An examination of thunderstorm-charging mechanisms using a two-dimensionalstorm electrification model

The early, prelightning, electrification of a storm resulting from noninduc tive (NI) charging involving graupel, cloud ice/snow, and supercooled cloud water in a riming environment is studied using a comparative approach in a two-dimensional storm electrification model. The primary schemes examined al-e NI charge transfers based on the laboratory work of Takahashi [1978] a nd Saunders et al. [1991]. The NI mechanism, based on Takahashi's work, dev elops a positive dipole (positive charge above negative) and electric field s approaching 185 kV m(-1) as the cloud enters the dissipating stage. Charg e transfers, based on the work of Saunders and colleagues, had to be reduce d in magnitude to produce electrification that is consistent with the obser vations. In addition, the Saunders scheme produces an initially inverted di pole (negative charge above positive) which resolves to a positive dipole i n the latter part of the simulation and produces electric fields approachin g 250 kV m(-1). Sensitivity tests show that the NI scheme, based on Takahas hi's work is sensitive to the number concentration of ice crystals, whereas the Saunders-based scheme is much less sensitive to ice crystal numbers. T he Saunders parameterization has strong positive charging of graupel at low effective liquid water content and low temperature. This positive charging can result in an unusual cloud-top charge structure when used at full valu e but is benign when the charging is reduced in magnitude. The charge struc ture resulting from the Saunders scheme is quite sensitive to the calculati on of the effective water content, which determines the level of charge rev ersal. Both of the NI schemes are capable of producing electrification that approaches thunderstorm levels.