The role of elevated cloud base height in the inverted electrical polarity of severe storms

Ordinary thunderclouds are characterized electrically by positive dipole structures, and a predominance of negative cloud-to-ground lightning. These conditions require the selective negative charging of larger ice particles by the widely recognized non-inductive process in ice particle collisions. Severe supercells, mostly in the LP (Low Precipitation) category, have been documented with clustered positive ground flashes and the inversion of the gross dipole structure compared with ordinary thunderclouds. Numerous cases were documented during STEPS 2000. Laboratory simulations by different workers are in agreement that in the limit of large cloud water contents, the larger ice particles charge positively, consistent with inverted polarity of the main cloud dipole. This study is concerned with thermodynamic conditions favorable to the occurrence of large large cloud water content aloft in the mixed phase region. A key condition is the presence of high cloud base height, which can promote large cloud water content aloft by (1) encouraging a broad, undilute updraft, and by (2) discouraging the warm rain coalescence process that deprives the updraft of cloud water. The climatology of cloud base height for the CONUS is compared with the known locations for +CG-clustered and inverted polarity thunderclouds. Dryline cases previously investigated in the literature will be reexamined in light of these ideas.