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Subject: G9) How much lightning occurs in
tropical cyclones?
Contributed by Chris Landsea
Surprisingly, not much lightning occurs in the inner core (within
about 100 km or 60 mi) of the tropical cyclone center. Only around a
dozen or less cloud-to-ground strikes per hour occur around the
eyewall of the storm, in strong contrast to an overland
mid-latitude mesoscale convective complex which may be observed
to have lightning flash rates of greater than 1000 per
hour maintained for several hours.
Hurricane Andrew's eyewall had less than 10 strikes per hour
from the time it was over the Bahamas until after it made
landfall along Louisiana, with several hours with no
cloud-to-ground lightning at all
(Molinari et al. 1994). However, lightning can be more
common in the outer cores of the storms (beyond around 100 km or
60 mi) with flash rates on the order of 100s per hour.
This lack of inner core lightning is due to the relative weak nature
of the eyewall thunderstorms. Because of the lack of surface heating
over the ocean ocean and the "warm core" nature of the tropical
cyclones, there is less buoyancy available to support the
updrafts. Weaker updrafts lack the super-cooled water (e.g.
water with a temperature less than 0° C or 32° F) that is
crucial in charging up a thunderstorm by the interaction of ice
crystals in the presence of liquid water
(Black and Hallett 1986).
The more common outer core lightning occurs in conjunction with the
presence of convectively-active rainbands
(Samsury and Orville 1994).
One of the exciting possibilities that recent lightning studies
have suggested is that changes in the inner core strikes - though the
number of strikes is usually quite low - may provide a useful forecast
tool for intensification of tropical cyclones.
Black (1975) suggested that bursts of inner core
convection which are accompanied by increases in electrical
activity may indicate that the tropical cyclone will soon
commence a deepening in intensity. Analyses of Hurricanes
Diana (1984), Florence (1988) and Andrew (1992), as well as an
unnamed tropical storm in 1987 indicate that this is often true
(Lyons and Keen 1994 and
Molinari et al. 1994).
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