Characteristics of Positive Cloud-to-Ground Lightning

1
Characteristics of Positive Cloud-to-Ground
Lightning
Scott D. Rudlosky Department of
Meteorology Florida State University
2
Outline

• Introduction
• Provide necessary background information on
  cloud-to-ground (CG) lightning.
• National Lightning Detection Network (NLDN) data
• Describe tools and methods used for measuring CG
  lightning.
• Results
• Large scale distribution
• Regional distributions
• Misclassification of cloud pulses
• Summary and Conclusions

• Future Work

3
Cloud-to-Ground (CG) Lightning

• Injuries and fatalities
• Structural damage
• Buildings
• Trees
• Utility lines
• Communications systems

www.lightningsafety.noaa.gov
4
Stroke vs. Flash vs. Strike

• A lightning flash can consist of one or more
  individual (return) stokes.
• Multiplicity of return strokes.
• A lightning strike is the point where CG
  lightning impacts the ground.

• A single CG flash can be responsible for more
  than one lighting strikes.
• Return strokes do not always follow the same
  channel to the ground.

Ron Holle (Oro Valley, AZ)
Johnny Autery
5
Positive CG (CG) Lightning

• Positive charge lowered from cloud to ground.
• First identified by Berger (1967) in Mount San
  Salvatore, Lugano, Switzerland.

• Account for 10 of CG lightning globally (Uman
  1987).
• Percentage varies by
• Season
• Geographical region
• Individual storm

• Greatest counts of CG in the contiguous U.S.
  occur in Florida (Orville and Huffines 2001)

6
CG Lightning

• Dispatchers at Florida Power and Light
  Corporation (FPL) have observed that damage to
  their facilities is often associated with CG
  lightning.

• CG lightning also has been considered for its
  role in the initiation of forest fires.
• The apparent increased damage is linked to
  physical characteristics of CG lightning.
• Greater peak current
• Smaller multiplicity (fewer return strokes)
• Long continuing current (LCC)

7
CG Physics

• CG is characterized by Rakov (2003)
• Greatest recorded lightning currents
• Largest charge transfer to the ground
• CG flashes usually consist of a single stroke,
  whereas 80 of CG flashes contain two or more
  (Rakov 2003).
• Positive return strokes tend to be followed by
  continuing currents that last for tens to
  hundreds of milliseconds (Rust et al. 1981).
• Continuing currents of CG are at least an order
  of magnitude greater than CG (Brook et al.
  1982).

8
Long Continuing Current (LCC)

• LCC associated with CG typically follows the
  first stroke.
• However, LCC also occurs between strokes in
  multiple return stroke CG or CG flashes (Ron
  Holle, personal communication).
• Image depicts LCC from multiple return stroke
  event.
• LCCs occur between each of the strokes (faint
  glow between return strokes).

Ron Holle (Holle Meteorology and Photography, Oro
Valley, AZ)
9
CG Lightning in Severe Storms

• Storms with gt 25 CG account for less than 10
  20 of all warm season severe storms in the
  eastern and southeastern U.S. (Carey and Buffalo
  2007).
• Some severe storms can produce gt 90 CG for
  short periods of time (Price and Murphy 2003
  Biggar 2002).
• Local mesoscale environment can influence the
  polarity of CG lightning by controlling a storms
  structure, dynamics, and microphysics (Carey and
  Buffalo 2007).

10
Charging Mechanism

• Non-inductive charging mechanism (NIC) occurs as
  cloud particles of varying size and phase collide
  in the charging zone (Saunders et al. 1991).
• Larger graupel or hail particles carry negative
  charge to the lower levels.

Classical Thunderstorm

• Updraft carries positively charged ice crystals
  to the upper levels (Saunders et al. 1991).
• Image obtained from Krehbiel (1986).

11
CG Mechanisms
Tilted Dipole (Tripole)

• Classical thunderstorms contain a dipole with
  positive charge over negative.
• Electrical structure of deep convection is more
  complex with three or more significant charge
  layers (Stolzenburg et al 1998).

Inverted Dipole (Tripole)
Precipitation Unshielding
12
CG Mechanisms

• Highly sheared environments lead to the advection
  of upper-level positive charge (Brook et al
  1982).
• Positive charge is then exposed to the ground.
• Explains CG occurring in the anvil region.
• Advection of charge also helps explain CG
  lightning that occurs in the stratiform region.

Tilted Dipole (Tripole)
13
CG Mechanisms

• Highly sheared environments lead to the advection
  of upper-level positive charge (Brook et al
  1982).
• Positive charge is then exposed to the ground.
• Explains CG occurring in the anvil region.
• Advection of charge also helps explain CG
  lightning that occurs in the stratiform region.

Tilted Dipole (Tripole)
14
CG Mechanisms

• After most of the heaviest precipitation has
  fallen from a cell, the upper positive charge is
  exposed to the ground.
• Abundance of positive charge is dependent on the
  duration and severity of an individual storm
  (Carey and Buffalo 2007).
• Explains CG which occurs in the dissipating
  stage of thunderstorms.

Precipitation Unshielding

• Severe storms can accumulate a large reservoir of
  positive charge (Carey and Buffalo 2007).

15
CG Mechanisms

• NIC mechanism is dependent on temperature, liquid
  water content (LWC), and collision velocity
  between particles.
• Under certain conditions, the riming graupel
  and/or hail particles are positively charged
  while smaller ice crystals are negatively charged
  (Saunders et al. 1991).
• Explains some of the CG occurring in the region
  of deepest convection and heaviest
  precipitation.

Inverted Dipole (Tripole)
16
Inverted Dipole (Tripole)

• Mesoscale environments can influence the polarity
  of CG within individual thunderstorms (Carey and
  Buffalo 2007).
• Higher cloud base
• Smaller warm cloud depth
• Greater conditional instability
• Greater buoyancy in the mixed phase zone
• Drier middle to lower troposphere
• Under these conditions, broad strong updrafts
  lead to larger LWC in the mixed phase zone and
  greater collision velocity, and in turn, the
  aforementioned modifications to the NIC mechanism
  (i.e. positive charge at low levels).

17
Seasonal Variability

• CG comprises a larger percentage of total CG
  during the cold season months (Engholm 1990).
• Greater wind shear
• Smaller warm cloud depth
• The tendency is for CG in more shallow
  convective regions, whereas CG is more prevalent
  in deep convection.
• This can be observed in bi-poles of individual
  thunderstorms.
• During the warm season in Florida, the lower to
  middle levels are moist, reducing the
  relative number of CG flashes.

18
Lightning Data

• CG lightning data were collected by the National
  Lightning Detection Network (Vaisala Inc.).
• Network consists of 113 sensors across the
  contiguous U.S.
• Began full time operation in 1989.
• Early applications included
• Directing spotter aircraft
• Electric utilities
• Insurance industry
• General aviation community

• Due in part to changing users and their specific
  needs, major upgrades were undertaken in
  1994-1995 and again in 2002- 2003.

Jerauld et al. 2005
19
1994-1995 Upgrade

• Goals (Grogan 2004)
• Report strokes in addition to flashes
• Improve location accuracy
• Increase percentage of CG flashes detected
• Report the peak current of CG flashes
• Results (Grogan 2004, Cummins 2006)
• Flash detection efficiency (DE) of 80-90
• 500 meter location accuracy
• Performance decreases near edges of the network
• This upgrade represented a major improvement.

20
2002-2003 Upgrade

• Goals
• Provide enhanced DE
• Improve location accuracy
• Increase network reliability
• Detect some cloud flashes
• Results
• Increased stroke DE from 40-50 to 60-80
• Provided flash DE of 90-95
• Jerauld et al. (2005) conducted a rocket
  triggered lightning study at Camp Blanding, FL
  from 2001-2003 and found

Cummins 2006

• Stroke DE near 100 for peak current (Ip) gt 30 kA
• Stroke DE 60-70 for 10 lt Ip lt 30 kA
• Stroke DE lt 30 for 5 lt Ip lt 10 kA
• Peak current underestimate of 18

21
2002-2003 Upgrade

• Only data since 2002 were used in this study.
• Prior to the most recent upgrade, it was
  suggested that all CG flashes with Ip lt 10 kA be
  removed because they were likely cloud pulses
  that were misclassified as weak positive flashes.
• A post upgrade study by Biagi et al. 2007 noted
• Clearly there is no unique threshold for
  classifying a small-positive report as a CG
  stroke, but and Ip of 15 kA appears to be the
  value where the number of false CG reports equals
  the number of correct reports.
• The more recent threshold (15 kA) is used for
  this study.
• Little is known about the characteristics of
  these misclassified weak positive events.

22
Goals

• Previous studies have shown Florida to be the
  lightning capital of the U.S.
• Describe the characteristics of CG Lighting in
  Florida.
• Annual flash densities are highly variable.
• Describe variations in CG and total CG flash
  densities across the state of Florida.
• No previous studies have compared the
  multiplicity and peak current by month and region
  in Florida.
• Describe regional and monthly variability in
  these characteristics.
• Investigate characteristics of apparently
  misclassified NLDN events (10 kA lt Ip lt 15 kA).

23
Procedures

• Current study analyzed 5 yrs of CG data
  2002-2006.
• 5 years of data are not sufficient to develop a
  true climatology.
• However, using years prior to 2002 was
  inappropriate.
• Flash densities were computed on a 2x2 km grid
  utilizing geographic information system (GIS)
  techniques.
• GIS provides a common spatial domain for
  computing flash densities and statistics (e.g.
  percentage positive).
• Flash densities have units of flashes km-2
  season-1 (warm and cold) and flashes km-2 year-1
  (annual).

• Warm season consists of May September, while
  cold season is the remaining months.

24
Analysis and Results
Photo Kane Quinnell
Photo Anonymous
25
Warm Season Total CG Flash Density
26
Warm Season Positive CG Flash Density
27
Cold Season Total CG Flash Density
28
Cold Season Positive CG Flash Density
29
Statistics for Entire Domain

• Composite months (i.e. January 2002, 2003, 2004,
  2005, and 2006).
• Statistics for CG are compared with those of
  total CG.

30
Annual Total CG Flash Densities
Orlando Area
Tampa / Saint Petersburg Region
Miami / West Palm Beach Corridor
31
Annual CG Flash Density Maximum
Tallahassee / Apalachicola Region
32
Regional Distribution

• The percentage of CG flashes that are positive
  varies by region and season. Percentages of CG
  are

• Greatest during the cold season
• Generally greater in the northwestern region and
  decrease southward along the peninsula

33
Regional Distribution

• Actual CG flash counts are more beneficial when
  accessing the threat which CG poses.
• Percentage of CG is highly dependent on the
  total number of CG flashes.
• Percentages and counts are within 100 km of
  sounding locations in Jacksonville, Tampa and
  Miami.

Composite Positive Flash Counts
34
Regional Distributions
Composite Mean Multiplicity
Composite Median Peak Current
Note the different scales for each figure.
35
Regional Distributions - Multiplicity
36
Regional Distributions Peak Current
Note the different scales for each region.
37
Daily Variability
Note the different scales for each month and
region.
38
NLDN Performance

• Sensors are more closely spaced in the Southeast
  U.S. and especially Florida.

• Closer spacing results in the detection of more
  weak positive events, and apparently more cloud
  pulses.

Jerauld et al. 2005
39
Misclassification of Cloud Pulses

• Prior to the 2002-2003 upgrade, a threshold of
  10 kA was recommended. Afterwards, the
  threshold was changed to 15 kA.
• The (small) population of positive discharges
  between 10-20 kA are a mix of CG and cloud
  discharges (Cummins et al. 2006).
• This population is far from small during the warm
  season in Florida.

40
Misclassification of Cloud Pulses

• Larger median peak current and smaller
  multiplicity of CG occur during the cold season
  (consistent with previous studies).
• However, during the warm season, CG flashes are
  characterized by smaller median peak current and
  greater multiplicity.
• Further research is needed to more accurately
  classify weak positive events.

41
Unusual Characteristics

• The increase in CG mean multiplicity has not
  been accounted for in previous studies.
• Side flashes responsible for the bolt from the
  blue are not always positive.

42
Summary and Conclusions

• Maximum annual flash density of 28.1 flashes km-2
  year-1.
• Warm season maxima total CG flash densities were
  located in
• The Tampa / Saint Petersburg region
• The greater Orlando area
• Between Lake Okeechobee and the Atlantic Ocean
• Cold season flash densities show a tendency
  toward the Northwestern portion of the domain.

43
Summary and Conclusions

• Percentage of CG was found to vary by season
• Maximum during January of 13.59
• Minimum during July of 2.41
• CG flashes showed
• Minimum mean multiplicity of 1.4 during the cold
  season
• Maximum median peak current of 35 kA during the
  cold season
• Maximum mean multiplicity of 1.7 during the warm
  season
• Minimum median peak current of 20 kA during the
  warm season
• -CG flashes showed
• Mean multiplicity fairly consistent throughout
  the year.
• Maximum median peak current of 18 kA during the
  warm season
• Minimum median peak current of 12 kA during the
  spring time.

44
Conclusions and Future Work

• March was characterized by episodic occurrence of
  CG, while July had almost daily lightning.
• On a given day, there is more CG in Jacksonville
  than Miami.
• Predicting this daily variability is the long
  term goal of the current research.
• Sounding parameters will be linked to the
  percentage of CG within 100 km radii of sounding
  locations in Jacksonville, Miami, Tallahassee,
  and Tampa.
• Isolate better methods for distinguishing between
  in-cloud and cloud-to-ground events.

45
Acknowledgments

• Prof. Henry E. Fuelberg
• Encouragement, confidence, and guidance
• Committee Members
• Dr. Phil Cunningham
• Dr. Mark Bourassa
• Mr. Irv Watson
• The Fuelberg Lab
• John Sullivan, Jeremy Halland, Steven Martinaitis
• Geoffrey Stano, Dr. Phillip Shafer

• Ron Holle (Vaisala Inc.)
• The Rudlosky Family
• Parents Bill and Carol
• Siblings Mark, Julie, and Kevin

46
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Questions or Comments?

• Self-portrait during the last month and a half.

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c)
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http//www.lightningsafety.noaa.gov/resources/Ligh
tning_Detection.pdf
NLDN sensor locations in the U.S.
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