Standardizing the Definition of a “Pulse” Thunderstorm
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STANDARDIZING THE DEFINITION OF A “PULSE” THUNDERSTORM PAUL W. MILLER AND THOMAS L. MOTE The lexical evolution of “pulse thunderstorm” within meteorological texts is chronicled, revealing two common, yet incompatible, present-day uses. A standard definition is proposed. ulse thunderstorm” is a widely recognized These storms are a staple feature of the summer term within the meteorological lexicon. climate across the central and eastern United States. “P Though its applications vary, contemporary Fueled by the diurnal instability, short-lived, isolated uses of “pulse” broadly reference a small, short- convection generally forms during the afternoon lived, and isolated updraft forming in a weakly in hot, humid, summertime air masses. Typically sheared environment. Aside from pulse (and its lasting between 30 min and 1 h, each cell consists sister term “pulse type”), the weather nomencla- of a three-stage life cycle (i.e., the cumulus, mature, ture also contains several other words to describe and dissipating stages) first described by Byers and disorganized convection (see www.spc.noaa.gov Braham (1949) during the Thunderstorm Project. /faq/#4.4 for a description of “organized” versus “dis- These cloud formations are almost a daily feature of organized” thunderstorms). “Airmass,” “ordinary,” the southeastern U.S. sky during the warm season. “garden variety,” and “single cell” are all commonly used While most disorganized thunderstorms cause to indicate unicellular, nonsupercellular convection. relatively little human inconvenience, the strongest Meanwhile, broadcast meteorologists frequently opt cells can produce surface conditions exceeding severe for the phrase “pop up” or “popcorn” thunderstorm to weather warning criteria. Pulse thunderstorms are communicate this convective mode to their audiences. generally not tornado producers (relative to supercell thunderstorms), but their associated large hail and AFFILIATIONS: MILLER AND MOTE—Department of Geography, high wind threats can be particularly troublesome to The University of Georgia, Athens, Georgia diagnose. Consequently, meteorologists experience CORRESPONDING AUTHOR: Paul W. Miller, considerable difficulty in issuing accurate severe [email protected] weather warnings for pulse thunderstorms. False The abstract for this article can be found in this issue, following the alarm ratios (FARs) are larger and probabilities of table of contents. detection (PODs) are smaller for warnings issued DOI:10.1175/BAMS-D-16-0064.1 on pulse thunderstorms than for other storm modes A supplement to this article is available online (10.1175/BAMS-D-16-0064.2) (Guillot et al. 2008). However, perhaps their great- est impact on human activity occurs in the absence In final form 7 September 2016 ©2017 American Meteorological Society of severe weather. Even when their outflow winds remain below severe criteria, the dangerous shear AMERICAN METEOROLOGICAL SOCIETY MAY 2017 | 905 Unauthenticated | Downloaded 10/09/21 05:12 AM UTC conditions created by pulse-storm microbursts can of the coauthors, “pulse” was intended to describe “a lead to aviation tragedy (e.g., NTSB 1986). Further, multicellular storm that is largely non-severe. However, more individuals are killed by lightning strikes from occasionally one cell within the multicellular complex pulse storms than any other convective mode (Ashley will briefly become severe.” The authors selected the and Gilson 2009). word “pulse” to reference a “local surge of the updraft Though many meteorologists are familiar with the portion of the cell” that was structured as a discrete term “pulse,” its applications in the meteorological bubble rather than a current (L. Lemon 2015, personal literature suggest it has multiple connotations within communication). The pulse was seen as the mechanism the field. “Pulse” was originally intended to reference responsible for the subsequent severe weather. a briefly severe member of a multicell thunderstorm This original meaning was essentially preserved complex, typically forming in a weakly sheared envi- in both educational and research texts for the first ronment (L. Lemon 2015, personal communication). 20 years of the term’s existence, as indicated by a chro- However, in recent decades the application of this nology of such resources in Table ES1 in the online term has broadened to also describe nonsevere storms supplement to this article (http://dx.doi.org/10.1175 while simultaneously narrowing to exclude multicel- /BAMS-D-16-0064.2). However, the application of lular structures. As the meteorological community the term broadened following the new millennium. focuses more attention on clearly and effectively com- Cerniglia and Snyder (2002) are the first to make an municating weather hazards to the public, the array of explicit reference to “non-severe pulse storms,” indi- terminology used to describe disorganized convection cating that all cases of short-lived, isolated convection is a self-inflicted handicap. This is a timely discussion may be termed “pulse.” In the ensuing decades, this given that general circulation models suggest that interpretation has become increasingly frequent. unstable, weakly sheared atmospheres will become Nine of the 18 textbooks, web tutorials, and research increasingly frequent in future climate scenarios papers in Table ES1 produced after 2000 appear to (Diffenbaugh et al. 2013; Gensini and Mote 2015), and either apply “pulse” as a synonym for all isolated references to pulse storms may become more com- ordinary-cell convection or abstain from including mon as a result. To facilitate new research into these a severe weather criterion in their definition. As the difficult-to-forecast storms, as well as to effectively meaning of “pulse” expanded to include nonsevere communicate their potential hazards, a standard no- thunderstorms, a new variation gained traction menclature for disorganized convection is needed. within the severe weather lexicon: “pulse severe” The purpose of this paper is to propose a common (e.g., Cerniglia and Snyder 2002; Miller and Petrolito definition for a pulse thunderstorm. However, in do- 2008). The need for an explicitly severe variation of ing so the nomenclature of all disorganized convec- the term further illustrates the evolved meaning of tion must also be addressed. In subsequent sections, “pulse” proper. “Pulse” also appears in many other this paper 1) summarizes the historical development journal articles and Internet resources, but the ref- of the term “pulse” and its use within academic and erence is too brief to confidently infer the authors’ educational contexts, 2) performs a content analysis concept of this storm mode. of Storm Prediction Center (SPC) text products to The definitions provided by the current National infer “pulse” applications in an operational setting, Weather Service (NWS) and American Meteorologi- and 3) describes the deficiencies with the current ter- cal Society (AMS) glossaries (Table ES2 in the online minology. We conclude by suggesting a consolidated supplement) presumably offer credible standards for nomenclature for disorganized convection, including disorganized convection terminology. Though the a standard application for “pulse thunderstorm.” NWS glossary (NWS 2016) defines more of the cur- rent lexicon than the AMS Glossary of Meteorology A BRIEF HISTORY OF THE TERM “PULSE.” (AMS 2016), its definitions demonstrate the same “Pulse” was first coined as a thunderstorm mode de- evolution evident in research texts. Separate entries scriptor by Wilk et al. (1979).1 This document, created are given to “pulse” and “pulse severe,” and these for a Federal Aviation Administration (FAA) training definitions are not similar. The NWS “pulse severe” program, was prepared by researchers at the National definition requires that the storm be a single cell, but Severe Storms Laboratory (NSSL). According to one “pulse” does not. According to the NWS glossary, a convective line segment or a supercell could qualify 1 Wilk et al. (1979) is no longer accessible, but readers seeking as a pulse storm if the period of severe weather were more information can reference Burgess and Lemon (1990) sufficiently brief. Although the NWS definition is for a similar definition. clear that severe weather is to be associated with 906 | MAY 2017 Unauthenticated | Downloaded 10/09/21 05:12 AM UTC this storm mode, the requirement that a pulse severe commonly used by forecasters in conjunction with storm adhere to single-cell expectations is inconsis- “pulse.” With single words serving as the unit of tent with Wilk et al. (1979). Despite the AMS’s formal analysis, an inductive dictionary of pulse-related procedure for fielding and reviewing user-suggested terms was developed following the five-step process definitions, the online open-access Glossary of Me- recommended by Short et al. (2009) to optimize teorology largely reflects definitions published in the content validity. All words within the body of pulse- original (Huschke 1959) and revised (Glickman 2000) referencing MDs were considered candidates for the hardcopy editions. Consequently, neither “pulse” nor dictionary. This inductive technique differs from a any of its variant forms are defined; only the defini- deductive approach by forming the dictionary from tion for “ordinary cell” is explicitly provided.2 Entries observed recurring words rather than conceptually for “airmass thunderstorm” and “convective cell” associated words that theoretically should recur. redirect to “airmass shower”