Threshold electric fields for streamer ignition from colliding charged hydrometeors in thunderstorms

How lightning initiates in thunderstorm fields well below the conventional breakdown electric field Ek, which is defined by the equality of the ionization and dissociative attachment coefficients in air [Raizer, 81 1991, p. 135], remains an outstanding question. We investigate a robust pathway for streamer ignition through the collision of charged hydrometeors. By extending a two-particle image-charge model [Cai et al., 2018, https://doi.org/10.1029/2018JD028407] to include an initial charge Q, we quantify how polarization, particle dimensions, and background fields control ignition thresholds. We identify a "diagonal valley" of optimal radius ratios where the required charge is minimized, and is significantly below the corona discharge limit of a single isolated hydrometeor. In ambient fields near 0.3Ek, where photoelectric feedback [Pasko et al., 2025, https://doi.org/10.1029/2025JD043897] can provide a sustained supply of seed electrons, this collision-mediated mechanism provides a pathway to overcome the charge-limiting constraints of isolated particles. These findings offer a consistent physical basis for the birth of lightning leaders in typical thundercloud environments.