Lightning detection is based on the concept that a magnetic wave in the form of concentric circles is generated and propagated outward from the return stroke. This magnetic field is the basis of many techniques of lightning detection. Because much of the stroke is not parallel to the ground, the direction determination is contaminated by signals arriving with a significant vertical component. Earlier spherics measurements were typically accurate to only about 15o. Technological advances have incorporated modifications to magnetic lightning detection by using a wide band (omni-frequency) receiver to identify only those waves associated with the surface portion of the flash so only surface strokes are detected.
Lightning detection equipment detects mainly cloud-to-ground lightning activity. Cloud-to-cloud and intra-cloud lightning flashes are not as easy to detected. The primary reason for this limitation is that CG lightning flashes emit a better-defined electro-magnetic wave form which can propagate hundreds or thousands of miles. Lightning flashes vary in magnitude and type (transfer of + or - charge to the ground). The amount of current also varies which affects the strength of the wave form generated.
The ability to detect a lightning flash is partially dependent upon the peak current of its first return stroke. The higher the peak current, the more likely the flash will be detected. Peak currents are typically higher for winter-time flashes (Orville et al. 1987), so a greater percentage of CG flashes should be detected outside the 250 mi. (400 km) nominal range in winter than in summer. Other factors which affect CG detection efficiency are listed below.
2. Different Types of Sensors.
In the past, lightning ground flash density was determined by "flash counters" which had limitations in the effective range and discrimination against intra-cloud flashes. Ground flash density was also determined by estimating the number of "thunder days" (number of days per year on which thunder is heard).
There are many different types of lightning location technologies such as flash counters, spherics monitors, tuned VHF interferometric techniques, time-of-arrival and wide band magnetic direction finding. Lightning has also been observed via conventional radar, lightning mapping satellites, aircraft and shuttles. At the Kennedy Space Center, the use of in situ EM meters has been proven useful in determining the potential for lightning to occur. The Melbourne (Florida) Forecast Office has been using lightning data from the Lightning Detection and Ranging (LDAR) system which detects all electrical discharges in a convective cloud.
Internationally, two very different types of lightning detection and location networks have been developed. The SAFIR two-dimensional VHF interferometer system developed by the French aerospace research organization ONERA and commercialized by Dimensions of France, is used to provide detailesysinformation on all types of lightning activity within a relatively small area. The VLF Arrival-Time Difference (ATD) system, designed and operated by the United Kingdom Meteorological Office, detects and locates lightning at very long range, but with less detection efficiency. In addition, other networks cover portions of Europe, Asia, Australia, China, and Canada.
For ranges up to 250 mi. (400 km), the two most prominent methods of lightning detection are the wide band magnetic Direction Finding (DF), sometimes referred to as IMPACT sensors and the Time-Of-Arrival (TOA) sensors. The current lightning detection network in the United States uses these two types of technology.
3. Detection Networks.
Regional networks to measure the ground flash locations began in the United States in 1976 (Kreider et al, 1980) with the establishment of the western lightning detection network operated by the Bureau of Land Management (BLM). This was followed in 1979 with a research network in Oklahoma operated by the National Severe Storms Laboratory (NSSL) and, in 1982 by a second research network in New York managed by the State University of New York at Albany (SUNYA). These regional networks grew until complete coverage of the contiguous United States was obtained in 1989. The Electric Power Research Institute (EPRI) commenced the network's original formation for the purpose of helping utilities make objective decisions regarding line maintenance priorities, effective crew dispatch, and future design and placement of utility lines.
In the past, Atmospheric Research Systems, Inc (ARSI) was a company using the TOA system to supply the data for the AFOS LDS graphic. Another company in Tucson, Arizona providesysinformation using the DF technology. Late in 1993, these two companies merged and the data that had been appearing on the AFOS graphic using TOA technology was entirely replaced by data using the DF technology. The new company, Global Atmospherics, Inc. (GAI) in Tucson, Arizona, is a corporation engaged in the research, design, manufacturing and marketing of lightning detection systems.
Side-by-side testing of the DF and TOA technologies were conducted to identify each system's strengths and weaknesses. Results from test networks in Florida indicated that the TOA sensors were considerably more sensitive than the DF sensors as well as more accurate in location. The TOA sensors could identify both CG and CC lightning, since there is a distinct difference in the rise time from ambient to peak charge in CC and CG lightning. However, many CG strikes were misidentified as CC strikes since the sensor detected the dart leader before it actually reached the ground. The DF technology had fast waveforms analysis circuits which prevented this misidentification. Low detection efficiencies are often found in thunderstorms that have weak CG flashes with waveforms of small amplitude, or in storms that have ground flashes with long intra-cloud channels which create complex waveforms that are often rejected by the direction-finder software.
As a result of these tests and new user requirements by EPRI, GAI decided to merge the two systems (TOA and DF) to attain at least 1 km strike accuracy and detection efficiencies exceeding 90% which falls very slowly with range.
Today, there is a network of over 100 sensors combining the direction finding and time of arrival techniques into one integrated system to form a National Lightning Detection Network (NLDN). This network is capable of detecting cloud-to-ground lightning flashes at distances up to 400 km across the contiguous United States and extending hundreds of miles into both oceans and beyond the borders of Canada and Mexico. Processesysinformation is transmitted to the GAI Network Control Center (NCC) in Tucson. The ground stroke lightning data includes information on latitude and longitude, date and time, polarity, and amplitude.
Soon there will be the creation of the North American Lightning Detection Network (NALDN) as the US National Lightning Detection Network (NLDN) is combined with the new Canadian Lightning Detection Network (CLDN).
A map showing the location of the Impact and TOA sensors making up the U.S. network is shown in the figure below:
Location of the lightning sensors comprising the NLDN. The red triangles are impact sites; blue circles are TOA sites.
1. Lightning detection is based on the concept that a magnetic wave in the form of concentric circles is generated and propagated outward from the return stroke.
2. Lightning detection equipment detects mainly cloud-to-ground lightning activity.
3, There are several factors which affect lightning detection efficiency, including strength of the waveform, unique character of the thunderstorm, distance of the thunderstorm from the network and geography.
4. The current lightning detection network in the United States uses the wide band magnetic Direction Finding (DF) or IMPACT and time-of-arrival (TOA) sensors.
5. The ground stroke lightning data includes information on latitude and longitude, date and time, polarity, and amplitude.