1. The Problem of Lightning Initiation Dr.-to-be Danyal Petersen Desert Research Institute Reno, NV

2. What is lightning? Large-scale spark-type discharge of thundercloud space charge

3. 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)

4. How is charge distributed in a typical thundercloud? from Stolzenburg (1998)

5. Where does lightning typically occur in a thundercloud? from Thomas (2001)

6. Lightning Initiation Observation: Maximum observed thundercloud electric fields are 1/10 th the dielectric strength of air Hypothesis: Lightning initiation begins with corona discharges on polarized hydrometeors

7. Corona Electron avalanches occur in high- field region near point electrode

8. Positive Streamer Electron avalanche liberates positive space charge Electron avalanches ahead of space charge effectively extend positive space into electrode gap

9. 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

10. Phelps and Griffiths model of positive streamer system

11. Electric field modification due to multiple overlapping streamer systems

12. Sounds Great! So, what’s the problem? Thundercloud fields still too weak!

13. New Idea: Runaway Breakdown

14. Local intensification of the thundercloud electric field near polarized plasma extremities One possible outcome of Runaway Breakdown

15. Another possible outcome of runaway breakdown Local intensification of the thundercloud electric field at the base of the discharge region

16. 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

17. Hybrid mechanism for local electric field intensification

18. 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?

19. Morphology of a lightning leader

20. Lightning as a bipolar leader Simultaneous positive and negative leaders, conductively connected Require large currents in order to maintain channel heating

21. Great! (waving hands) So a piping hot bipolar leader suddenly forms! Piece of cake, right? (well, not quite…)

22. Pilot Process

23. ‘Space’ leader One of the pilots becomes heated enough to remain conductive Continued bipolar streamers further heat and extend pilot, becoming ‘space’ leader

24. 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 A pilot/space leader process could develop in intensified field region such as near positive streamer system or strong runaway avalanche

25. Recap Runaway breakdown Hydrometeor- initiated positive streamer systems Pilot/space leader process

26. How can this hypothesis be verified? In-situ studies – observation of physical processes in thunderstorms Laboratory studies – reproduction of physical processes in controlled environment

27. An idea Large parallel- plate electrode gap with suspended simulation ice crystal Interested in reproducing streamer system mechanism followed by space leader formation