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THE BOW ECHO and MCV EXPERIMENT Observations and Opportunities

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THE BOW ECHO and MCV EXPERIMENT Observations and Opportunities THE BOW ECHO AND MCV EXPERIMENT Observations and Opportunities BY CHRISTOPHER DAVIS, NOLAN ATKINS, DIANA BARTELS, LANCE BOSART, MICHAEL CONIGLIO, GEORGE BRYAN, WILLIAM COTTON, DAVID DOWELL, BRIAN JEWETT, ROBERT JOHNS,* DAVID JORGENSEN, JASON KNIEVEL, KEVIN KNUPP, WEN-CHAU LEE, GREGORY MCFARQUHAR, JAMES MOORE, RON PRZYBYLINSKI, ROBERT RAUBER, BRADLEY SMULL, ROBERT TRAPP, STANLEY TRIER, ROGER WAKIMOTO, MORRIS WEISMAN, AND CONRAD ZIEGLER The field campaign, involving multiple aircraft and ground-based instruments, documented numerous long-lived mesoscale convective systems, many producing strong surface winds and exhibiting mesoscale rotation. hile the observational study of mesoscale Kansas Preliminary Regional Experiment for convective systems (MCSs) has been active Stormscale Operational and Research Meteorology W since the 1940s (e.g., Newton 1950 and ref- (STORM)-Central (PRE-STORM) (Cunning 1986) erences within), until the Bow Echo and Mesoscale were geographically fixed by the ground-based instru- Convective Vortex Experiment (BAMEX) there were ment networks employed. The unique observing no studies designed to sample multiscale aspects of strategy of BAMEX relied on the deployment of these systems throughout the majority of their life highly mobile observing systems, both airborne and cycles. Previous field studies such as the Oklahoma- ground based, supported by the enhanced operational AFFILIATIONS: DAVIS, BRYAN, DOWELL, KNIEVEL, BARTELS, LEE, Missouri; SMULL—NSSL, and University of Washington, Seattle, TRIER, AND WEISMAN—National Center for Atmospheric Research,+ Washington; TRAPP—Purdue University, West Lafayette, Indiana; Boulder, Colorado; ATKINS—Lyndon State College, Lyndon, WAKIMOTO—University of California, Los Angeles, Los Angeles, Vermont; BARTELS—NOAA/Forecast Systems Laboratory, Boulder, California Colorado; BOSART—The University at Albany, State University of *Retired New York, Albany, New York; CONIGLIO—University of Oklahoma, +The National Center for Atmospheric Research is sponsored by Norman, Oklahoma; COTTON—Colorado State University, Fort the National Science Foundation Collins, Colorado; JEWETT, MCFARQUHAR, AND RAUBER—University CORRESPONDING AUTHOR: Christopher A. Davis, P.O. Box of Illinois at Urbana–Champaign, Urbana, Illinois; JOHNS—Storm 3000, Boulder, CO 80307 Prediction Center, Norman, Oklahoma; JORGENSEN AND ZIEGLER— E-mail: [email protected] National Severe Storms Laboratory, Norman, Oklahoma; KNUPP— DOI: 10.1175/BAMS-85-8-1075 University of Alabama in Huntsville, Huntsville, Alabama; MOORE— In final form 25 May 2004 Joint Office of Science Support, UCAR, Boulder, Colorado; ©2004 American Meteorological Society PRZYBLINSKI—National Weather Service Forecast Office, St. Louis, AMERICAN METEOROLOGICAL SOCIETY AUGUST 2004 | 1075 observing networks [e.g., Weather Surveillance lometers in horizontal extent (Klimowski et al. 2000), Radar-1988 Doppler (WSR-88D)] over the central and can occur as isolated entities or embedded within United States. Unprecedented high-density kinematic larger squall lines. Lifetimes vary from tens of min- and, especially, thermodynamic observations were utes to several hours. obtained within and near MCSs as they evolved. The severe, straight-line winds in bow echoes may The field phase of BAMEX was conducted between result from local acceleration of the surface wind 20 May and 6 July 2003 and was based at MidAmerica within the pressure gradient created by the mesoscale, St. Louis Airport (MAA) in Mascoutah, Illinois (near high pressure region (the “mesohigh”) within the St. Louis, Missouri). During the field phase, life cycles surface cold pool. The cold pool, in turn, depends on of MCSs occurring over a large portion of the central the structure and strength of the rear-inflow jet (e.g., United States were examined. Two types of systems Smull and Houze 1987) as it entrains drier midlevel were emphasized—one producing severe surface air into the mesoscale downdraft, enhancing evapo- winds (bow echoes), and the other producing long- rative effects. The production of negative buoyancy lived mesoscale convective vortices (MCVs) capable also depends on the microphysical composition of the of initiating subsequent convection. Although these stratiform precipitation region (where the rear-inflow two phenomena represent useful conceptual arche- jet resides). An alternative explanation is that dam- types of organized convection, many bow echoes fea- aging winds result from meso-g -scale vortices near ture pronounced mesoscale rotation. In fact, the the surface along the leading edge of the bow echo broad objective of BAMEX may be summarized as the (Trapp and Weisman 2003). Yet another possible study of rotationally dominated MCSs. mechanism, purported to explain the intriguing in- This article presents an overview of the objectives stances of nocturnal wind events, involves internal and experimental design of BAMEX (section 2), fol- gravity waves produced from perturbations of the lowed by a brief summary of selected observations to stable boundary layer by deep convection (Bernadet highlight both the phenomena sampled and the ca- and Cotton 1998). However, the relative magnitudes pabilities of different observing platforms (section 3). and importance of these various contributions to sur- We conclude with a discussion of some of the lessons face winds have not yet been quantified. learned from operations and the scientific opportu- Few Doppler radar precursors of damaging wind nities that lay ahead. with bow echoes exist to date. Schmocker et al. (1996) found that near the forward (downwind) flank of the SCIENCE OBJECTIVES AND EXPERIMEN- convective line, a midaltitude radial convergence TAL DESIGN. Bow echoes. Convectively produced (MARC) signature often preceded the bowing of the windstorms pose a significant hazard to life and prop- reflectivity field and subsequent severe straight-line erty over much of the United States during the spring outflow at the surface. Miller and Johns (2000) no- and summer months. The longer-lived, larger-scale ticed that extreme damaging winds in derechoes were events have been given the generic name of “derecho” at times due to high-precipitation (HP) supercells (Johns and Hirt 1987). Most derechoes are manifes- embedded within a convective line. Recent observa- tations of “bow echoes,” first described in detail by tional studies also suggest the importance of preex- Fujita (1978; see also Weisman 2001) and most eas- isting, line-normal surface boundaries to the forma- ily identified by their characteristic bow-shaped pat- tion, propagation, and severe weather associated with tern of high reflectivity on radar images. Key kine- bow echoes (e.g., Przybylinski et al. 2000; Schmocker matic features of bow echoes include a strong et al. 2000). Such boundaries locally augment hori- leading-line updraft followed by an intense zontal convergence and the horizontal and vertical downdraft and divergent, cold outflow at the surface. components of vorticity. This local modification can The outflow is often accompanied by an intense, rear- favor more vigorous convective cells with attendant inflow jet and a weak-reflectivity region (a “notch”) rotation and stronger surface winds. behind the apex of the bow (Burgess and Smull 1993; In addition to their association with strong, Przybylinski 1995). Additionally, a dominant cy- straight-line surface winds, there is also a link between clonic vortex and weaker anticyclonic vortex are usu- bow echoes and tornadoes (e.g., Fujita 1978; Forbes ally evident in the lower to middle troposphere be- and Wakimoto 1983; Przybylinski 1995; Pfost and hind the northern and southern ends of the bow, Gerard 1997; Funk et al. 1999). A recent study by respectively (referred to as “bookend” or “line end” Trapp et al. (2004, manuscript submitted to Wea. vortices; Weisman 1993). Bow echoes are observed Forecasting) suggests that tornadoes within convec- over a range of scales, from tens to a few hundred ki- tive lines account for up to 20% of all tornadic events 1076 | AUGUST 2004 nationwide. Also, contrary to popular belief, such tor- The process of MCV formation has yet to be well nadoes can be quite strong and long-lived. However, observed; hence, collecting observations adequate to unlike the case of tornadoes spawned from supercell understand mechanisms of MCV formation is an storms, there are no systematic radar signatures pre- important objective of BAMEX. Idealized convection- ceding tornadoes within convective lines. The incipi- resolving numerical simulations (e.g., Skamarock ent rotation usually appears first near the ground just et al. 1994; Davis and Weisman 1994; Weisman and a short time before the tornado forms (e.g., Trapp Davis 1998; Cram et al. 2002) have shown that sys- et al. 1999). The fact that nonsupercell tornadoes de- tematic tilting of horizontal vorticity associated with velop primarily northward of the apex of the bow MCS-generated buoyancy gradients is responsible for (Forbes and Wakimoto 1983) has not yet been the development of vertical vorticity that is subse- explained. quently enhanced by midtropospheric convergence of Numerical modeling studies to date have repro- planetary vorticity. Here the simulated convection re- duced with some success the observed spectrum of sembles the frequently observed asymmetric squall- bow-echo phenomena, ranging from strong, bow- type MCS (e.g., Houze et al. 1989, 1990; Blanchard shaped lines of cells, forced by a strong surface cold 1990; Loehrer and Johnson
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