AMERICAN METEOROLOGICAL SOCIETY Bulletin of the American Meteorological Society EARLY ONLINE RELEASE This is a preliminary PDF of the author-produced manuscript that has been peer-reviewed and accepted for publication. Since it is being posted so soon after acceptance, it has not yet been copyedited, formatted, or processed by AMS Publications. This preliminary version of the manuscript may be downloaded, distributed, and cited, but please be aware that there will be visual differences and possibly some content differences between this version and the final published version. The DOI for this manuscript is doi: 10.1175/BAMS-D-16-0061.1 The final published version of this manuscript will replace the preliminary version at the above DOI once it is available. If you would like to cite this EOR in a separate work, please use the following full citation: Lang, T., S. Pédeboy, W. Rison, R. Cerveny, J. Montanyà, S. Chauzy, D. MacGorman, R. Holle, E. Ávila, Y. Zhang, G. Carbin, E. Mansell, Y. Kuleshov, T. Peterson, M. Brunet, F. Driouech, and D. Krahenbuhl, 2016: WMO World Record Lightning Extremes: Longest Reported Flash Distance and Longest Reported Flash Duration. Bull. Amer. Meteor. Soc. doi:10.1175/BAMS-D-16-0061.1, in press. © 2016 American Meteorological Society Manuscript (non-LaTeX) Click here to download Manuscript (non-LaTeX) WMOlightning_lang_etal_REVB3.docx 1 WMO World Record Lightning Extremes: 2 Longest Reported Flash Distance and Longest Reported Flash Duration 3 Timothy J. Lang1 4 Stéphane Pédeboy2 5 William Rison3 6 Randall S. Cerveny4* 7 Joan Montanyà5 8 Serge Chauzy6 9 Donald R. MacGorman7 10 Ronald L. Holle8 11 Eldo E. Ávila9 12 Yijun Zhang10 13 Gregory Carbin11 14 Edward R.Mansell7 15 Yuriy Kuleshov12 16 Thomas C. Peterson13 17 Manola Brunet14 18 Fatima Driouech15 19 Daniel S. Krahenbuhl4 20 1. NASA Marshall Space Flight Center 21 2. Météorage 22 3. Electrical Engineering, New Mexico Institute of Mining and Technology 23 4. School of Geographical Sciences, Arizona State University Page 1 24 5. Polytechnic University of Catalonia 25 6. Laboratoire d'Aérologie, University of Toulouse/CNRS 26 7. National Severe Storms Laboratory, NOAA, Norman, Oklahoma 27 8. Vaisala, Inc., Tucson, Arizona 28 9. FaMAF, Universidad Nacional de Cordoba, Argentina. IFEG-CONICET 29 10. Laboratory of Lightning Physics and Protection Engineering, Chinese Academy of 30 Meteorological Sciences 31 11. Storm Prediction Center, NOAA, Norman Oklahoma 32 12. Australian Bureau of Meteorology & School of Mathematical and Geospatial Sciences, 33 Royal Melbourne Institute of Technology (RMIT) University 34 13. World Meteorological Organization Commission for Climatology 35 14. Centre for Climate Change, Dept. of Geography, University Rovira i Virgili & Climatic 36 Research Unit, School of Environmental Sciences, University of East Anglia 37 15. Climate Studies Service at the Direction de la Météorologie nationale of Morocco 38 *Corresponding Author, email: [email protected]; School of Geographical Sciences, Arizona 39 State University, Tempe AZ 85287-5302 40 41 Capstone Statement: A World Meteorological Organization committee has judged that the 42 world’s longest reported distance for a single lightning flash occurred with a horizontal distance 43 of 321 km (199.5 mi) over Oklahoma in 2007, while the world’s longest reported duration for a 44 single lightning flash is an event that lasted continuously for 7.74 seconds over southern France 45 in 2012. 46 Page 2 47 Abstract. A World Meteorological Organization weather and climate extremes committee has 48 judged that the world’s longest reported distance for a single lightning flash occurred with a 49 horizontal distance of 321 km (199.5 mi) over Oklahoma in 2007, while the world’s longest 50 reported duration for a single lightning flash is an event that lasted continuously for 7.74 seconds 51 over southern France in 2012. In addition, the committee has unanimously recommended 52 amendment of the AMS Glossary of Meteorology definition of lightning discharge as a "series of 53 electrical processes taking place within 1 second" by removing the phrase “within one second” 54 and replacing with “continuously.” Validation of these new world extremes (a) demonstrates the 55 recent and on-going dramatic augmentations and improvements to regional lightning detection 56 and measurement networks, (b) provides reinforcement regarding the dangers of lightning, and 57 (c) provides new information for lightning engineering concerns. 58 59 1. Introduction and Suggested Modification of “Lightning Flash” Definition 60 61 Dramatic augmentations and improvements to lightning remote sensing techniques have 62 allowed the detection of previous unobserved extremes in lightning occurrence. As part of the 63 ongoing work of the World Meteorological Organization (WMO) Commission for Climatology 64 (CCl) in detection and documentation of global weather extremes (e.g., El Fadli et al. 2013), a 65 critical evaluation of two recent lightning extremes has been undertaken. These two extremes 66 are: (1) the world’s longest detected distance for a single lightning flash, and (2) the world’s 67 longest detected duration for a single lightning flash. Specifically, a WMO CCl evaluation 68 committee has adjudicated that the world’s longest detected distance for a single lightning flash 69 occurred over a horizontal distance of 321 km (199.5 mi) using a maximum great circle distance Page 3 70 between individual detected VHF lightning sources. The event occurred on 20 June 2007 across 71 parts of Oklahoma. They accepted the world’s longest detected duration for a single lightning 72 flash is a single event that lasted continuously for 7.74 seconds on 30 August 2012 over parts of 73 southern France. It should be noted that, as with all WMO evaluations of extremes (e.g., 74 temperature, pressure, wind, etc.), the proposed extremes are identified based on only those 75 events with available quality data and brought to the WMO’s attention by the meteorological 76 community. It is possible, indeed likely, that greater extremes can and have occurred. For 77 example, it is likely that the current highest recorded wind gust extreme of 113.2 m s-1 (253 mph; 78 220 kt) [Barrow Island Australia, 1055 UTC 10/4/1996] can be exceeded by winds in a tornado 79 or similar phenomena. However, the Australian wind gust has been the highest recorded event 80 placed before the WMO for adjudication. When higher extreme events are effectively recorded 81 and brought to the attention of the WMO, subsequent evaluations of those extremes can occur. 82 A critical element in the discussion of these extremes is the fundamental definition of a 83 lightning flash. The American Meteorological Society (AMS) Glossary of Meteorology 84 (American Meteorological Society 2015) defines a lightning flash as “a transient, high-current 85 electric discharge with path lengths measured in kilometers” while a lightning discharge is 86 defined (American Meteorological Society 2015) as “the series of electrical processes taking 87 place within 1 s by which charge is transferred along a discharge channel between electric charge 88 centers of opposite sign within a thundercloud (intracloud flash), between a cloud charge center 89 and the earth's surface (cloud-to-ground flash or ground-to-cloud discharge), between two 90 different clouds (intercloud or cloud-to-cloud discharge), or between a cloud charge and the air 91 (air discharge). It is a very large-scale form of the common spark discharge. A single lightning 92 discharge is called a lightning flash.” Page 4 93 Debate on an updated precise definition of a lighting flash was initiated by the committee 94 and through the review process. Specifically, after careful deliberation by the WMO evaluation 95 committee, comprised in part of international users and operators of lightning locating systems 96 (LLS), the unanimous consensus was that this lightning discharge definition has not been 97 adapted to fit with physical characteristics and processes as revealed by modern technologies. At 98 this time, the committee recommends only small revisions to the AMS Glossary of Meteorology 99 definitions to bring the definition to more current conformance with improved technologies. 100 For the broad meteorological community, it is useful to review a few relevant features of 101 lightning formation. This discussion generally follows materials from Rakov and Uman (2007), 102 WMO (2014), Albrecht et al. (2014), and UCAR MetEd (2016). A lightning flash is initiated 103 through the occurrence of bi-directional leaders between two oppositely charged regions of a 104 cloud. Lightning initiates at altitudes colder than freezing where a mixture of hail particles 105 called graupel, supercooled water droplets, and various forms of ice crystals occur in the 106 presence of an updraft. The updraft separates the different charges associated with these variably 107 sized particles resulting in initiation of a lightning event. Negative stepped leaders move 108 downward in steps of around 50 meters that can be detected by high-speed cameras and through 109 the high-frequency radio emissions received by ground-based detection networks such as a 110 Lightning Mapping Array (LMA). 111 For simplicity and because 90 percent of lightning strikes are of this type, consider the 112 typical phenomenology of a negative cloud-to-ground (CG) flash. As a negative stepped leader 113 approaches the ground, positive charges are induced at the ground and by tall conducting 114 features, thereby maintaining the electrical potential between leader and ground. The electric 115 potential difference between a downward‐moving stepped‐leader tip and ground is probably on Page 5 116 the order of tens of megavolts. This allows an upward streamer of positive charge to develop 117 from tall plants, artificial structures, or from flat ground or water surfaces. Typically, an LMA 118 misses these upward streamers near the surface of the earth because they occur at a lower altitude 119 than the detection network’s line of sight.