Review of Recent Lightning Research at the University of Florida
Vladimir A. Rakov, Martin A. Uman, Rajeev Thottappillil
Presented first is a brief survey of the lightning studies performed by the Lightning Research Laboratory at the University of Florida (UF), often in collaboration with other research groups, during the last ten years (1984-1994). The studies are discussed in Section 2 and are grouped into three categories:
(1) Parameters of the lightning discharge that are considered important
for lightning protection;
(2) Modeling of the lightning return stroke;
(3) Lightning-induced voltages on power distribution lines.
Second, we give in Section 3 an overview of the triggered lightning discharge and the lightning-triggering facility at Camp Blanding, Florida, now operated by UF.
The review includes a comprehensive list of references that contain more detailed information on the research we outline here.
2. Primary Results of the UF Lightning Studies in 1984-1994
2.1 Parameters of the Lightning Discharge that are Considered
Important for Lightning Protection
Number of strokes per flash [1-3]. More than 80% of cloud-to-ground discharges contain more than one stroke, with the maximum number of strokes per flash observed in Florida being 18. The observed percentage of single-stroke flashes in Florida (17%) is similar to the 14% found for New Mexico thunderstorms, and both figures are significantly lower than those presently found in the lightning protection literature (for example, 45% is given in ).
Lightning termination on ground [5, 6]. Roughly half of all lightning discharges to earth, both single- and multiple-stroke flashes, strike ground at more than one point with the spatial separation between the channel terminations being up to many kilometers. This result has important and generally unrecognized implications for presently accepted lightning protection concepts.
Relative stroke intensity within a flash [5, 6]. About one third of cloud-to-ground flashes contain at least one subsequent stroke with electric field peak, and, by theory, current peak, greater than the first-stroke peak. This relatively high percentage suggests that such flashes are not unusual, contrary to the implication of most lightning protection and lightning test standards.
Submillisecond interstroke intervals [7, 8]. It appears that two leader-return stroke sequences may occur in the same lightning channel within 1 ms of less. This finding raises questions regarding the adequacy of the short-duration end of the interstroke interval distribution presently found in the literature.
Lightning parameters in different geographical locations [9, 10]. Return-stroke current waveforms, peak currents, charge transfer, action integral, and interstroke intervals for artificially-initiated lightning discharges in Florida and Alabama are similar, but the lightning peak currents and interstroke intervals from Florida and Alabama are different from those for artificially-initiated lightning in New Mexico (TRIP '81 experiment).
Estimation of lightning currents from measured electromagnetic fields [11, 12]. Theoretical relations between lightning current and the radiation component of the vertical electric field have been derived using lightning return-stroke models found in the literature. An improved empirical formula for predicting the return-stroke peak current I from the measured, at a range D, peak electric field E has been obtained: I = 1.5 - 0.037 DE, where I is in kA and taken as negative, E is positive and in V/m, and D is in km.
Close electric fields produced by dart leaders [13, 14]. The leader field change at 30 m may be as fast as that of the return-stroke field change, whereas most studies on lightning-induced overvoltages on power lines ignore any electric field changes prior to the return stroke.
Additional information regarding recent findings on lightning parameters important for lightning protection can be found in [15-20].
2.2 Modeling of the Lightning Return Stroke
There has been considerable interest lately in lightning return-stroke
modeling. This interest has been motivated by the desire to have relatively
straightforward techniques (1) for deriving lightning current parameters
from remotely-measured electromagnetic fields (the so-called inverse source
problem), and (2) for predicting the coupling and resultant effects of
the fields of nearby lightning on airborne vehicles and on ground-based
objects like power lines. The UF group has made significant progress in
the development and experimental evaluation of new lightning return-stroke
models which both allow a better understanding of the physical processes
occurring in the return stroke and make possible more accurate calculation
of return-stroke electric and magnetic fields, given the channel-base current
and other pertinent parameters. Additionally, five of the return-stroke
models found in literature have been evaluated using experimental data,
and recommendations are made regarding suitability of those models [21,
Further information regarding our recent work on lightning return-stroke modeling, some in collaboration with other research groups, can be found in [23-27].
2.3 Lightning-Induced Voltages on Power Distribution Lines
The University of Florida lightning research group has performed three major experiments on lightning-induced effects in overhead test lines: in Tampa, Florida in 1979 and at the NASA Kennedy Space Center in 1985 and in 1986. The data acquired include induced line voltages and the causative electric and magnetic fields for the case of (1) cloud-to-ground lightning more distant than a few kilometers [28, 29], (2) overhead cloud lightning , and (3) very close (20 m) artificially-initiated lightning [31-33]. Both time domain and frequency domain techniques have been used to model the observed voltages. The models tested can predict reasonably well the measured waveshapes of the induced voltages for categories 1 and 2 above. Work is still in progress on category 3. It is important to note that in the case of close lightning the fields at different points along the line can not be inferred from a single-station field measurement, as can be done in the case of distant lightning, since the fields vary strongly with distance. A model of return stroke from which to calculate the fields at different distances from the lightning channel therefore must be used in addition to any available field measurements.
More information regarding our work on lightning induced voltages on power distribution lines, some in collaboration with other research groups, can be found in [34-37].
3. Triggered Lightning Discharge for Lightning Research and Lightning Testing
3.1 Why Triggered Lightning?
An understanding of the physical properties and deleterious effects of lightning is critical to the optimum protection of power and communication lines, aircraft, spacecraft, and other active and passive structures, as well as to the prediction and tracking of lightning. Many aspects of lightning are not yet well understood and are in need of research that often requires the termination of lightning channel at or in the immediate vicinity. The probability for a natural lightning to strike a given point on the earth's surface or an object or structure of interest is very low, even in areas of relatively high lightning activity, such as Florida. Simulation of the lightning channel in a high-voltage laboratory has very limited application, since it does not allow the reproduction of many lightning features important for lightning protection and it does not allow the testing of large distributed systems such as overhead power lines. The most promising tool for studying both the direct and the induced effects of lightning is an artificially initiated (or triggered) lightning discharge from a thunderstorm cloud to a designated point on ground. In most respects the triggered lightning is a controllable analog of natural lightning.
3.2 What is Triggered Lightning?
The most effective technique for artificial lightning initiation involves launching a small rocket trailing a thin grounded wire toward the charged cloud overhead. The cloud charge is indirectly sensed through measuring the electric field at ground, with values of 4 to 10 kV/m being generally a good indicator of favorable lightning triggering conditions. When the rocket, ascending at about 200 m/s, is about 200-300 m high, the field enhancement near the rocket tip launches a positively charged (in the most common case of predominantly negative charge at the bottom of the cloud) leader propagating toward the cloud. This leader vaporizes the trailing wire and initiates, on its arrival at the cloud, a current (so-called continuous current) of the order of several hundred amperes flowing along the wire vapor to the instrumented triggering facility. There often follows, after the cessation of the continuous current, several leader-return stroke sequences traversing the same path to the triggering facility. The return strokes in triggered lightning are similar if not identical to subsequent return strokes in natural lightning, although the initial processes in natural and classical triggered lightning are distinctly different. Recent advances now make possible the reproduction of even the initial processes in natural lightning by triggering lightning at higher altitudes, but this technique produces a ground strike point that is only predictably within a few hundred meters rather than precisely as in the case of classical triggering technique.
3.3 The Camp Blanding Triggering Facility
The Camp Blanding lightning triggering facility is located half-way between Gainesville and Jacksonville, Florida, about 40 km north-east of Gainesville. The facility is operated by the Lightning Research Laboratory of the University of Florida (Gainesville). The Laboratory is one of the leading lightning research groups in the world. The Laboratory was founded in the 1970s and is well known for its contributions to various areas of lightning research and protection (see Section 2 above).
The triggering site occupies a flat, open field with dimensions approximately 400 m by 700 m, under agreement between the University of Florida and the Camp Blanding Florida Army National Guard Base. The site includes a 1 km open-loop test underground power distribution system, a 1 km test overhead power line, two rocket launchers, a launch control complex, an office trailer, storage facilities, and four instrumentation buildings located along the underground distribution system. Air space is controlled by Camp Blanding so that rocket launching can be done at essentially any time. Several areas are available at the site to build new structures for testing and research purposes, as needed. The facility was originally designed by the Electric Power Research Institute (EPRI) and Power Technologies, Inc. (PTI) to study the responses (voltages and currents) of the test underground distribution system and the test overhead power line to lightning strikes. Besides the capability of measuring voltages and currents in buried and overhead conductors, electric and magnetic field measurements, video recording, still photography, and current measurements at the rocket launcher are generally available.
3.4 What Can be Studied?
— Physical properties of the lightning discharge.
— Interaction of lightning (both direct and induced effects) with underground and overhead systems, electronic devices, various structures and materials.
— Effectiveness of various lightning protection schemes, including the direct comparison between schemes.
— Fidelity of the laboratory simulation of lightning.
— Accuracy and detection efficiency of existing and future lightning locating systems.
— Evaluation of proposed techniques for the estimation of lightning parameters from remotely measured electric and magnetic fields.
— Effectiveness of various types of lightning protection grounding.
— Evaluation of various mathematical models of the lightning discharge, the lightning striking mechanism, lightning induced effects, etc.
3.5 What Can be Tested?
— Complete power and communication systems containing or
not containing lightning protection.
— Power system equipment
Switchgear and fuses
Electronic power controls
Power quality devices
— Communication system equipment.
— Aircraft and spacecraft equipment.
The research results reviewed in Section 2 show that a number of properties of lightning important for lightning protection, as well as the detailed mechanism of lightning interaction with power lines, are in need of further investigation. Many properties of natural lightning and its effects on various objects can be studied using lightning discharges artificially initiated from the thundercloud to a designated point on ground. Such studies are presently being performed by the University of Florida at Camp Blanding, Florida.
Parameters of the Lightning Discharge
[l] RAKOV, V. A., UMAN, M. A.: Some Properties of Negative Cloud-to-Ground Lightning. Proc. of 20th International Conference on Lightning Protection, Paper 6.4, Interlaken, Switzerland, 1990.
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 RAKOV, V. A., UMAN, M. A., THOTTAPPILLIL: Review of Lightning Properties from Electric Field and TV Observations. J. Geophys. Res., 99, 1994, pp. 10,745-10,750.
 ANDERSON, R. B., ERIKSSON, A. J.: Lightning Parameters for Engineering Application. Electra, 69, 1980, pp. 65-102.
 RAKOV, V. A., THOTTAPPILLIL, R., UMAN, M. A.: First vs. Subsequent Stroke Intensity and Multiple Channel Terminations in Cloud-to-Ground Lightning. Proc. of 21st International Conference on Lightning Protection, Berlin, Germany, September 22-25, 1992. 1992, pp. 13-18.
 THOTTAPPILLIL, R., RAKOV, V. A., UMAN, M. A., BEASLEY, W. H., MASTER, M. J., SHELUKHIN, D. V.: Lightning Subsequent Stroke Electric Field Peak Greater than the First Stroke Peak and Multiple Ground Terminations. J. Geophys. Res. 97, 1992, pp. 7503-7509.
 RAKOV, V. A., UMAN, M. A.: Origin of Lightning Electric Field Signatures Showing Two Return-Stroke Waveforms Separated in Time by a Millisecond or Less. J. Geophys. Res. 99, 1994a, pp. 8157-8165.
 RAKOV, V. A., UMAN, M. A.: On the Duration of Time Intervals Between Lightning Return Strokes. Proc. of 22nd International Conference on Lightning Protection (ICLP), September 19-23, 1994. Budapest, Hungary, Paper la-04. published by Technical University of Budapest, Budapest, Hungary, 1994b, 5 p.
 JORDAN, D. M., IDONE, V. P., RAKOV, V. A., UMAN, M. A., BEASLEY, W. H., JURENKA, H.: Observed Dart Leader Speed in Natural and Triggered Lightning. J. Geophys. Res. 97, 1992, pp. 9951-9957.
 FISHER, R. J., SCHNETZER, G. H., THOTTAPPILLIL, R., RAKOV, V. A., UMAN, M. A., GOLDBERG, J.: Parameters of Triggered Lightning Flashes in Florida and Alabama. J. Geophys. Res. 98, 1993, pp. 22,887-22,902.
 RAKOV, V. A., THOTTAPPILLIL, R., UMAN, M. A.: On the Empirical Formula of Willett et al. Relating Lightning Return Stroke Peak Current and Peak Electric Field. J. Geophys. Res. 97, 1992b, pp. 11,527-11,533.
 RACHIDI, F., THOTTAPPILLIL, R.: Determination of Lightning Currents from Far Electromagnetic Fields. J. Geophys. Res. 98, 1993, pp. 18,315-18,321.
 RUBINSTEIN, M., UMAN, M. A., THOMSON, E. M., MEDELIUS, P., RACHIDI, F.: Measurements and Characterization of Ground Level Vertical Electric Fields 500 m and 30 m from Triggered Lightning. Proc. of 9th International Conference on Atmospheric Electricity, June 15-19, 1992, St. Petersburg, Russia. 1992, pp. 276-278.
 UMAN, M. A., RAKOV, V. A., VERSAGGI, J. A., THOTTAPPILLIL, R., EYBERT-BERARD, A., BARRET, L., BERLANIDIS, J.-P., BADOR, B., BARKER, P. P., HNAT, S. P., ORAVSKY, J. P., SHORT, T. A., WARREN, C. A., BERNSTEIN, R.: Electric Fields Close to Triggered Lightning. Proc. of International Symposium on Electromagnetic Compatibility EMC '94 ROMA, September 13-16, 1994, Rome, Italy. 1994, pp. 33-37.
 RAKOV, V. A., UMAN, M. A.: Long Continuing Current in Negative Lightning Ground Flashes. J. Geophys. Res. 95, 1990a, pp. 5455-5470.
 RAKOV, V. A., UMAN, M. A.: Some Properties of Negative Cloud-to-Ground Lightning Flashes versus Stroke Order. J. Geophys. Res., 95, 1990b, pp. 5447-5453.
 RAKOV, V. A., UMAN, M. A.: Waveforms of First ind Subsequent Leaders in Negative Lightning Flashes. J. Geophys. Res., 95, 1990c, pp. 16,561-16,577.
 RAKOV, V. A., UMAN, M. A., JORDAN, D. M., PRIORI III, C. A.: Ratio of Leader to Return-Stroke Electric Field Change for First and Subsequent Lightning Strokes. J. Geophys. Res. 95, 1990, pp. 16,579-16,587.
 RAKOV, V. A., UMAN, M. A.: Long Continuing Currents in Negative Cloud-to-Ground Lightning Flashes: Occurrence Statistics and Hypothetical Mechanism (in Russian). Proceedings of the USSR Academy of Sciences (Izvestiya AN SSSR, ser. Fizika Atmosferi i Okeana), 27, No. 4, 1991, pp. 376-390.
 FISHER, R. J., SCHNETZER, G. H., THOTTAPPILLIL, R., RAKOV, V. A., UMAN, M. A., GOLDBERG,, J. D.: Negative Subsequent Strokes: Natural Versus Triggered Lightning. Proc. of 22nd International Conference on Lightning Protection (ICLP), September 19-23, 1994, Budapest, Hungary. Paper Ic-02, published by Technical University of Budapest, Budapest, Hungary, 1994, 6 p.
Return Stroke Modeling
 THOTTAPPILLIL, R., UMAN, M. A., DIENDORFER, G.: Influence of Channel
Base Current and Varying Return Stroke Speed on the Calculated Fields of
Three Important Return Stroke Models. Proc. of 1991 International Aerospace
and Ground Conference on Lightning and Static Electricity, Cocoa Beach,
Florida, April 16-19, 1991. 1991a, pp. 118.1-118.9.
 THOTTAPPILLIL, R., UMAN, M. A.: Comparison of Lightning Return Stroke Models. J. Geophys. Res. 98, 1993, pp. 22,903-22,914.
 DIENDORFER, G., UMAN, M. A.: An Improved Return Stroke Model with Specified Channel-Base Current. J. Geophys. Res. 95, 1990, pp. 13,621-13,644.
 NUCCI, C. A., DIENDORFER, G., UMAN, M. A., RACHIDI, F., IANOZ, M., MAZZETTI, C.: Lightning Return Stroke Current Models with Specified Channel-Base Current: A Review and Comparison. J. Geophys. Res. 95, 1990, pp. 20,395-20,408.
 RAKOV, V. A., DULZON, A. A.: A Modified Transmission Line Model for Lightning Return Stroke Field Calculations. Proc. of 9th International Symposium on Electromagnetic Compatibility, March 12-14, 1991, Zurich Switzerland, Paper 44H1, published by ETH Zentrum-IKT, Zurich, Switzerland. 1991, pp. 229-235.
 THOTTAPPILLIL, R., McLAIN, D. K., DIENDORFER, G., UMAN, M. A.: Extension of the Diendorfer-Uman Lightning Return Stroke Model to the Case of a Variable Upward Return Stroke Speed and a Variable Downward Discharge Current Speed. J. Geophys. Res. 96, 1991b, pp. 17,143-17,150.
 THOTTAPPILLIL, R., UMAN, M. A.: Lightning Return Stroke Model with Height-Variable Discharge Time Constant. J. Geophys. Res. 99, 1994, pp. 22,773-22,780.
Lightning-induced Voltages on Power Distribution Lines
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E. M.: An Experimental Test of a Theory of Lightning Induced Voltages on
an Overhead Wire. IEEE Trans. EMC, Vol. 31, 1989, pp. 376-383.
 GEORGIADIS, N,, RUBINSTEIN, M., UMAN, M. A., MEDELIUS, P. J., THOMSON, E. M.: Lightning-Induced Voltages at Both Ends of a 448-m Power-Distribution Line. IEEE Trans. EMC, Vol. 34, 1992, pp. 451-460.
 YACOUB, Z., RUBINSTEIN, M., UMAN, M. A., THOMSON, E. M., MEDELIUS, P.: Voltages Induced on a Power Distribution Line by Overhead Cloud Lightning. Proc. of 1991 International Conference on Lightning and Static Electricity, Cocoa Beach, Florida, April 16-19, 1991.
 RUBINSTEIN, M., UMAN, M. A., THOMSON, E. M., MEDELIUS, P.J.: Voltages Induced on a Test Distribution Line by Artificially Initiated Lightning at Close Range: Measurement and Theory. Proc. of 20th International Conference on Lightning Protection, Interlaken, Switzerland, September 24-28, 1990.
 RUBINSTEIN, M., UMAN, M. A., THOMSON, E. M., MEDELIUS, P. J.: Characterization of Vertical Electric Fields and Associated Voltages Induced on an Overhead Power Line from Close Artificially-Initiated Lightning. Proc. of 1991 International Conference on Lightning and Static Electricity, Cocoa Beach, Florida, April 16-19, 1991.
 RUBINSTEIN, M., UMAN, M. A., MEDELIUS, P. J., THOMSON, E. M.: Measurements of the Voltage Induced on an Overhead Power Line 20 m from Triggered Lightning. IEEE Trans., EMC, Vol. 36, No. 2, 1994, pp. 134-140.
 MASTER, M. J., UMAN, M. A.: Lightning Induced Voltage on Power Lines: Theory. IEEE Trans. PAS, PAS-103, 1984, pp. 2502-2518.
 MASTER, M. J., UMAN, M. A., BEASLEY, W. H., DARVENIZA, M.: Lightning Induced Voltages on Power Lines: Experiment. IEEE Trans. PAS, PAS-103, 1984, pp. 2519-2529.
 RUBINSTEIN, M., UMAN, M. A.: Review of the University of Florida Research on Lightning Induced Voltages on Power Distribution Lines. Proc. of 21st International Conference on Lightning Protection, Berlin, Germany, September 21-25, 1992. 1992, pp. 189-193.
 NUCCI, C. A., IANOZ, M., RACHIDI, F., RUBINSTEIN, M., TESCHE., F. M., UMAN, M. A., MAZZETTI, C.: Modeling of Lightning Induced Voltages on Overhead Lines: Recent Developments. Proc. of International Symposium on Electromagnetic Compatibility EMC '94 ROMA, September 13-16, 1994, Rome, Italy. 1994, pp. 44- 49.
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