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C. B. Moore, G. D. Aulich, and William Rison

Abstract

Conventional lightning rods used in the United States have sharp tips, a practice derived from Benjamin Franklin's discovery of a means to obtain protection from lightning. However, the virtue of sharp tips for strike reception has never been established. An examination of the relevant physics shows that very strong electric fields are required above the tips of rods in order that they function as strike receptors but that the gradients of the field strength over sharp-tipped rods are so great that, at distances of a few millimeters, the local fields are often too weak for the development of upward-going streamers. In field tests, rods with rounded tips have been found to be better strike receptors than were nearby sharp-tipped rods.

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C. B. Moore, William Rison, James Mathis, and Graydon Aulich

Abstract

Although lightning rods have long been used to limit damage from lightning, there are currently no American standards for the shape and form of these devices. Following tradition, however, sharp-tipped Franklin rods are widely installed despite evidence that, on occasion, lightning strikes objects in their vicinity. In recent tests of various tip configurations to determine which were preferentially struck by lightning, several hemispherically tipped, blunt rods were struck but none of the nearby, sharper rods were “hit” by lightning.

Measurements of the currents from the tips of lightning rods exposed to strong electric fields under negatively charged cloud bases show that the emissions consist of periodic ion charge bursts that act to reduce the strength of the local fields. After a burst of charge, no further emissions occur until that charge has moved away from the tip. Laboratory measurements of the emissions from a wide range of electrodes exposed to strong, normal-polarity thunderstorm electric fields show that positive ions are formed and move more readily over sharp-tipped electrodes than over blunter ones. From these findings, it appears that the electric field rates of intensification over sharp rods must be much greater than those over similarly exposed blunt rods for the initiation of upward-going leaders.

Calculations of the relative strengths of the electric fields above similarly exposed sharp and blunt rods show that although the fields, prior to any emissions, are much stronger at the tip of a sharp rod, they decrease more rapidly with distance. As a result, at a few centimeters above the tip of a 20-mm-diameter blunt rod, the strength of the field is greater than that over an otherwise similar, sharper rod at the same height. Since the field strength at the tip of a sharpened rod tends to be limited by the easy formation of ions in the surrounding air, the field strengths over blunt rods can be much stronger than those at distances greater than 1 cm over sharper ones.

The results of this study suggest that moderately blunt metal rods (with tip height–to–tip radius of curvature ratios of about 680:1) are better lightning strike receptors than are sharper rods or very blunt ones.

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Richard D. Scott, Paul R. Krehbiel, and William Rison

Abstract

Observations are presented in which the standard dual-polarization meteorological quantities (Z DR, ϕ dp, and ρ HV) are determined from simultaneous horizontal (H) and vertical (V) transmissions. The return signals are measured in parallel H and V receiving channels. Because the parameters are determined from simultaneous measurements they are not affected by Doppler phase shifts that increase the variance of ϕ dp and ρ HV when alternating H and V polarizations are transmitted. The approach has the additional advantage that a high-power polarization switch is not needed. The relative phases of the H and V components were such that the transmitted polarization was circular. Circular polarization is shown to detect horizontally oriented particles such as rain with the same effectiveness as linearly polarized transmissions, and optimally detects randomly oriented or shaped particles such as hail. Circular polarization also optimally senses nonhorizontally oriented particles such as electrically aligned ice crystals. By not needing to alternate between H and V transmissions it becomes practical to make polarization-diverse measurements by transmitting other orthogonal polarizations on successive pulses (e.g., left-hand circular and +45° slant linear) to aid in identifying precipitation types. It is shown that ρ HV from simultaneous transmissions provides the same information on randomly oriented scatterers as the linear depolarization ratio LDR from H or V transmissions, and that LDR does not need to be measured when information on particle canting is not important or is not needed.

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Timothy J. Lang, Steven A. Rutledge, Brenda Dolan, Paul Krehbiel, William Rison, and Daniel T. Lindsey

Abstract

Pyrocumulus clouds above three Colorado wildfires (Hewlett Gulch, High Park, and Waldo Canyon; all during the summer of 2012) electrified and produced localized intracloud discharges whenever the smoke plumes grew above 10 km MSL (approximately −45°C). Vertical development occurred during periods of rapid wildfire growth, as indicated by the shortwave infrared channel on a geostationary satellite, as well as by incident reports. The lightning discharges were detected by a three-dimensional lightning mapping network. Based on Doppler and polarimetric radar observations, they likely were caused by ice-based electrification processes that did not involve significant amounts of high-density graupel. Plumes that did not feature significant amounts of radar-inferred ice at high altitudes did not produce lightning, which means lightning observations may assist in diagnosing pyrocumulus features that could affect the radiative characteristics and chemical composition of the upper troposphere. The lightning was not detected by the National Lightning Detection Network, implying that pyrocumulus lightning may occur more frequently than past studies (which lacked access to detailed intracloud information) might suggest. Given the known spatial and temporal advantages provided by lightning networks over radar and satellite data, the results also indicate a possible new application for lightning data in monitoring wildfire state.

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Donald R. MacGorman, W. David Rust, Paul Krehbiel, William Rison, Eric Bruning, and Kyle Wiens

Abstract

Balloon soundings were made through two supercell storms during the Severe Thunderstorm Electrification and Precipitation Study (STEPS) in summer 2000. Instruments measured the vector electric field, temperature, pressure, relative humidity, and balloon location. For the first time, soundings penetrated both the strong updraft and the rainy downdraft region of the same supercell storm. In both storms, the strong updraft had fewer vertically separated charge regions than found near the rainy downdraft, and the updraft’s lowest charge was elevated higher, its bottom being near the 40-dBZ boundary of the weak-echo vault. The simpler, elevated charge structure is consistent with the noninductive graupel–ice mechanism dominating charge generation in updrafts. In the weak-echo vault, the amount of frozen precipitation and the time for particle interactions are too small for significant charging. Inductive charging mechanisms and lightning may contribute to the additional charge regions found at lower altitudes outside the updraft. Lightning mapping showed that the in-cloud channels of a positive ground flash could be in any one of the three vertically separated positive charge regions found outside the updraft, but were in the middle region, at 6–8 km MSL, for most positive ground flashes. The observations are consistent with the electrical structure of these storms having been inverted in polarity from that of most storms elsewhere. It is hypothesized that the observed inverted-polarity cloud flashes and positive ground flashes were caused by inverted-polarity storm structure, possibly due to a larger than usual rime accretion rate for graupel in a strong updraft.

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Eric C. Bruning, W. David Rust, Terry J. Schuur, Donald R. MacGorman, Paul R. Krehbiel, and William Rison

Abstract

On 28–29 June 2004 a multicellular thunderstorm west of Oklahoma City, Oklahoma, was probed as part of the Thunderstorm Electrification and Lightning Experiment field program. This study makes use of radar observations from the Norman, Oklahoma, polarimetric Weather Surveillance Radar-1988 Doppler, three-dimensional lightning mapping data from the Oklahoma Lightning Mapping Array (LMA), and balloon-borne vector electric field meter (EFM) measurements. The storm had a low flash rate (30 flashes in 40 min). Four charge regions were inferred from a combination of LMA and EFM data. Lower positive charge near 4 km and midlevel negative charge from 4.5 to 6 km MSL (from 0° to −6.5°C) were generated in and adjacent to a vigorous updraft pulse. Further midlevel negative charge from 4.5 to 6 km MSL and upper positive charge from 6 to 8 km (from −6.5° to −19°C) were generated later in quantity sufficient to initiate lightning as the updraft decayed. A negative screening layer was present near the storm top (8.5 km MSL, −25°C). Initial lightning flashes were between lower positive and midlevel negative charge and started occurring shortly after a cell began lofting hydrometeors into the mixed phase region, where graupel was formed. A leader from the storm’s first flash avoided a region where polarimetric radar suggested wet growth and the resultant absence of noninductive charging of those hydrometeors. Initiation locations of later flashes that propagated into the upper positive charge tracked the descending location of a polarimetric signature of graupel. As the storm decayed, electric fields greater than 160 kV m−1 exceeded the minimum threshold for lightning initiation suggested by the hypothesized runaway breakdown process at 5.5 km MSL, but lightning did not occur. The small spatial extent (≈100 m) of the large electric field may not have been sufficient to allow runaway breakdown to fully develop and initiate lightning.

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Christopher Maggio, Lee Coleman, Thomas Marshall, Maribeth Stolzenburg, Mark Stanley, Timothy Hamlin, Paul Krehbiel, William Rison, and Ron Thomas

Abstract

The lightning data that are recorded with a three-dimensional lightning mapping array (LMA) are compared with data from an electric field change sensor (in this case a flat-plate antenna operated both as a “slow” and a “fast” antenna). The goal of these comparisons is to quantify any time difference that may exist between the initial responses of the two instruments to a lightning flash. The data consist of 136 flashes from two New Mexico thunderstorms. It is found that the initial radiation source detected by the LMA usually precedes the initial response of both the slow and fast antennas. In a small number of cases, the flat-plate antenna response precedes the initial LMA source, but by no more than 2 ms. The observations of such a close time coincidence suggest that the first LMA radiation source of each flash was located at or very near the flash-initiation point. Thus, the first LMA radiation source and the initial sequence of sources from a lightning flash can be used as remote sensing tools to give information about the magnitude of the electric field (relative to lightning-initiation thresholds) and the direction of the electric field at the initiation location.

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Donald R. MacGorman, W. David Rust, Terry J. Schuur, Michael I. Biggerstaff, Jerry M. Straka, Conrad L. Ziegler, Edward R. Mansell, Eric C. Bruning, Kristin M. Kuhlman, Nicole R. Lund, Nicholas S. Biermann, Clark Payne, Larry D. Carey, Paul R. Krehbiel, William Rison, Kenneth B. Eack, and William H. Beasley

The field program of the Thunderstorm Electrification and Lightning Experiment (TELEX) took place in central Oklahoma, May–June 2003 and 2004. It aimed to improve understanding of the interrelationships among microphysics, kinematics, electrification, and lightning in a broad spectrum of storms, particularly squall lines and storms whose electrical structure is inverted from the usual vertical polarity. The field program was built around two permanent facilities: the KOUN polarimetric radar and the Oklahoma Lightning Mapping Array. In addition, balloon-borne electric-field meters and radiosondes were launched together from a mobile laboratory to measure electric fields, winds, and standard thermodynamic parameters inside storms. In 2004, two mobile C-band Doppler radars provided high-resolution coordinated volume scans, and another mobile facility provided the environmental soundings required for modeling studies. Data were obtained from 22 storm episodes, including several small isolated thunderstorms, mesoscale convective systems, and supercell storms. Examples are presented from three storms. A heavy-precipitation supercell storm on 29 May 2004 produced greater than three flashes per second for 1.5 h. Holes in the lightning density formed and dissipated sequentially in the very strong updraft and bounded weak echo region of the mesocyclone. In a small squall line on 19 June 2004, most lightning flashes in the stratiform region were initiated in or near strong updrafts in the convective line and involved positive charge in the upper part of the radar bright band. In a small thunderstorm on 29 June 2004, lightning activity began as polarimetric signatures of graupel first appeared near lightning initiation regions.

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Timothy J. Lang, L. Jay Miller, Morris Weisman, Steven A. Rutledge, Llyle J. Barker III, V. N. Bringi, V. Chandrasekar, Andrew Detwiler, Nolan Doesken, John Helsdon, Charles Knight, Paul Krehbiel, Walter A. Lyons, Don MacGorman, Erik Rasmussen, William Rison, W. David Rust, and Ronald J. Thomas

During May–July 2000, the Severe Thunderstorm Electrification and Precipitation Study (STEPS) occurred in the High Plains, near the Colorado–Kansas border. STEPS aimed to achieve a better understanding of the interactions between kinematics, precipitation, and electrification in severe thunderstorms. Specific scientific objectives included 1) understanding the apparent major differences in precipitation output from supercells that have led to them being classified as low precipitation (LP), classic or medium precipitation, and high precipitation; 2) understanding lightning formation and behavior in storms, and how lightning differs among storm types, particularly to better understand the mechanisms by which storms produce predominantly positive cloud-to-ground (CG) lightning; and 3) verifying and improving microphysical interpretations from polarimetric radar. The project involved the use of a multiple-Doppler polarimetric radar network, as well as a time-of-arrival very high frequency (VHF) lightning mapping system, an armored research aircraft, electric field meters carried on balloons, mobile mesonet vehicles, instruments to detect and classify transient luminous events (TLEs; e.g., sprites and blue jets) over thunderstorms, and mobile atmospheric sounding equipment. The project featured significant collaboration with the local National Weather Service office in Goodland, Kansas, as well as outreach to the general public. The project gathered data on a number of different cases, including LP storms, supercells, and mesoscale convective systems, among others. Many of the storms produced mostly positive CG lightning during significant portions of their lifetimes and also exhibited unusual electrical structures with opposite polarity to ordinary thunderstorms. The field data from STEPS is expected to bring new advances to understanding of supercells, positive CG lightning, TLEs, and precipitation formation in convective storms.

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Timothy J. Lang, Stéphane Pédeboy, William Rison, Randall S. Cerveny, Joan Montanyà, Serge Chauzy, Donald R. MacGorman, Ronald L. Holle, Eldo E. Ávila, Yijun Zhang, Gregory Carbin, Edward R. Mansell, Yuriy Kuleshov, Thomas C. Peterson, Manola Brunet, Fatima Driouech, and Daniel S. Krahenbuhl

Abstract

A World Meteorological Organization weather and climate extremes committee has judged that the world’s longest reported distance for a single lightning flash occurred with a horizontal distance of 321 km (199.5 mi) over Oklahoma in 2007, while the world’s longest reported duration for a single lightning flash is an event that lasted continuously for 7.74 s over southern France in 2012. In addition, the committee has unanimously recommended amendment of the AMS Glossary of Meteorology definition of lightning discharge as a “series of electrical processes taking place within 1 s” by removing the phrase “within 1 s” and replacing it with “continuously.” Validation of these new world extremes 1) demonstrates the recent and ongoing dramatic augmentations and improvements to regional lightning detection and measurement networks, 2) provides reinforcement regarding the dangers of lightning, and 3) provides new information for lightning engineering concerns.

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