triggered-lightning properties inferred from measured currents and

161 Pages·2005·1.3 MB·English

TRIGGERED-LIGHTNING PROPERTIES INFERRED FROM MEASURED
CURRENTS AND VERY CLOSE MAGNETIC FIELDS

By

ASHWIN B. JHAVAR

A THESIS PRESENTED TO THE GRADUATE SCHOOL
OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF
MASTER OF SCIENCE

UNIVERSITY OF FLORIDA

2005
Copyright 2005

by

Ashwin B. Jhavar
ACKNOWLEDGMENTS
I would like to thank Dr. Vladimir A. Rakov for his infinite patience, guidance, and
support throughout my graduate studies at the University of Florida. I would like to thank
Dr. Martin A. Uman and Dr. Douglas M. Jordan for their valuable suggestions during the
weekly lightning meetings. I sincerely thank Jens Schoene, Jason Jerauld, Rob Olsen,
Brian DeCarlo, and Vinod Jayakumar for helping me with the data and software, and for
other innumerable favors (without which I would not have been able to complete my
thesis). Research in my thesis was funded in part by National Science Foundation. The
data analyzed in the thesis were originally acquired with NSF and FAA funding.

iii
TABLE OF CONTENTS

page
ACKNOWLEDGMENTS.................................................................................................iii
LIST OF TABLES.............................................................................................................vi
LIST OF FIGURES..........................................................................................................vii
ABSTRACT.....................................................................................................................xiii
CHAPTER
1 INTRODUCTION........................................................................................................1
2 LITERATURE REVIEW.............................................................................................3
2.1 Cumulonimbus........................................................................................................3
2.2 Cloud Charge Distribution......................................................................................4
2.3 Mechanisms of Cloud Electrification.....................................................................6
2.3.1 Convection Mechanism................................................................................7
2.3.2 Graupel-ice Mechanism................................................................................7
2.4 Downward Negative Lightning Discharges to Ground........................................10
2.5 Artificial Initiation (Triggering) of Lightning Using the Rocket-and-Wire
Technique...............................................................................................................15
2.5.1 Classical Triggering....................................................................................16
2.5.2 Altitude Triggering.....................................................................................18
2.6 Previous Studies of Displacement Current Associated with Triggered
Lightning................................................................................................................19
2.6.1 Theory.........................................................................................................19
2.6.2 Estimation of Displacement Current Contribution at 50 m........................21
3 CHARACTERIZATION OF EXPERIMENTAL DATA USED IN THIS STUDY.25
3.1 Magnetic Field Measuring Techniques................................................................25
3.2 Experimental Setup...............................................................................................27
3.2.1 ICLRT Overview........................................................................................27
3.2.2 1997 Experiments.......................................................................................29
3.2.3 1999 Experiments.......................................................................................30
3.2.3.1 Instrumentation for Current Measurements.....................................32
3.2.3.2 Instrumentation for Electric Field Measurements............................33
iv
3.2.3.3 Instrumentation for Electric Field Derivative Measurement............33
3.2.4 2000 Experiments.......................................................................................33
3.2.5 2001 Experiments.......................................................................................35
3.3 Data Presentation..................................................................................................36
3.3.1 General Information...................................................................................36
3.3.2 Channel-base current..................................................................................40
3.3.3 Magnetic Field............................................................................................42
3.3.4 Electric field derivative (dE/dt)..................................................................47
4 ESTIMATION OF LEADER AND RETURN-STROKE CURRENTS FROM
MEASURED MAGNETIC FIELDS..........................................................................51
4.1 Introduction...........................................................................................................51
4.2 Estimation of Currents Using Ampere’s Law......................................................51
4.3 Discussion and Summary.....................................................................................77
5 DISPLACEMENT CURRENT ASSOCIATED WITH LEADER/RETurN
STROKE SEQUENCES IN TRIGGERED LIGHTNING.........................................80
5.1 Displacement Current Estimates from Measured Magnetic Fields and
Channel-Base Currents..........................................................................................80
5.2 Displacement Current Estimates from dE/dt Signatures at 15 and 30 m.............92
5.3 Discussion and Summary...................................................................................106
6 SUMMARY..............................................................................................................108
7 RECOMMENDATIONS FOR FUTURE RESEARCH..........................................112
APPENDIX DISPLACEMENT CURRENT GRAPHS.................................................113
LIST OF REFERENCES.................................................................................................144
BIOGRAPHICAL SKETCH...........................................................................................147

v
LIST OF TABLES
Table page

2-1 Displacement currents I estimated from measured electric field data at times
d
prior to and at the onset of a 20 kA peak stroke current..........................................22
3-1 Summary of mean, standard deviation, GM (geometric mean), and sample sizes
of measured peak current, peak magnetic field and electric field derivative...........37
5-1 Displacement currents estimated using eq. 5.4 (step-wise approximation of dE/dt
distance dependence) and eq. 5.5 (linear approximations of dE/dt distance
dependence)..............................................................................................................95
5-1 (Contd.) Displacement currents estimated using eq. 5.4 (step-wise
approximation of dE/dt distance dependence) and eq. 5.5 (linear approximations
of dE/dt distance dependence)..................................................................................96

vi
LIST OF FIGURES
Figure page

2-1 An isolated thundercloud in central New Mexico, with a rudimentary indication
of how electric charge is thought to be distributed inside and around the
thundercloud, as inferred from the remote and in situ observations..........................5
2-2 Balloon measurements of corona current and the inferred vertical electric field E
versus altitude and air temperature inside a small storm in New Mexico on 16
August 1981, which produced no lightning...............................................................6
2-3 Illustration of the convection mechanism of cloud electrification.............................8
2-4 Charge transfer by collision in the graupel-ice mechanism of cloud
electrification. It is assumed that the reversal temperature TR is -15 oC and that
it occurs at a height of 6 km.......................................................................................9
2-5 A vertical tripole representing the idealized gross charge structure of a
thundercloud such as that shown in Figure 2-1; the negative screening layer
charges at the cloud top and the positive corona space charge produced at
ground are ignored here............................................................................................10
2-6 Various processes comprising a negative cloud-to-ground lightning flash.............11
2-7 Sequence of events in classical triggered lightning. The upward positive leader
and initial continuous current constitute the initial stage.........................................16
2-8 Sequence of events in altitude-triggered lightning leading to the establishment of
a relatively low-resistance connection between the upward-moving positive
leader tip and the ground. The processes that follow the sequence shown, an
initial continuous-current and possibly one or more downward-leader--upward-
return-stroke sequences, are similar to their counterparts in classical triggered
lightning. The rocket speed is of the order of 102 m s-1...........................................18
2-9 Qualitative illustration of the shape of the time derivative of the ground level
vertical electric field of a nearby return stroke . Since displacement current
density is simply related to the derivative of electric field by a constant, it has
the same waveshape.................................................................................................24
vii
2-10 From top to bottom, magnetic field measured at 50 m during stroke 5 of Flash
96-23, the corresponding field according to Ampere’s law for magnetostatics, as
applied to the measured channel-base current, and the difference between the
two............................................................................................................................24
3-1 Thevenin equivalent circuit of a loop antenna.........................................................25
3-2 Norton equivalent circuit for a loop antenna............................................................26
3-3 An overview of the ICLRT at Camp Blanding, Florida, 1999-2001. Not all test
objects are shown.....................................................................................................28
3-4 Photograph of lightning flash S0012 triggered from the underground launcher.....28
3-5 Locations of different instrumentation stations for 1997 multiple station
experiment................................................................................................................30
3-6 Data acquisition system used in the 1997 multiple-station experiment...................31
3-7 Experimental setup (placement of electric and magnetic field antennas) used in
SATTLIF for 2000...................................................................................................34
3-8 Setup of strike rod and ring mounted over launch tubes in 2000.............................35
3-9 Measured channel-base current, Flash S9901, Stroke 3...........................................37
3-10 Magnetic field at 15 m, Flash S9901, Stroke 3........................................................38
3-11 Magnetic field at 30 m, Flash S9901, Stroke 3........................................................38
3-12 Electric field derivative (dE/dt) at 15 m, Flash S9901, Stroke 3.............................39
3-13 Electric field derivative (dE/dt) at 30 m, Flash S9901, Stroke 3.............................39
3-14 Return-stroke peak currents in (a) 1997, (b) 1999, (c) 2000, and (d) 2001.............40
3-15 Return-stroke peak currents in 1997, 1999, 2000 and 2001.....................................41
3-16 Peak magnetic fields measured at (a) 5 m, (b) 10 m, (c) 20 m, and (d) 30 m in
1997..........................................................................................................................42
3-17 Peak magnetic fields measured at (a) 15 m and (b) 30 m in 1999...........................43
3-18 Peak magnetic fields measured at (a) 15 m and (b) 30 m in 2000...........................44
3-19 Peak magnetic fields measured at (a) 15 m and (b) 30 m in 2001...........................45
3-20 Peak magnetic fields measured at (a) 15 m and (b) 30 m in 1997, 1999, 2000 and
2001..........................................................................................................................46
viii
3-21 Peak dE/dt fields measured at (a) 15 m and (b) 30 m in 1999.................................47
3-22 Peak dE/dt fields measured at (a) 15 m and (b) 30 m in 2000.................................48
3-23 Peak dE/dt fields measured at (a) 15 m and (b) 30 m in 2001.................................49
3-24 Peak dE/dt fields measured at (a) 15 m and (b) 30 m in 1999, 2000 and 2001.......50
4-1 A straight current channel of infinite length and B at a distance r...........................51
4-2 A streaked-image diagram of a dart leader—return-stroke sequence in a rocket-
triggered lightning flash...........................................................................................52
4-3 Magnetic field at 15 m, Flash S9901, Stroke 3........................................................54
4-4 Magnetic field at 30 m, Flash S9901, Stroke 3........................................................54
4-5 Dart leader current inferred using Ampere’s Law for magnetostatics from
measured magnetic fields at (a) 5 m, (b) 10 m, (c) 20 m, and (d) 30 m in 1997......55
4-6 Dart leader current inferred using Ampere’s Law for magnetostatics from
measured magnetic fields at (a) 15 m and (b) 30 m in 1999....................................56
4-7 Dart leader current inferred using Ampere’s Law for magnetostatics from
measured magnetic fields at (a) 15 m and (b) 30 m in 2000....................................57
4-8 Dart leader current inferred using Ampere’s Law for magnetostatics from
measured magnetic fields at (a) 15 m and (b) 30 m in 2001....................................58
4-9 Dart leader current inferred using Ampere’s Law for magnetostatics from
measured magnetic fields at 15 m in 1999, 2000, and 2001....................................59
4-10 Dart leader current inferred using Ampere’s Law for magnetostatics from
measured magnetic fields at 30 m in 1997, 1999, 2000, and 2001..........................60
4-11 Dart leader current inferred using Ampere’s Law for magnetostatics from
measured magnetic fields measured at 30 m vs. that at 15 m in 1999, 2000, and
2001..........................................................................................................................61
4-12 Dart leader current inferred using Ampere’s Law for magnetostatics from
magnetic fields measured at 15 m vs. leader current inferred from dE/dt
measurements in 1999, 2000, and 2001...................................................................62
4-13 Dart leader current inferred using Ampere’s Law for magnetostatics from
magnetic fields measured at 30 m vs. leader current inferred from dE/dt
measurements in 1999, 2000, and 2001...................................................................63
ix
4-14 Return-stroke peak currents inferred using Ampere’s Law for magnetostatics
from measured magnetic fields at (a) 5 m, (b) 10 m, (c) 20 m, and (d) 30 m in
1997..........................................................................................................................64
4-15 Return-stroke peak currents inferred using Ampere’s Law for magnetostatics
from measured magnetic fields at (a) 15 m and (b) 30 m in 1999...........................65
4-16 Return-stroke peak currents inferred using Ampere’s Law for magnetostatics
from measured magnetic fields at (a) 15 m and (b) 30 m in 2000...........................66
4-17 Return-stroke peak currents inferred using Ampere’s Law for magnetostatics
from measured magnetic fields at (a) 15 m and (b) 30 m in 2001...........................67
4-18 Return-stroke peak currents inferred using Ampere’s Law for magnetostatics
from measured magnetic fields at 15 m in 1999, 2000, and 2001...........................68
4-19 Return-stroke peak currents inferred using Ampere’s Law for magnetostatics
from measured magnetic fields at 30 m in 1997, 1999, 2000, and 2001.................69
4-20 Return-stroke peak current inferred using Ampere’s Law for magnetostatics
from measured magnetic fields at 30 m vs. that inferred from measured
magnetic fields at 15 m in 1999, 2000, and 2001....................................................70
4-21 Return-stroke peak current inferred using Ampere’s Law for magnetostatics
from measured magnetic fields at 15 m vs. measured return-stroke peak current
in 1999, 2000, and 2001...........................................................................................71
4-22 Return-stroke peak current inferred using Ampere’s Law for magnetostatics
from measured magnetic fields at 30 m vs. measured return-stroke peak current
in 1997, 1999, 2000, and 2001.................................................................................72
4-23 Leader vs. return-stroke currents inferred using Ampere’s Law for
magnetostatics from magnetic fields measured at 15 m in 1999, 2000, and 2001...73
4-24 Leader vs. return-stroke currents inferred using Ampere’s Law for
magnetostatics from magnetic fields measured at 30 m in 1999, 2000, and 2001...74
4-25 Comparison of I from (B +B ) and I from (B +B ) for 1999, 2000,
RS L RS 30m RS L RS 15m
and 2001...................................................................................................................75
4-26 Comparison of I from (B +B ) inferred using Ampere’s Law for
RS L RS 15m
magnetostatics vs. I measured at channel base in 1999, 2000, and 2001.............76
RS
4-27 Comparison of I from (B +B ) inferred using Ampere’s Law for
RS L RS 30m
magnetostatics vs. I measured at channel base in 1997, 1999, 2000, and 2001...77
RS
5-1 Superposition of measured magnetic field and channel-base current for Flash
S9903, stroke 3.........................................................................................................81
x