How do collisions between ice particles charge thunderclouds?

Laboratory experiments since the 1970s show that rebounding collisions between ice crystals and riming graupel transfer charges of order $3 \times 10^{-6} \text{ to } 3 \times 10^{-4}\,\mathrm{statC}$ per collision, with the sign of graupel charge reversing near $253 \text{ to } 263\,\mathrm{K}$ depending on liquid water content (review: Saunders, Space Science Reviews 2008, https://doi.org/10.1007/s11214-008-9345-0). Identify the microphysical mechanism (relative vapor growth rates of the colliding surfaces, mass transfer through the quasi-liquid surface layer, ionic impurities) and derive a charge-transfer law as a function of temperature, liquid water content, impact speed and particle size. An answer is a physical law that reproduces the laboratory charging diagrams and the observed tripole charge structure of storms.

Single-collision experiments with controlled surface growth rates and impurity content compared with a molecular-level model of charge transfer between ice surfaces.