Transcript The Problem of Lightning Initiation

The Problem of
Lightning Initiation
Dr.-to-be Danyal Petersen
Desert Research Institute
Reno, NV
What is lightning?
• Large-scale
spark-type
discharge of
thundercloud
space charge
How does a thundercloud become
charged?
• Charging occurs at
updraft/downdraft
interface, where
mixed phase exists
• Involves collisions
between rimed ice
(graupel) and pristine
ice
Rime charging
Saunders (1998)
How is charge distributed in a
typical thundercloud?
from Stolzenburg (1998)
Where does lightning typically
occur in a thundercloud?
from Thomas (2001)
Lightning Initiation
Observation: Maximum observed
thundercloud electric fields are 1/10th the
dielectric strength of air
Hypothesis: Lightning initiation begins with
corona discharges on polarized hydrometeors
Corona
Electron
avalanches
occur in highfield region
near point
electrode
Positive Streamer
•Electron avalanche
liberates positive
space charge
•Electron avalanches
ahead of space
charge effectively
extend positive space
into electrode gap
Positive Streamer System
• In strong E fields,
positive streamers
intensify and branch
• As streamer system
accumulates positive
charge, equal
negative charge is
deposited in wake
Phelps and Griffiths model of
positive streamer system
Electric field
modification
due to
multiple
overlapping
streamer
systems
Sounds Great! So, what’s the
problem?
12.0
Altitude (km)
9.0
6.0
3.0
0.0
0.0
100.0
200.0
300.0
E (kV/m)
Thundercloud
fields still too
weak!
400.0
New Idea:
Runaway
Breakdown
One possible outcome of Runaway
Breakdown
Local intensification
of the thundercloud
electric field near
polarized plasma
extremities
Another possible outcome of
runaway breakdown
Local intensification
of the thundercloud
electric field at the
base of the
discharge region
A ‘Hybrid’ Hypothesis
• Runaway breakdown followed by
hydrometeor-initiated positive streamer
systems
• This relaxes the required field
intensification due to runaway breakdown,
allowing positive streamer systems to
make up the difference
Hybrid mechanism for local electric
field intensification
So, how does this initiate lightning?
• This is the ‘hand waving’ part of most
lightning initiation explanations, going
something like …”And then a miracle
(lightning) happens”.
• Clearly, local intensification of the electric
field can happen. But is it really that
simple?
Morphology of a lightning leader
Lightning as a bipolar leader
• Simultaneous
positive and
negative leaders,
conductively
connected
• Require large
currents in order to
maintain channel
heating
Great! (waving hands)
So a piping hot bipolar
leader suddenly forms!
Piece of cake, right?
(well, not quite…)
Pilot Process
‘Space’ leader
• One of the
pilots becomes
heated enough
to remain
conductive
• Continued
bipolar
streamers
further heat and
extend pilot,
becoming
‘space’ leader
Hypothesis: Embryonic lightning
leader as an isolated space leader
A pilot/space leader
process could
develop in intensified
field region such as
near positive
streamer system or
strong runaway
avalanche
Runaway
breakdown
Recap
Hydrometeorinitiated positive
streamer
systems
Pilot/space
leader process
How can this hypothesis be
verified?
• In-situ studies – observation of physical
processes in thunderstorms
• Laboratory studies – reproduction of
physical processes in controlled
environment
An idea
• Large parallelplate electrode gap
with suspended
simulation ice
crystal
• Interested in
reproducing
streamer system
mechanism
followed by space
leader formation