d-to-Ground Lightning Activity in the Contiguous United States from 1995 to 1999 M
AY
2001
999
Z A J A C A N D R U T L E D G E
2001 American Meteorological Society
Cloud-to-Ground Lightning Activity in the Contiguous
United States from 1995 to 1999
B
ARD
A. Z
AJAC
*
AND
S
TEVEN
A. R
UTLEDGE
Department of Atmospheric Science, Colorado State University, Fort Collins, Colorado
(Manuscript received 16 February 2000, in nal form 7 August 2000)
ABSTRACT
The spatial and temporal distributions of cloud-to-ground lightning are examined over the contiguous United
States from 1995 to 1999 using data from the National Lightning Detection Network. Annual ash density,
annual lightning days, cumulative frequency distributions of daily ash counts, and annual and summertime
diurnal distributions of lightning are documented. The spatial, annual, and summertime diurnal distributions of
positive and negative polarity cloud-to-ground lightning are also documented. Over the same ve-year period,
the production of positive and negative lightning is examined over two case study areas located in the north-
central United States and along the Gulf Coast, centered on Sioux Falls, South Dakota, and Fort Rucker, Alabama,
respectively. Case studies include radarlightning analyses of signicant lightning episodes from 1996.
Maximum ash densities and lightning days are found over coastal regions of the southeastern United States.
Other prominent maxima are seen over parts of the southern Rocky Mountains and adjacent High Plains. Cumulative
frequency distributions indicate that throughout the contiguous United States roughly 10% of the days with lightning
accounted for 50% of lightning production. The majority of lightning was produced during summer (JuneAugust)
throughout the contiguous United States, except over the south-central United States and along and near the Pacic
coast. Summertime lightning activity over the western and eastern United States exhibited a diurnal cycle with
maximum frequencies in the afternoon to early evening. Over the central United States, summertime lightning
activity was complex with signicant longitudinal variations in daily activity and a tendency to occur at night.
Over most of the contiguous United States, a larger fraction of negative lightning was produced during summer
than positive lightning, and the diurnal cycle of positive lightning lagged the diurnal cycle of negative lightning
by up to two hours during summer. The main exception to these behaviors occurred over an area in the north-
central United States extending from the ColoradoKansas border to western Minnesota. Over this area, positive
lightning peaked during midsummer versus late summer for negative lightning, and the diurnal cycle of positive
lightning also peaked up to several hours prior to the maximum in the diurnal cycle of negative lightning during
summer. In addition, this area was characterized by maxima in the percentage of positive lightning and positive
mean peak current. The maximum in the percentage of positive lightning over the north-central United States
was caused by a dramatic increase in positive ash density to the east of the Rocky Mountains and a local
minimum in negative ash density over the area described above.
Results from the Sioux Falls case study indicate that positive lightning was produced primarily during summer
in the hours around sunset by isolated storms and convective lines in various stages of mesoscale convective system
(MCS) development. These convective events usually contained one or more storms that were characterized by
predominantly positive lightning, high positive ash rate, and large positive peak currents. Negative lightning
activity was produced later in the summer and throughout the night by more mature convective systems arranged
in lines or clusters. Over Fort Rucker, positive and negative lightning was produced throughout the year, by diurnally
forced storms during the warm season and by MCSs with areally extensive stratiform regions during the cold
season. Diurnally forced storms (MCSs) were characterized by a low (high) percentage of positive lightning.
1. Introduction
Thunderstorms play an important role in many human
societies. Thunderstorms produce benecial rainfall as
* Current afliation: Cooperative Institute for Research in the At-
mosphere, Colorado State University, Fort Collins, Colorado.
Corresponding author address: Bard Zajac, Cooperative Institute for
Research in the Atmosphere, Colorado State University, Fort Collins,
CO 80523.
E-mail: zajac@cira.colostate.edu
well as ooding, tornadoes, hail, strong winds, and light-
ning. Knowledge of the distribution of these weather
phenomena in space and time can be used to 1) infer
what physical processes control the occurrence of thun-
derstorms, 2) predict the benecial and destructive ef-
fects of thunderstorms, and 3) verify output from nu-
merical models that resolve the effects of thunderstorms.
Climatologies based on the weather phenomena men-
tioned above have been developed over the contiguous
United States for these and other reasons (Table 1). Col-
lectively, these climatologies document signicant spa-
tial, annual, and diurnal variations in thunderstorm ac-
tivity. However, the annual and diurnal distributions of 1000
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OLUME
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M O N T H L Y W E A T H E R R E V I E W
T
ABLE
1. Studies related to thunderstorm and convective activity in the contiguous United States. List is separated into national studies
(phenomena listed) and regional studies of lightning activity (region listed). Methods of documentation are summarized: spatial distributions
(S; e.g., map of thunderstorm days), annual or seasonal distributions (A), and diurnal distributions (D). A subscript s indicates whether
temporal distributions are documented over different areas (D
S
or A
S
). For lightning studies, S
FD
and S
LD
indicate that maps of ash density
and lightning days are presented, respectively, and P indicates that distinctions are made between positive and negative polarity cloud-to-
ground lightning.
National studies
Precipitation
Wallace (1975)
Dai et al. (1999)
D
S
D
S
Flash oods
Tornadoes
Large hail and strong winds
Audible thunder
Maddox et al. (1979)
Kelly et al. (1978)
Kelly et al. (1985)
Wallace (1975)
Court and Grifths (1981)
Easterling and Robinson (1985)
S, A
S,
D
S, A, D
S
S, A
S,
D
D
S
S, A, D
S
D
S
Lightning
Orville (1991)
Orville (1994)
Orville and Silver (1997)
Lyons et al. (1998)
Hufnes and Orville (1999)
Orville and Hufnes (1999)
Boccippio et al. (2001)
S
FD,
A
S
FD,
P
S
FD,
A, P
S
FD,
D, P
S
FD
S
FD,
A, P
S
FD
Regional studies of lightning activity
Florida peninsula
Western United States
Colorado and Florida
Northeastern United States
Oklahoma and Kansas
Maier et al. (1984)
Reap (1986)
Lopez and Holle (1986)
Orville et al. (1987)
Reap and MacGorman (1989)
D
S
S
FD,
S
LD,
D
S
FD,
S
LD,
A
S
, D
S
A, P
S
FD,
D, P
Northeastern United States
Southern Appalachians
Gulf Stream
Arizona
Arizona
Reap and Orville (1990)
Weisman (1990)
Biswas and Hobbs (1990)
King and Balling (1994)
Watson et al. (1994a,b)
S
FD,
D
S
FD,
D
S
S
FD,
P
S
FD,
D
S
S
FD,
A, D
S
Florida peninsula
New Mexico
Southern Great Lakes
Southeastern United States
Georgia
Reap (1994)
Fosdick and Watson (1995)
Clodman and Chisholm (1996)
Watson and Holle (1996)
Livingston et al. (1996)
S
FD,
D
S
FD,
S
LD,
D
S
S
FD,
S
LD,
D
S,
P
S
FD,
S
LD,
D
S
S
FD,
D
Arizona
Florida
Gulf Coast/Florida Panhandle
Lopez et al. (1997)
Hodanish et al. (1997)
Camp et al. (1998)
S
FD
S
FD,
A
S
S
FD,
D
cloud-to-ground (CG) lightning have not been docu-
mented over the contiguous United States in a consistent
manner. Therefore, in section 3a, the spatial, annual,
and summertime diurnal distributions of CG lightning
are examined over the contiguous United States from
1995 to 1999 using data from the National Lightning
Detection Network (NLDN). Results from this section
complement other climatologies and demonstrate the
spatial continuity of NLDN lightning observations.
In section 3b, the spatial, annual, and summertime
diurnal distributions of positive and negative polarity
CG lightning are examined over the contiguous United
States from 1995 to 1999. Documentation of these dis-
tributions provides a large-scale context for case studies
of thunderstorms and associated positive and negative
lightning production. Emphasis is placed on positive
lightning due to its anomalous nature. On average, pos-
itive lightning accounts for roughly 10% of CG light-
ning (10.6 million of the 128.9 million ashes detected
by the NLDN during 199599 were of positive polarity).
However, the absolute and/or relative number of positive
ashes tends to increase 1) during the dissipating stage
of nonsevere storms (Fuquay 1982) and severe storms
(Kane 1991), 2) over stratiform regions of mesoscale
convective systems (MCSs; Orville et al. 1988; Rut-
ledge and MacGorman 1988; Engholm et al. 1990; Rut-
ledge et al. 1990; Holle et al. 1994; Schuur and Rutledge
2000a,b), 3) during the mature stage of some hailstorms
and tornadic storms (MacGorman and Nielsen 1991;
Curran and Rust 1992; Branick and Doswell 1992; Sei-
mon 1993; Knapp 1994; MacGorman and Burgess 1994;
Stolzenburg 1994; Perez et al. 1997; Carey and Rutledge
1998; Bluestein and MacGorman 1998; Smith et al.
2000), and 4) during the cold season. The tendency for
positive lightning to occur during the dissipating stage
of nonsevere and severe storms has been tentatively
explained using the titled dipole (Brook et al. 1982) and
precipitation unshie