Three‐dimensional radio images of winter lightning in japan and characteristics of associated charge structure

Abstract

We conducted lightning observational campaigns using a three‐dimensional (3D) lightning mapper called the Broadband Observation network for Lightning and Thunderstorms (BOLT) and C‐band radar to further understand the distinct characteristics of winter lightning in the coastal area of the Sea of Japan. We succeeded in locating 3D intracloud (IC) flashes, cloud‐to‐ground (CG) flashes, and upward lightning during winter. The basic characteristics of winter lightning, such as leader speeds and charge structure associated with an IC discharge, are similar to summer lightning. A noticeable difference between the winter lightning and the summer lightning is that the altitudes of the source locations associated with winter lightning are substantially lower than those associated with summer lightning. We performed a statistical comparison of winter and summer lightning. This comparison shows that the BOLT source altitude distributions during winter and summer peak at 2.1 and 7.5 km, respectively. Conversely, the distributions of the BOLT temperatures, which are the temperatures corresponding to the BOLT source altitudes, during both winter and summer show similar trends and have large values near −10 °C. These results indicate that a similar charge separation process, likely a noninductive charging mechanism, primarily contributes to thunderstorm electrification during both winter and summer. We show that the horizontal extent of the winter flashes is substantially larger than that of the summer flashes. This result implies that the charge regions in winter thunderstorms cover a wide area and, therefore, produce CG flashes with large charge transfers. © 2018 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.

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

“…The heights of the charge regions in winter thunderstorms are understandably lower than those in summer thunderstorms because of the low-altitude isotherms in winter. The same patterns regarding the comparison of charge region heights in winter and summer have been reported in previous studies (e.g., Brook et al, 1982;Caicedo et al, 2018;Yoshida et al, 2018). On the other hand, the charged cores corresponded to isotherms ranging from 2 to -31°C, which is consistent with those in summer thunderstorms in previous reports (e.g., Krehbiel, 1986;Yoshida et al, 2018).…”

“…However, nearly all the charge regions occurred in altitude below the –30 °C level in this study (referring to the analysis in sections – and Figure ), which was different from some strong summer thunderstorms, in which many flash source could occur even above the –40‐°C level (e.g., Zhang et al, ; Zheng et al, , , ; Zheng & MacGorman, ). In the comparison of sources in summer and winter, Yoshida et al () also noted that the ratios of sources at temperatures colder than –10 °C to the total sources in winter thunderstorms were substantially lower than those in summer thunderstorms.…”

“…The above discussion indicates that convection plays a crucial role in shaping the diversity of the charge patterns in Hokuriku's winter thunderstorms. With relatively strong convection, the main charge regions tend to be vertically aligned in a positive dipole, such as those shown in Figures 21c1 and 21c2, following the typical charge structure (Brook et al, 1982;Krehbiel, 1986;Takahashi et al, 1999Takahashi et al, , 2017Takahashi et al, , 2018Yoshida et al, 2018). Relatively weak convection tends to cause the main charging between graupel and ice crystals or their aggregations to occur at a warmer level, thus forming a predominately lower positive charge region and a middle negative charge region (i.e., an inverted dipole, as in Figures 21a1, 21a2, 21a4, and 21b2).…”

“…The properties of flash including duration, horizontal distance, and area of the convex hull are also investigated. The flash spatial size can be a proxy for the extent scale of valid charge regions for lightning propagation, a concept employed in previous studies on electrical summer storms (e.g., Bruning & MacGorman, 2013;Calhoun et al, 2013;Zhang et al, 2017a;Zheng et al, 2018;Zheng & MacGorman, 2016) and winter storms (e.g., López et al, 2017;Schultz et al, 2018;Yoshida et al, 2018). The flash duration indicates the time difference between the last source and the first source.…”

“…The charge structure plays a crucial role in determining the characteristics of lightning and, therefore, gets special interest. Most previous studies have suggested that Japanese winter thunderstorms have normal charge structures similar to summer thunderstorms (i.e., a positively charged region above a negatively charged region (dipolar structure) and possibly an additional positively charged region at the lower level (tripolar structure); e.g., Brook et al, 1982;Kitagawa & Michimoto, 1994;Takahashi et al, 1999Takahashi et al, , 2017Takahashi et al, , 2018Yoshida et al, 2018). Takahashi et al (1999Takahashi et al ( , 2018 described the charge evolution in Hokuriku winter clouds.…”