Riming Electrification in Hokuriku Winter Clouds and Comparison with Laboratory Observations

Abstract

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.

Charge Regions Indicated by LMA Lightning Flashes in Hokuriku's Winter Thunderstorms

“…The second bottom negative charge regions in Figures a1 and b2 that were located at ~0 °C and corresponded to a strong reflectivity of >40 dBZ might be associated with falling negative graupel. In general, the charge region distribution with temperature was analogous to that suggested by Takahashi et al (, , ).…”

“…The LMA sources can only indicate the charge regions taking part in lightning discharges, which is more different from the observations of sounding through the storm (e.g., Takahashi et al, , , ) that may provide evidence for charge region not involved during the discharges of their paths. If only referring to the vertical arrangements of the charge regions included in discharges, the charge structures contained quad‐polar structures (Figures a1 and b2; the charges from top to bottom were positive, negative, positive, and negative), tripolar structures (Figures a2 and b3; upper positive, middle negative, and lower positive), dipolar structures (Figures b1 (the main part), 21c1, 21c2, and 21c3; upper positive and lower negative), inverted dipolar structures (Figures a4, c4, and c5; upper negative and lower positive) and inverted tripolar structures (Figures a3, b4, b5, and c6; upper negative, middle positive, and lower negative).…”

“…If only referring to the vertical arrangements of the charge regions included in discharges, the charge structures contained quad‐polar structures (Figures a1 and b2; the charges from top to bottom were positive, negative, positive, and negative), tripolar structures (Figures a2 and b3; upper positive, middle negative, and lower positive), dipolar structures (Figures b1 (the main part), 21c1, 21c2, and 21c3; upper positive and lower negative), inverted dipolar structures (Figures a4, c4, and c5; upper negative and lower positive) and inverted tripolar structures (Figures a3, b4, b5, and c6; upper negative, middle positive, and lower negative). In contrast, in most previous studies, winter thunderstorms were suggested to have dipolar or tripolar charge structures (Brook et al, ; Caicedo et al, ; Kumjian & Deierling, ; Schultz et al, ; Takahashi et al, , , ), which was similar to normal summer thunderstorms (Williams, ). As we mentioned in the beginning of section , due to the limit of the observation range of the LMA and the rapid movement of thunderstorms, the cells producing the investigated flashes might be in different development stages; therefore, the diverse patterns of the charge distributions, as shown in sections – and summarized in Figure , might have resulted from both the differences in diverse cells and distinct evolution stages.…”

Charge Separation in Collisions Between Ice Crystals and a Spherical Simulated Graupel of Centimeter Size

“…There were attempts to explain their discrepancies by recalling that the impact velocities and simulated graupel sizes used were different, but it is apparent that these differences could not be the only cause. The cause or causes of the differences between the results are still being debated (Takahashi et al, ).…”

“…There were attempts to explain their discrepancies by recalling that the impact velocities and simulated graupel sizes used were different, but it is apparent that these differences could not be the only cause. The cause or causes of the differences between the results are still being debated (Takahashi et al, 2017). Pereyra et al (2000) carried out measurements of the charge transfer when vapor grown ice crystals rebound from a riming target representing a graupel pellet falling in a thunderstorm.…”