A digital camera capable of 5400 frame-per-second time resolution will be equipped with diffraction gratings previously built by the investigator to document optical emissions from naturally occurring lightning flashes beneath thunderstorms. Spectral data will be obtained in the 380-850 nm range from the entire visible portion of lightning channels encompassing flash elements that may include return strokes, stepped and dart leaders, M-strokes, and episodes of "continuing current" (thought to be responsible for a disproportionate amount of lightning damage to structures and equipment) at spatial and temporal resolutions of 10's of meters and microseconds, respectively. These data will be analyzed to provide measures of channel temperature, electron density, and other thermodynamic information. Observing activities will be conducted during the climatological springtime thunderstorm maximum at a primary site located at the National Weather Center in Norman, Oklahoma, and during the off-season at the principal investigator's home institution (Texas A&M University) in southern Texas. Extensive supporting meteorological facilities and special measurements are available at the Oklahoma site, and include finely resolved (nanosecond-scale) optical imaging of lightning channels recorded by another NSF-supported investigator based at the University of Oklahoma. Both sites are encompassed by the National Lightning Detection Network (NLDN) as well as more specialized lightning mapping array (LMA) equipment. Together, these ancillary systems will provide contextual 4D information on lightning channel location, geometry and propagation (both visible and in-cloud), as well as the polarity, estimated peak current and flash multiplicity (i.e. number of return strokes) for lightning channels reaching the ground.
The intellectual merit of this research centers upon improved understanding of the spectroscopically-derived properties of natural lighting viewed at fine spatial and temporal resolution, which will lead to improved understanding of the physics of natural lightning. Additional insights are also possible concerning lightning's contribution to key atmospheric chemical processes.
Broader impacts of this work will include training of university graduate and undergraduate students, outreach to K-12 educational institutions, and increased collaboration between multiple university and government facilities involved in lighting research.
supported in this project are part of the Upward Lightning Triggering Study (UPLIGHTS). This latter study is a three-year National Science Foundation-funded field project that is taking place in Rapid City, South Dakota, from April 2012 to September 2014. Since 2004, GPS time-synchronized optical observations of upward lightning have been conducted from 10 towers in Rapid City. These towers range in height from 91 to 191 m and are situated along an elevated ridge line that runs north–south through Rapid City. The height of the ridge reaches approximately 180 m above the surrounding terrain. The natural conditions around Rapid City (open-sky country, low-height urban area, and tower locations) are favorable for optical observations of multiple towers. The working hypothesis of UPLIGHTS is that upward leaders from the towers are primarily triggered by 1) the approach of horizontally propagating negative stepped leaders associated with either intracloud development or following a positive cloud-to-ground (+CG) return stroke, and/or 2) a +CG return stroke as it propagates through a previously formed leader network that is near the towers, and that specific storm types are favorable for the occurrence of upward lightning. There are a number of reasons why it is important to understand how upward lightning is triggered by nearby flashes and to quantify the types and components of flashes responsible for this triggering. With the increasing number of tall structures being built, there will be a corresponding increase in the number of upward lightning flashes from these structures. Quantifying the contribution of upward lightning to the total flash production in the vicinity of a tower will show how anthropogenic activity likely is increasing the total number of lightning flashes to ground in the vicinity of tall towers and to what scale. A better understanding of how nearby flashes trigger upward lightning will help to quantify the increased rates of lightning events and the increased exposure of these objects to lightning current. It may also result in methods to reduce or eliminate the initiation of upward leaders from tall structures or in improved lightning protection standards, since existing protection standards are based on attachment of downward, CG lightning. In order to understand the conditions for triggering upward leaders from tall objects by nearby lightning flashes and to determine a quantifiable flash component (e.g., return stroke, horizontal negative leader development) responsible for this triggering, the following questions will be addressed: What types of flashes (intracloud or CG) and their properties (spatial development relative to the towers, electrical potential, polarity, and current) affect or are critical for the initiation of upward leaders from tall towers? What types of storms (e.g., mesoscale convective systems, supercells, multicells), region of storms (e.g., anvil region, convective core, trailing stratiform area), and storm development stage (e.g., mature, dissipating) are present when upward lightning occurs? What conditions are required for triggering upward leaders on multiple tall objects during the same flash? As in the past, optical observations will be obtained using GPS time-synchronized high-speed cameras operating from 1,000 to 100,000 ips, along with standard-speed video and digital still-image camera systems. The electric field environment will be sampled using sensors measuring the ambient electric field (electric-field meter) and electric-field change (fast and slow antennae). Furthermore, two VHF interferometers will be used to locate three-dimensional leader development. These electromagnetic data will be time-correlated with optical observations along with NLDN data, which will provide flash type, timing, location, polarity, and peak current for those flashes detected by the network. Our resulting findings may benefit society by increasing understanding of, and safety from, upward lightning from tall buildings, which may contain people, and tall structures that provide services to society such as telecommunications and energy production (in the case of wind turbines). Since extant lightning-protection standards are based on downward lightning, this research may identify unique hazards associated with upward lightning that presently are not known or are not being addressed by the standards. This research may allow for the quantification of upward lightning as a percentage of total lightning and determination of whether there is a meaningful increase in upward lightning by anthropogenic activity. It will help visualization of all types of lightning and therefore improve safety through public education and outreach. The research may result in improved detection of upward lightning by lightning location systems. These improvements would directly benefit lightning location system data users such as power companies, fire managers, and meteorologists, and therefore improve the services they provide to society. Furthermore, high-speed video segments have already been provided to National Weather Service forecast offices for public outreach and education.
