Riming electrification in Hokuriku winter clouds and comparison with laboratory observations

Riming electrification is the main charge separation mechanism of thunderstorms, occurring mainly during graupel particle-ice crystal collisions. Laboratory experiments have found that charge separation polarity and magnitude depend critically on cloud water content and temperature. Several groups have mapped this dependence, but there are substantial differences between their results. These conflicting laboratory-derived riming electrification topographies can be tested by comparing them to field observations. Here, direct and simultaneous sonde-based measurement of both precipitation particle type and charge (videosonde) and cloud water content [hydrometeor videosonde (HYVIS)] in lightning-active Hokuriku winter clouds at Kashiwazaki, Niigata Prefecture, Japan, are reported. With decreasing height, summed graupel charge transitioned from negative to positive at a mean temperature of ≥11°C, and the mean peak cloud water content in the positive graupel domain was 0.4 g m^-3. Thus, in cloud regions of relatively high temperature (≥-11°C) and low cloud water content (CWC; ≤0.4 g m^-3), graupel particles were mainly positively charged. This result can be compared with those of laboratory riming experiments; for example, in this temperature/cloud water content domain, graupel electrification has been reported to be positive by Takahashi, largely negative in early reports using the Manchester cloud chamber, positive in later reports using the Cordoba and Manchester modified cloud chambers, and partially positive in a more recent report using the Cordoba cloud chamber.