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The Severe Thunderstorm Electrification and Precipitation Study

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The Severe Thunderstorm Electrification and Precipitation Study THE SEVERE T HUN DERST ORM ELEC TRIFIC A TIO N A N D PRECIPIT A TIO N STUDY BY TIM O T HY J . L A N G , L. J AY MILLER, M O RRIS W E I S M A N , STEVEN A . RUTLE D GE, LLYLE J . BARKER III, V . N . BRINGI, V . C H A N D R ASEKAR , A N D R E W D E TWILER , N O L A N D O E SKE N , J O H N HELS D O N , CHARLES K N IG H T , PAUL KREHBIEL, W A L T E R A . LY O NS, D O N M A C G O R M A N , ERIK RASMUSSEN, W I L L I A M RIS O N , W . D A VI D RUST , A N D R O N A L D J . T H O M A S Data from a field project on the Colorado - Kansas border in summer 2000 is helping to improve our understanding of positive cloud-to-ground lightning and low- precipitation storms. evere thunderstorms, because of their propensity as they exhibit not only a wide range of electrical ac- to injure, kill, and cause extensive property dam- tivity, but also diversity in precipitation type and Sage, are a primary concern to not only weather amount. One of the more intriguing severe storms forecasters but also the public. However, these storms types in this regard is the supercell thunderstorm remain a puzzling scientific and forecasting problem, (Browning 1964). In its most pristine state, a supercell is a unicellular thunderstorm comprised of a single, long-lived, rotating updraft, and it frequently pro- AFFILIATIONS: LANG, RUTLEDGE, BRINGI, AND CHANDRASEKAR— duces large hail, high winds, prolific lightning, and Colorado State University, Fort Collins, Colorado; MILLER, WEISMAN, occasionally tornadoes. While the basic dynamics of AND KNIGH T—National Center for Atmospheric Research, Boulder, supercells seem well understood (e.g., Klemp 1987), Colorado; BARKER—National Weather Service, Lincoln, Illinois; these storms exhibit a wide variety of precipitation DETWILER AND HELSD O N—South Dakota School of Mines and Technology, Rapid City, South Dakota; D OESKEN—Colorado Climate characteristics that are not understood. For instance, Center, Fort Collins, Colorado; KREHBIEL, RISON, AND TH OMAS— supercells have been classified as either low precipi- New Mexico Institute of Mining and Technology, Socorro, New tation (LP; Donaldson et al. 1965; Davies-Jones et al. Mexico; LY O NS—FMA Research, Inc., Fort Collins, Colorado; 1976; Burgess and Davies-Jones 1979; Bluestein and MACGORMAN, RASMUSSEN, AND RUST—National Severe Storms Parks 1983), classic or medium precipitation (MP), Laboratory, Norman, Oklahoma or heavy precipitation (HP; Doswell and Burgess CORRESPONDING AUTHOR: Timothy J. Lang, Department of 1993; Rasmussen and Straka 1998) based on visual Atmospheric Science, Colorado State University, Fort Collins, C O 80523 observations of the cloud and precipitation shafts. E-mail: [email protected] Perhaps the least-understood among these storms are D OI: 10.1175/BAMS-85-8-1 107 LP supercells, which characteristically produce some In final form 26 March 2004 large hail but little rain. Potentially because of the dry ©2004 American Meteorological Society environment and lack of visible precipitation, the vis- ible cloud below the anvil is a skeleton compared with A M ERI C A N M E T E O R O L O G I C A L S O C IE T Y A U G U S T 2004 BAfft I 1107 other supercell storms (Bluestein and Parks 1983; began with efforts to discriminate between hail and Bluestein and Woodall 1990). rain, but as these radars became more sophisticated, Another unusual aspect of some convective storms, the number of observed variables and thus the num- including supercells, is their tendency to produce ber of potential discriminants increased. Some algo- copious positive cloud-to-ground (+CG) lightning rithms distinguish between such diverse hydrometeor (e.g., Branick and Doswell 1992; Curran and Rust types as large and small hail, graupel, snow, and 1992; Carey and Rutledge 1998; Williams 2001), in mixed-phase precipitation. Hydrometeor identifica- contrast with most warm-season thunderstorms that tion can be useful in various applications to weather produce predominantly negative CG lightning (-90% forecasting and aviation weather warnings, as well as of all warm-season CGs are negative; Orville 1994; in fundamental studies of storm structure and evolu- Orville and Silver 1997; Orville and Huffines 2001). tion. However, like all remote sensing techniques, po- In fact, the percentage of CG lightning that is posi- larimetric hydrometeor classification needs in situ veri- tive in these storms [predominantly positive CG fication to establish and improve the scope of its validity. (PPCG) storms] can be far greater than 50%, some- During May-July 2000, the Severe Thunderstorm times approaching 100%. Many PPCG storms are Electrification and Precipitation Study (STEPS; severe, but not all severe storms are PPCG (Carey and Weisman and Miller 2000; http://box.mmm.ucar.edu/ Rutledge 2003; Carey et al. 2003), and we currently pdas/STEPS.html) took place near the Colorado- do not understand what exactly distinguishes PPCG Kansas border in order to achieve a better under- storms from other storms. standing of the interactions between kinematics, pre- Researchers typically observe two major positive cipitation production, and electrification in severe charge regions within the updrafts of ordinary thun- thunderstorms. Specific scientific objectives included derstorms, an upper one (above the -20°C isotherm 1) understanding the apparent major differences in altitude; the main reservoir for positive charge) and precipitation output from supercells that have led to a lower one (near 0°C altitude), with a major nega- their being classified as LP, MP, and HP; 2) under- tive charge region (thought to be the source region standing lightning occurrence and behavior in storms, for most -CGs) in between (i.e., between -10° and and how it differs among storm types, particularly to -20°C altitude; e.g., Krehbiel 1986; Koshak and better understand the mechanisms by which storms Krider 1989; Stolzenburg et al. 1998a,b). This typical produce predominantly +CG lightning; and 3) veri- structure is often thought of as a dipole (main upper fying and improving microphysical interpretations positive charge over midlevel negative) or tripole from polarimetric radar. Additionally, the emphasis (considering the commonly observed lower positive on +CG lightning enabled research into what is dif- charge; Williams 1989), although thunderstorm ferent about the small subset of +CGs, usually from charge structures can be more complex than a simple large thunderstorms known as mesoscale convective dipole or tripole, particularly outside the main updraft systems (MCSs), that trigger mesospheric transient (Stolzenburg 1998a,b). However, PPCG storms may luminous events (TLEs) such as sprites (Lyons et al. not have typical electrical structures. Indeed, out- 2000, 2003a,b; Williams 1998). This latter problem standing issues in +CG research include identifying has important implications for our understanding of the positive charge region that is the source for +CGs, how TLEs occur. and in addition, understanding if, how, and why the This paper is intended to be an overview of the charge structures in PPCG storms differ from ordi- STEPS field campaign and in addition provides a brief nary thunderstorms. Williams (2001) reviewed hy- examination of STEPS research in order to demon- potheses for positive CG storms, and testing these strate how the design and execution of the field cam- hypotheses requires more information on relation- paign helped address the project goals. Because of ships between precipitation formation, airflow kine- length constraints, this paper mainly, though not ex- matics, electrification, and lightning production in clusively, focuses on the positive CG issue (STEPS PPCG thunderstorms. goal no. 2) in discussing the STEPS project. The ability to understand these relationships, how- ever, requires sophisticated tools to observe and ana- S T E P S D E S I G N A N D E X E C U T I O N . T h e lyze thunderstorm characteristics. In particular, for STEPS project brought together a unique suite of precipitation, research with polarimetric radars has complementary observing platforms in eastern Colo- led to an emerging capability for identifying hydrom- rado and western Kansas. The basic geographical lay- eteor types remotely (Vivekandan et al. 1999; Liu and out of the project is shown in Fig. 1. This portion of Chandrasekar 2000; Straka et al. 2000). Such work the High Plains region of the United States has been 1 1 0 8 I BAF1 5 - AUGUST 2004 observed climatologically to favor supercell storms, ized, providing information on the size, shape, orien- particularly the LP variety (Bluestein and Parks 1983). tation, and thermodynamic phase of hydrometeors. This is primarily due to the warm-season presence in The network of three Doppler radars provided the this region of the dryline, the boundary between moist opportunity to examine the three-dimensional inter- air from the Gulf of Mexico and drier continental air, which has been strongly associated with the occur- rence of LP storms. This association exists possibly because LP storms form in environments that are drier and have less low-level shear than traditional supercell environments, characteristics that drylines could provide via proximity to dry air, as well as en- hanced mixing to reduce shear values (Bluestein and Parks 1983). However, little is understood about how exactly dryline proximity affects the kinematic and microphysical structure of LP storms. The STEPS region also is favorable for thunderstorms that pro- duce predominantly +CG lightning (Zajac and Rutledge 2001; Carey and Rutledge 2003; Carey et al. 2003) as well as severe hailstorms (Changnon 1977). Thus, the STEPS domain was ideal for studying the storms of interest.
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