Comment on “Thunderstorm electrification laboratory experiments and charging mechanisms” by C. P. R. Saunders

The physical basis for thunderstorm electrification remains poorly understood at the particle scale.Laboratory experiments designed to simulate charging conditions at the particle scale have made substantial advances in recent years.By comparing the laboratory simulations on charging with large-scale observations of the thermodynamic, microphysical, and electrical structure of thunderclouds and the electrified particles within them, one can then proceed to judge the validity of the laboratory simulations and to understand the microphysical conditions associated with the selective acquisition of positive and negative charge by particles [Baker et al., 1987;Williams et al., 1991; 1994].Saunders [1994] successfully reviews the laboratory results but unfortunately does not judge them in the context of the large-scale observations of thunderclouds.By neglecting the latter important discussion, Saunders avoids a critical comparison of two conflicting sets of laboratory results: those of his own [Jayaratne et al., 1983; Saunders et al., 1991] and those of Takahashi [1978].The conflicts are nontrivial and have been a source of puzzlement both within and outside the community of thunderstorm investigators for about a decade.By giving substantially greater emphasis to his own results throughout his review paper (and particularly in the discussion of the single table and figure of the paper), the reader is left with the impression that Saunders' results are correct and applicable to thunderstorms and Takahashi's are not.The purpose of this comment is to present an alternative view primarily by appeal to observations at the large scale.In making comparisons between the large scale and the laboratory results, it is important to consider the entire range of electrical, kinematic, and microphysical conditions.Presently, available observations are consistent with three general categories for thunderstorms, as summarized in Table 1.These three categories, stratiform precipitation, ordinary thunderstorms, and super hailstorms, may be distinguished on the basis of lightning flash rate, updraft speed, liquid water content, or predominant electrical polarity of the cloud and the polarity of the ground flashes the cloud produces.Approximate values for these various parameters are listed in Table 1 and generally range over at least 2 orders of magnitude.The vast majority of terrestrial thunderstorms are "ordinary" and exhibit a predominant positive dipole structure (positive charge uppermost) and almost exclusively negative ground flashes.