Download - The Problem of Lightning Initiation Dr.-to-be Danyal Petersen Desert Research Institute Reno, NV
The Problem of The Problem of Lightning InitiationLightning Initiation
Dr.-to-be Danyal Petersen
Desert Research Institute
Reno, NV
What is lightning?What is lightning?
• Large-scale spark-type discharge of thundercloud space charge
How does a thundercloud become How does a thundercloud become charged?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 How is charge distributed in a typical thundercloud?typical thundercloud?
from Stolzenburg (1998)
Where does lightning typically Where does lightning typically occur in a thundercloud?occur in a thundercloud?
from Thomas (2001)
Lightning InitiationLightning 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
Positive StreamerPositive Streamer
•Electron avalanche liberates positive space charge
•Electron avalanches ahead of space charge effectively extend positive space into electrode gap
Positive Streamer SystemPositive 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 Phelps and Griffiths model of positive streamer systempositive streamer system
Electric field Electric field modification modification
due to due to multiple multiple
overlapping overlapping streamer streamer systemssystems
Sounds Great! So, what’s the Sounds Great! So, what’s the problem?problem?
0.0
3.0
6.0
9.0
12.0
0.0 100.0 200.0 300.0 400.0
E (kV/m)
Alti
tude (
km)
Thundercloud fields still too weak!
Local intensification Local intensification of the thundercloud of the thundercloud electric field near electric field near polarized plasma polarized plasma extremitiesextremities
One possible outcome of Runaway Breakdown
Another possible outcome of Another possible outcome of runaway breakdownrunaway breakdown
Local intensification Local intensification of the thundercloud of the thundercloud electric field at the electric field at the base of the base of the discharge regiondischarge region
A ‘Hybrid’ HypothesisA ‘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 Hybrid mechanism for local electric field intensificationfield intensification
So, how does this initiate lightning?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?
Lightning as a bipolar leaderLightning as a bipolar leader
• Simultaneous positive and negative leaders, conductively connected
• Require large currents in order to maintain channel heating
Great! (waving hands)Great! (waving hands) So a piping hot bipolar So a piping hot bipolar leader suddenly forms! leader suddenly forms!
Piece of cake, right? Piece of cake, right? (well, not quite…)(well, not quite…)
‘‘Space’ leaderSpace’ 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 Hypothesis: Embryonic lightning leader as an isolated space leaderleader as an isolated space leader
A pilot/space leader A pilot/space leader process could process could develop in intensified develop in intensified field region such as field region such as near positive near positive streamer system or streamer system or strong runaway strong runaway avalancheavalanche
RecapRecapRunaway breakdown
Hydrometeor-initiated positive streamer systems
Pilot/space leader process
How can this hypothesis be How can this hypothesis be verified?verified?
• In-situ studies – observation of physical processes in thunderstorms
• Laboratory studies – reproduction of physical processes in controlled environment