Synopsis of Positive Lightning Research

1
Synopsis of Positive Lightning Research

• Scott D. Rudlosky
• Henry E. Fuelberg

2
CG Physics

• The apparent increased damage associated with CG
  lightning is linked to its physical
  characteristics
• Greater peak current
• Smaller multiplicity (fewer return strokes)
• Long continuing current (LCC)
• 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 strokes
  (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 those of CG (Brook et
  al. 1982).

3
Long Continuing Current (LCC)

• LCC associated with CG typically follows the
  first stroke.
• LCC also occurs between strokes in multiple
  return stroke CG or CG flashes (Ron Holle,
  personal communication).
• Image depicts LCC from a multiple return stroke
  event.
• LCCs occur between each of the strokes (faint
  glow between return strokes).
• LCC is not directly measured by the NLDN.

Ron Holle (Holle Meteorology and Photography, Oro
Valley, AZ)
4
CG Mechanisms
Tilted Dipole (Tripole)

• Classic 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
5
Regional Distribution

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

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

6
Regional Distributions
7
CG Flash Densities (Flashes per km2 per season
(warm or cool)
8
CG Flash Densities (Flashes per km2 per year/warm
season)

• Total annual CG flash density (left) showing
  maxima near Tampa and along the Miami - West Palm
  Beach corridor.
• Warm season CG flash density (right), South
  Florida is compared with the panhandle region.
• The magnitude of the southwest Florida CG
  maximum is similar to that of northwest Florida,
  but the areal coverage is smaller.

9
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 cool
  season
• Maximum median peak current of 35 kA during the
  cool 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

10
Conclusions and Future Work

• The overall goals of this research were
  identifying patterns and characteristics of CG
  lightning and predicting its daily variability.
• Improvements to NLDN technology have presented
  new challenges that must be addressed before
  solid conclusions can be drawn from the data (see
  next slide).
• Sounding parameters will be linked to the
  percentage of CG within 100 km radii of sounding
  locations in Jacksonville, Miami, Tallahassee,
  and Tampa (several tools for this analysis
  already have been developed).
• Analysis techniques also are in development to
  obtain model derived proximity soundings to
  better identify the relationships between storm
  structure, CG lightning, and the local
  mesoscale environment (i.e., sounding
  parameters).

11
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 is a mix of CG and cloud
  discharges (Cummins et al. 2006).
• This population is far from small during the warm
  season in Florida.
• 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.

12
Forensic Studies

• Long continuing current most likely is
  responsible for the increased damage associated
  with CG flashes, but it is not directly measured
  by the NLDN.
• To relate individual CG (or CG) flashes to
  increased damage, a forensic study which links or
  relates CG flashes with damage to facilities is
  needed.
• Also, the mesoscale environment and individual
  storms can be examined for their roles on days
  that contain an unusually large number of these
  extremely damaging CG flashes (e.g., damage to a
  number of facilities or several lightning related
  house fires on a given day).