Numerical Simulation of the Formation of a Large Lower Positive Charge Center in a Tibetan Plateau Thunderstorm

Abstract

Numerical modeling is applied to elucidate the formation mechanism of the large lower positive charge centers (LPCCs) observed during thunderstorms over the Tibetan Plateau based on the simulation of a storm at the northeastern boundary of the plateau. Four sensitivity tests were carried out to explore the impacts of inductive charging, reversal temperature, and the choice of noninductive charging scheme. The results show that the unique environmental conditions of the Qinghai‐Tibet Plateau, which include weak convection and low freezing level, are fundamental to the formation of large LPCC. A weakened charge density in the upper positive charge center highlights the role of a LPCC in lightning initiation although the charge density of the LPCC has no obvious change compared to that in the LPCC of the typical tripole structure. This accounts for Tibetan Plateau thunderstorms having low frequencies of lightning flashes, which occur mainly in the lower dipole. Inductive electrification, which provided more than 50% of the positive charge on graupel and increased the positive and negative charge on cloud drops by 2 orders of magnitude, is an important complement to the lower dipole of the tripole charge structure originally established by noninductive electrification. The inductive electrification also evidently enhances the LPCC and the middle negative charge center while slightly reducing the upper positive charge center. Subsequently, the lightning activity is strengthened, and lightning flashes are more likely to be initiated at the lower dipole. Varying the reversal temperatures and noninductive charging scheme does not fundamentally affect the formation of the LPCC.

Spatiotemporal Lightning Activity Detected by WWLLN over the Tibetan Plateau and Its Comparison with LIS Lightning

“…This is possible, considering some recent studies suggested that the thunderstorms with strong convection could yield high-frequency lightning flashes (they are typically associated with the upper two main charge regions), but the flash size and energy (expressed by the radiation, peak current, etc.) tend to be small because strong convection and turbulence could cause the charge regions in the cloud to be small in size (Bruning and MacGorman 2013;Bruning and Thomas 2015;Wang et al 2016;Zhang et al 2017;Zheng et al 2018Zheng et al , 2019You et al 2019). The LIS data and NTBB 2528C suggest earlier peaks than those of the WWLLN data and CGLLS data in terms of their seasonal variations, which may mean that there exists significant seasonal change in the thunderstorm property in the proportions of CG lightning and strong-discharge lightning to the total lightning and the dominant position at which the lightning occurs in the vertical direction (associated with the variation in the main positive charge region, determined by the convection intensity).…”

“…Second, recent studies have revealed that lightning discharge intensity is affected by dynamic and microphysical processes in thunderstorms. In simple terms, small (large) charged regions tend to cause small (large) or weak-intensity (strong-intensity) lightning flashes, while the size of the charged regions is affected by the convection intensity and horizontal extent (Bruning and MacGorman 2013;Beirle et al 2014;Peterson and Liu 2013;Wang et al 2016;You et al 2019;Zheng et al 2018Zheng et al , 2019. The differences in the thunderstorm properties in different regions of the TP may lead to different intensities of the lightning discharge.…”

Evolution of the Charge Structure and Lightning Discharge Characteristics of a Qinghai‐Tibet Plateau Thunderstorm Dominated by Negative Cloud‐to‐Ground Flashes

“…Therefore, the tripole charge structure in the Qinghai‐Tibet Plateau is different from that of normal thunderstorm. Wang et al () used numerical modeling to simulate the formation of a large lower positive charge center (LPCC) in a Qinghai‐Tibet Plateau thunderstorm. The results showed that the unique environmental conditions of the Qinghai‐Tibet Plateau, which include weak convection and a low freezing level, are fundamental to the formation of a large LPCC.…”

Triggered Negative Lightning‐Leaders That Propagated Into Thunderstorm Lower Positive Charge

“…With this mechanism, the more compact graupel would fall away from the ice crystals to become the lower positive charge. Graupel also plays a role in thunderstorms on the central Tibetan Plateau and the Qinghai-Tibet Plateau (Qie, Kong, et al, 2005;Wang et al, 2019).…”