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OCR for page 41
2
INTRODUCTION
Positive Cloud-to-Grounc! Lightning
W. DAVID RUST
NOAA National Severe Storms Laboratory
Of the two common types of lightning flash, cloud-to-
ground (CG) and intracloud (IC), the CG flashes have
historically received more attention and study. This is
undoubtedly because they not only have more visible
channels that lend themselves to quantitative observa-
tions but also because they are responsible for most
death and damage caused by lightning. There are vari-
ous common names for several visually different fea-
tures of CG lightning, e.g., streak, forked, and ribbon.
However, the parameters often used in the scientific lit-
erature to categorize CG lightning are the polarity of
charge lowered, the magnitude of the current, and the
direction of propagation of the initiating leader and/or
ensuing return stroke. While most CG flashes transfer
negative charge from the cloud to the ground, early doc-
umentation of flashes that lower positive charge from
cloud to ground ~ + CG) is found in Berger's classical
study (1967) of lightning at Mount San Salvatore above
Lake Lugano in Switzerland. Berger used instruments
and photography to document strikes to towers on the
mountain and found a minority of + CG flashes. All but
one of the + CG flashes had an upward-propagating
leader followed by a downward-moving return stroke,
altogether opposite to the common negative CG flashes,
consisting of downward-propagating leaders followed
41
by upward return strokes. Because these + CG flashes
originated from tall towers, and apparently not even
from the mountain peaks without tall structures, they
have been termed "triggered" lightning, in contrast to
those occurring naturally. They nearly always had only
one return stroke. Berger's documentation of + CG
flashes to Mount San Salvatore established their devas-
tating nature; they had larger peak currents, charge
transfer, action integral, and duration than most nega-
tive flashes.
In general the instrumentation used to observe + CG
flashes is the same as that for other lightning. Typical
measurements include the electrostatic-field change for
the entire flash, fast electric- and magnetic-radiation-
field wave forms for the return stroke, optical tran-
sients, and thunder. Photographic and television re-
cordings are also often made. In addition, one type of
modern automatic lightning-strike locating system is be-
ing used experimentally in several parts of the world to
locate + CG flashes. However, its ability to distinguish
between intracloud and + CG flashes and its resulting
detection efficiency are still unknown.
RECENT AND ONGOING STUDIES
The recent and renewed interest in + CG flashes has
been stimulated by observations of winter storms in Ja
OCR for page 42
42
pan, where there is a large percentage (although low
total number) of CG flashes that are positive but not
triggered by tall structures. These storms have low cloud
bases and cloud tops at about 5 km and are much colder
in their lower region than typical summer thunder-
storms. Although they produce fewer flashes, there have
been several reports of strikes to aircraft. Whether the
strikes to aircraft were from + CG flashes is unknown,
but the prevalence of + CG flashes in these storms has
led to questions concerning their correlation to storm
structure and their relevance to aviation safety.
The existence of + CG flashes in Japan was first re-
ported by Japanese scientists, who have since conducted
collaborative research programs with scientists from the
United States and Sweden. The recently published
results by Brook et al. (1982) include the best quantita-
tive information on charge transfer for naturally occur-
ring + CG flashes. From their multistation network of
electrostatic field-change sensors, they calculated
charge transfer and the magnitude of continuing cur-
rent. As in Berger's earlier work on + CG flashes trig-
gered by tall towers, the naturally occurring + CG
flashes in Japan have large currents. While the largest
negative flashes are comparable with + CG flashes, it
appears that + CG flashes as a group tend to have signif-
icantly greater charge transfer and currents.
The combined data from seven Japanese winter
storms show a remarkably good correlation between the
percentage of + CG flashes per storm and the vertical
shear in the horizontal wind in the cloud layer. This sug-
gests that if a storm has its upper positive charge dis-
placed horizontally from its lower negative charge, the
production of + CG flashes is facilitated. This may well
hold true for such storms that are relatively shallow and
only mildly convective; preliminary results from other
investigations indicate that the correlation may not be
universal. Studies of this are in progress elsewhere.
The finding of naturally occurring + CG flashes in
Japan inspired the search for these flashes in the highly
sheared, large, and often severe springtime storms over
the Great Plains of the United States. The occurrence of
+ CG flashes during the mature and later stages of se-
vere storms has been verified in the observational pro-
gram at the National Severe Storms Laboratory (NSSL)
in Oklahoma (Rust et al., 1981a). Shown in Figure 3.1 is
a sketch of an isolated supercell thunderstorm, which
contains an intense updraft, wind shear, turbulence, a
large anvil, and a mesocyclone (rotation that can pro-
duce a tornado). Flashes to ground in the regions of
heavy precipitation have always been observed to lower
negative charge, while those from the upshear and
downshear anvil near the main storm tower can lower
either polarity of charge. Only a relatively few CG
W. DAVID RUST
flashes have been observed to emerge from beneath the
mesocyclone wall cloud (i.e., the visible manifestation
of a mesocyclone; see Figure 3.1), and two have been
documented as positive. Rust et al. (1981b) also ob-
served and obtained electric-field change records for 16
+ CG flashes in 30 minutes from a downshear anvil,
well away from the main tower of a severe storm shortly
after it produced a wall cloud. No other flashes to
ground were seen from the anvil during that time. In
most cases, + CG flashes appear visually to emanate
from high in the storm. The + CG flashes can cluster in
time and dominate CG activity for certain periods.
Positive CG flashes are observed both in isolated su-
percells and in squall-line storms. Preliminary analysis
of two squall lines indicates that + CG flashes often oc-
cur on the back side (relative to squall-line movement)
after the squall line has been in existence for several
hours. Acoustic recordings of thunder have been ana-
lyzed for two of six + CG flashes that were detected
within an 8-minute period in the back side of such a
squall line. They show a significant number of acoustic
sources, and thus channels, throughout a depth of 15 km
(the freezing level was about 4 km above ground). Dop-
pler radar data show that these + CG flashes were im-
bedded in the low radar reflectivities (less than 17 dBZ)
associated with very light precipitation behind the
squall line. Visual observations indicate that +CG
flashes may propagate horizontally through tens of kilo-
meters along the back side of the squall line before com-
ing to ground. Relatively large horizontal extent is also
apparent in observations of a few confirmed + CG
flashes during the final stage in some of the smaller
thunderstorms over the Rocky Mountains where usually
there are no or only a few + CG flashes (Fuquay, 1982~.
-CG AND
+CG FROM BACK O:
MAIN TOWER ~\
,4;
~ 1
Cam
WALL CLOUD-
+CG FROM
DOWNSHEAR ANVIL
FIGURE 3.1 Sketch of observed locations and polarities of CG
flashes from severe thunderstorms. The spiral denotes the updraft re-
gion and rotation (mesocyclone). Only negative CG flashes have been
observed in the intense precipitation. The horizontal scale of this storm
sketch is greatly compressed. After Rust et al. (1981a).
OCR for page 43
POSITIVE CLOUD-TO-GROUND LIGHTNING
The apparent correlation between + CG flashes and
storm development and severity suggested above are
quite tentative. This is due to the paucity of + CG flash
data, caused in part by the relative infrequency of + CG
flashes and in part by the need for corrobative verifica-
tion of the occurrence of each + CG flash until simpler
detection techniques are proven.
Nowhere is this more obvious than in the large data
bases recently acquired with a few of the automatic
strike-locating systems that have been modified but as
Vet unproved (as stated by the manufacturer, Lightning
Location and Protection, Inc.) for detection of positive
as well as negative ground flashes. If we had confidence
in the data bases obtained with these systems, much
could readily be learned, owing to the large numbers of
supposed + CG flashes that have and continue to be re-
corded. Samples of results from these as yet unproven
systems include the following:
1. Orville et al. (1983) reported a case study of a cy-
clone that produced several convective cells that moved
through their East Coast strike-locating network. They
found that while only 4 percent of all flashes to ground
in the storm are identified as + CG flashes, the percent-
age increased to 37 percent in the last hour of significant
CG flash activity. The also reported observations of a
higher percentage of + CG flashes in the later stages of
other storms.
2. The NSSL strike-Iocating system has been used to
study the diurnal variation of + CG flashes for summer-
time storms in Oklahoma, which tend to be less severe
than storms in the spring. The fraction of CG flashes
that were positive, averaged for 1 month, peaks about 2
hours later than the total CG flash activity.
3. Attempts have been made to ascertain if the + CG
flashes observed in severe storms are related to storm
severity or tornado occurrence. Two tornadic storms
have been analyzed using the strike-locating system in
Oklahoma. The ratio of + CG to all CG flashes appears
to be greater before and during tornadoes than after-
ward. This result is preliminary, not only because of un-
certainties in the performance of the strike locating sys-
tem but also because only two tornadic storms have been
analyzed. It also remains to be shown just what severe
storm parameters are related to the production of + CG
flashes. Some possibilities that are being examined are
mesocyclone strength, updraft speed, shear, and precip-
itation structure.
PHYSICAL CHARACTERISTICS
Although a few of the characteristics of + CG flashes
have been described above, it is worth considering what
43
we know in total about their characteristics, irrespective
of the storm conditions in which they occur. There are
two characteristics that appear consistently in all the re-
ported observations of + CG flashes: (1) the vast major-
ity have only a single return stroke, and (2) the return
stroke is often followed by continuing current. A repre-
sentative electrostatic field change for a + CG flash is
shown in Figure 3.2. Before the return stroke, there is
usually lengthy preliminary activity, which averages
about 0.25 see but can be as long as about 0.8 sec. If in-
cloud channels for + CG flashes are primarily horizon-
tal, as suggested by observations in several locations,
and if progression speeds are 105 m/see as typically ob-
served, then there may be large horizontal extent to
many + CG flashes. Indeed, horizontal movement of
+ CG flashes before they come to ground has been de-
termined from analysis of multistation field-change
data in Florida (Brook et al., 1983) and is indicated also
by visual observations of squall lines in Oklahoma.
The field change for the leader to ground has not yet
been extensively studied; however, both multistation
analysis for several flashes in Japan and photographic
evidence for a few flashes in the Rocky Mountains and
Oklahoma show that the leader propagates down from
the cloud to the ground, in contrast to the initially up-
ward-moving, triggered flashes to Mount San Salva-
tore. Recent studies in Japan indicate that + CG flashes
can be preceded by either a stepped or a nonstepped
leader.
The return-stroke wave form (Figure 3.3) is similar in
shape to that for negative flashes (Rust et al., 1981b),
with a relatively slow initial ramp followed by a faster
R
T
2 kV/m
1:
I
~C
I l
0.5
-
o
2115:37.27
I.Os
FIGURE 3.2 Typical electric-field change for + CG flash, recorded
at 2115:37.270 CST on May SO, 1982. The distance to the flash was
about 4 km. Time at the center of the scale (at 0.5 see) is 2115:37.300
CST. Interval a is the preliminary breakdown and leader; b is the time
from the return stroke through the end of the continuing current seen
on the streak photograph in Figure 3.4: and c is a larger interval of
possible continuing current or additional intracloud breakdown. The
return stroke (see Figure 3.3) is labeled R.
OCR for page 44
44
~ x H}H
,~
1 1 1 1 1
0 25 50 75 1 00 As
FIGURE 3.3 Electric-field wave form for return stroke of the + CG
flash in Figure 3. 2. The slow ramp, interval x, is followed in interval y
by ~ faster transition to peak, which is typical of both negative and
positive return strokes.
transition to peak. Thus far, the wave forms obtained in
several widely separated locations are essentially the
same. For 15 visually confirmed + CG flashes in Okla-
homa, the average zero-to-peak rise time is 6.9 ,usec. In
Florida, the average for three visually observed + CG
flashes at distances of 20-40 km is about 4 ,usec, a value
comparable with negative flashes in the same storm. In
both locations, some + CG flashes have been observed
with fast transition portions of the wave form having
rise times of less than 1 ,usec. While there was no visual
or photographic documentation, apparent + CG
flashes in Sweden have yielded zero-to-peak times of 5-
25 User for flashes at ranges of approximately 100 km;
the mean zero-to-peak times for + CG flashes was re
FIGURE 3.4 Streak-film photograph of
+ CG flash recorded on May SO, 1982 (see
Figures 3.2 and 3.3). Continuing current is
evident from the smearing of luminosity. It is
visible in the photograph for about 60 msec
and occurs during interval b in Figure 3. 2.
W. DAVID RUST
ported to be about twice that for negatives. The peak
amplitude of the electrostatic-field change due to the
return stroke itself averages about one tenth of the total
change for the entire flash. This appears due to the large
preliminary breakdown and continuing current, which
dominate the field change.
Analysis of multistation electric-field change mea-
surements by Brook et al. (1982) reveal + CG flashes
with continuing currents up to 105 A and positive charge
transfer to ground of up to several hundred coulombs.
Nakahori et al. (1982) made direct measurements of
currents in + CG flashes and found peak stroke currents
of 31 kA and total charge transfer of 164 coulombs in one
flash. The largest magnitudes of charge transfer are of-
ten more than 10 times greater than these for negative
flashes to ground in summer storms.
The duration of continuing currents in + CG flashes
has been reported to vary from a few milliseconds to
about 250 msec. However, the longer durations were
obtained from single-station field-change measure-
ments. The few streak-film and TV recordings of con-
tinuing current obtained thus far indicate that later por-
tions of the slow field change may not always be from
continuing current in the channel to ground but may be
additional intracloud activity. For example, the streak
photograph in Figure 3.4 (for the field change in Figure
3. 2) indicates a continuing current duration of 60 msec,
but Figure 3.2 alone could be interpreted as indicating
at least 200 msec of current flow. Either the luminosity
decreased below the threshold for the film, or the cur
OCR for page 45
POSITIVE CLOUD-TO-GROUND LIGHTNING
rent in the channel ceased and the remaining activity
was intracloud.
PRACTICAL IMPLICATIONS
The destructive nature and practical importance of
+ CG flashes to the electrical power industry are at least
partially documented. The multiline grounding and
wire-strand fusing in high-voltage transmission lines
cannot be explained by normal negative flashes to
ground in Japan.
Because of the usual occurrence of continuing cur-
rent, + CG flashes may ignite a disproportionate num-
ber of fires, especially in grasslands and forests. Of the
75 + CG flashes reported in the Rocky Mountain study,
all had field changes indicative of continuing current.
The apparent pattern is for + CG flashes to strike out-
side the rainfall, further enhancing the likelihood of
their starting a fire. One third of all storm days in a 3-
year period had + CG flashes within a 30-km radius of
the U.S. Forest Service observing site at Missoula, Mon-
tana. Thus on any given day the fire-starting probability
from + CG flashes in mountain thunderstorms appears
significant.
The results to date of the observations in severe storms
suggest that + CG flashes may be correlated with storm
severity and tornado occurrence. If this hypothesis were
shown to be true, it would enhance existing severe-
storm detection and warning capabilities. Additional
observations to test this hypothesis are in progress.
An area of increasing importance and study is the ef-
fect of lightning on new-generation aircraft (see Chap-
ter 5, this volume). The relevance of + CG flashes to this
problem is currently unknown; however, the reports of
strikes to aircraft in the winter storms in Japan suggest
that + CG flashes were involved. There are several as-
pects of + CG flashes needing additional clarification to
ascertain whether these flashes pose an unusual threat to
aircraft. They include the presence of fast return-stroke
rise times and large peak currents; the frequent occur-
rence of continuing current; and the apparent ten-
dency, especially in squall lines, for + CG flashes to
propagate through large horizontal and vertical extents
and to be in low radar reflectivity regions, which can
appear "innocent" on radar.
Because + CG flashes are a small percentage of the
total flashes that most storms produce, appear to have
large spatial extent, and do not seem to have a unique
electric-field change, it is difficult to verify + CG
flashes. However, because of their potentially devastat-
ing nature and their possible link to storm severitv~
+ CG flashes have become an important research topic
in several parts of the world.
45
AREAS NEEDING ADDITIONAL RESEARCH
It is worth noting those general areas of research that
are needed to increase our understanding of + CG
flashes and our ability to determine and cope with their
effects on important technologies. These research areas
are listed below but not in order of priority.
1. Measure electric-field changes and wave forms for
a large number of confirmed + CG flashes to establish
their typical characteristics with greater certainty.
2. Determine what storm types and environmental
conditions are conducive to + CG flashes for storms
throughout the year.
3. Relate the production of + CG flashes to storm
evolution and structure, including also flash initiation
and propagation characteristics.
4. Determine typical and extreme peak currents.
5. Evaluate the capabilities of automatic strike-lo-
cating systems in identifying + CG flashes, especially
their detection efficiency and false identification rate.
6. Determine the significance of + CG flashes to avi-
ation, especially to new-generation aircraft (typified by
composite structures and computer-controlled flight).
7. Determine the importance of the threat posed by
+ CG flashes to power distribution systems, including
whether they are the cause of the unexpected large num-
ber of faults on power lines in various parts of the United
States.
REFERENCES
Berger, K. (1967). Novelobservationsoflightning discharges: Results
of research on Mount San Salvatore, J. Franklin Inst. 283, 478-525.
Brook, M., M. Nikano, P. Krehbiel, and T. Takeuti (1982~. The elec-
trical structure of the Hokuriku winter thunderstorms, J. Geophys.
Res. 87, 1207-1215.
Brook, M., P. Krehbiel, D. MacLaughlan, T. Takeuti, and M. Na-
kano (1983). Positive ground stroke observations in Japanese and
Florida storms, in Proceedings in Atmospheric Electricity, L. H.
Ruhnke and J. Latham, eds., A. Deepak Publishing, Hampton,
Va., pp. 365-369.
Fuquay, D. M. (1982). Positive cloud-to-ground lightning in summer
thunderstorms, J. Geophys. Res. 87, 7131-7140.
Nakahori, K., T. Egawa, and H. Mitani (1982). Characteristics of
winter lightning currents in Hokuriku district, manuscript
82WM205-3, IEEE Power Engineering Society 1982 Winter Meet-
ing (Jan. 31-Feb. 5, 1982, New York).
Orville, R. E., R. W. Henderson, and L. F. Bosart (1983). An East
Coast lightning detection network, Bull. Am. Meteorol. Soc. 64,
1029-1037.
Rust, W. D., W. L. Taylor, D. R. MacGorman, and R. T. Arnold
(1981a). Research on electrical properties of severe thunderstorms in
the Great Plains, Bull. Am. Meteorol. Soc. 62, 1286-1293.
Rust, W. D., D. R. MacGorman, and R. T. Arnold (1981b). Positive
cloud-to-ground lightning flashes in severe storms, Geophys. Res.
Lett. 8, 791-794.
Representative terms from entire chapter:
charge transfer
