How does lightning start in fields far below breakdown?
In plain words
Air normally conducts only in very strong electric fields, about ten times stronger than any measured inside a storm. Something lets lightning start anyway.
Precise statement
Maximum measured in-cloud fields are of order 5 to 10 statV/cm, roughly an order of magnitude below the conventional breakdown field at cloud altitudes. Identify the initiation sequence: field enhancement at hydrometeors launching streamers, seeding by cosmic-ray relativistic runaway electrons, or fast positive breakdown. An answer is a quantitative model reproducing radio-mapped initiation events (timing, extent, current) from measured fields and hydrometeor properties.
What would settle it
Simultaneous in-cloud field and hydrometeor measurements with high-resolution radio interferometry (e.g. LOFAR) at the initiation point.