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

摘要

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.