within the updraft. In contrast to the work of Rotunno and Klemp (1985), the simulations by Walko (1993) demonstrate that en- vironmental horizontal vorticity can be the source for low-level rotation. (Although environmental horizontal vorticity may itself have been generated by baroclinic effects, it is distinguished from the type of baroclinically generated horizontal vorticity described by Rotunno and Klemp by its existence prior to storm formation.) Wicker (1996) stresses the importance of phasing of horizontal vorticity of both types in the development of the low- level mesocyclone. An issue that remains even more ambiguous is how the low-level mesocyclone and tornado are related. Tor- nadic vortex signatures (TVSs) in Doppler radar data FIG. 1. Map of the locations of the instrumented tower, sounding (Brown et al. 1978) provide observational evidence of (Edmond), radar sites (Norman and Cimarron), and town of Arcadia, the vastly different magnitudes of size and vorticity in Oklahoma. The Norman radar is located at the origin. The tornado mesocyclones and tornadoes. Furthermore, the behavior occurred south of Arcadia between approximately 1700 and 1710 CST 17 May 1981. Contours of the re¯ectivity factor (dBZ) measured of the TVS within the mesocyclone varies from storm by the Cimarron radar at 1.0 km AGL at 1704 CST are also shown. to storm; it may appear ®rst aloft, ®rst at low levels, or The crescent-shaped region indicates where the dual-Doppler be- simultaneously over a large depth (Brown et al. 1978; tween-beam angle is greater than 458 (and less than 1358). Trapp and Mitchell 1995). The diagnosis of how meso- cyclones and tornadoes are related continues to stretch the limits of observational capabilities. Another issue Oklahoma City. A high-resolution cross section of the that remains unclear is how much the processes that wind and temperature structure both within the pretor- produce low-level rotation vary from case to case; the nadic storm (including the low-level updraft and me- variety of observed storm types (Doswell and Burgess socyclone) and its environment were obtained, affording 1993) hints that not all tornadogenesis mechanisms are a unique opportunity for intercomparison of multiplat- alike. Investigation of such issues was a major moti- form observations. The tower measurements document vation for VORTEX (Rasmussen et al. 1994). details of the boundary layer ¯ow that are relevant to In this paper, we examine a dataset that has awaited the development of low-level rotation but that cannot analysis many years but that relates directly to the goals be resolved by distant Doppler radars. of VORTEX. On 17 May 1981, an extensive dual-Dopp- The main purposes of the research described in this ler dataset of an isolated, tornadic supercell was col- paper are twofold: to compare independent measure- lected with the National Severe Storms Laboratory ments of the wind ®eld within the 17 May 1981 su- (NSSL) 10-cm research radars (Fig. 1). Dual-Doppler percell and to look for clues about the processes of low- volumes span a period beginning 30 min before an F2 level mesocyclogenesis and tornadogenesis. tornado formed near Arcadia, Oklahoma, and ending 15 min after the tornado had dissipated. Previous studies of tornadic supercells using ground-based multiple- 2. Description of the 17 May 1981 dataset Doppler radar have not bene®ted from such a complete a. The Arcadia storm and its environment coverage of the supercell life cycle surrounding and including the tornadic stage (Brandes 1978, 1981; Ray Conditions on 17 May 1981 were characteristic of a et al. 1981; Brandes et al. 1988). In addition, the mean classic tornado outbreak in Oklahoma (Taylor 1982). A interval between consecutive dual-Doppler volumes (5 short-wave trough at 500 mb over the southern Rockies min) affords better time resolution than is typically during the morning moved into the Plains during the achieved in airborne Doppler radar studies (Dowell et day. Ahead of the trough, a warm front at the surface al. 1997; Wakimoto et al. 1996). raced northward through Oklahoma, and a dryline ad- A 444-m-tall instrumented tower sampled the Arcadia vanced eastward into west-central Oklahoma. Afternoon supercell as the storm moved over the north side of temperatures near 308C and dewpoints over 208C yield- Unauthenticated | Downloaded 09/25/21 04:09 PM UTC 2564 MONTHLY WEATHER REVIEW VOLUME 125 ed a potentially unstable environment over central Okla- homa. Thunderstorm development began near the intersec- tion of the warm front and dryline in northern Oklahoma around 1400 CST (all times in CST); later convective initiation occurred progressively farther south along the dryline (Brewster 1984). First echoes of what was to become the Arcadia storm appeared on the NSSL Nor- man radar display before 1500. A sounding taken from Edmond, Oklahoma (less than 10 km west of where the tornado later occurred), at 1430 (Figs.
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