Intellectual Merit: Under this award the Principal Investigator (PI) will operate and analyze data from a twelve-station total lightning detection network (Lightning Detection and Ranging, LDAR II) around Houston, Texas. The National Science Foundation under a Major Research Initiative (MRI) grant funded the purchase of the equipment for the LDAR II network. A high cloud-to-ground (CG) lightning density anomaly had been previously identified over Houston, which raised a number of scientific questions. The primary scientific goal of this research is to address many outstanding questions about CG and intracloud (IC) flashes that characterize the total lightning occurring over Houston and the surrounding areas using the LDAR II network and the National Lightning Detection Network (NLDN). The combination of data from the LDAR II and the NLDN will allow the PI to determine 1) the location and amplitude of the first lightning flash within storms, 2) cloud flash rates, 3) cloud-to-ground flash rates, 4) the evolution of cloud/CG flash ratios, 5) the geometric extent of cloud discharge channels, 6) the initiation points of cloud and CG flashes, 7) the altitude from which the primary charge is lowered, 8) the area and volume in which charge is altered by lightning, 9) the polarity of CG strokes, 10) estimates of the peak currents in CG strokes, and 11) measurements of the multiplicity of strokes in CG flashes in the thunderstorms.
Broader impacts of the research: Data from the LDAR II will have an immediate impact on the research programs of scientists and students who are performing research outside the bounds of this award. At the moment, more than ten cooperating institutions have expressed an interest in this project and the associated information. The continuing operation of the network will provide instrumentation and enhance the infrastructure of understanding and forecasting weather in Houston, the fourth largest city in population in the United States. In addition, general public access to the experimental LDAR II real-time lightning displays is provided at no cost through the following web site: www.met.tamu.edu/ciams/ldar/index.html. Informal reports suggest that this is a popular free web site for the Houston community.
This research will contribute to the education of several graduate students who will directly participate in the research, which will include their sharing in data acquisition and analysis. Research results will be incorporated into advanced courses on cloud physics and mesoscale phenomena. Finally, this research will take advantage of ongoing outreach and educational Texas A&M University programs to engage in mentorship of undergraduate and high school students with special effort to involve under-represented students.
The didgital high-speed spectroscopic lightning studies 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.
