以區域標記法進行閃電放電過程之模擬

Abstract

本研究利用天氣研究與預報模式(WRF)輔以大氣電學模組探討強降水與活躍閃電現象的關係。WRF中的電學模組對於放電過程僅以基本圓柱狀結構描述，不能有效區分雲對地與雲內閃電放電過程。本研究改進放電通路的設定方式，以區域標記法對高電場區域進行連通，容許複雜幾何通路的放電計算，並且區分雲對地與雲內閃電，進一步對於不同放電特性進行計算。模擬結果顯示新方案能有效改進閃電放電頻率過高的現象，且閃電極性與雷暴雲微物理過程之間具有強烈關聯性:雲對地正閃電好發於軟雹初生的對流前期，雲內閃電好發於具有較強上升氣流的對流成熟期，雲對地負閃電則伴隨層狀區降水發生於對流消散期。以上閃電與對流結構發展的關聯性有助於強降水事件的即時預警。

This study investigated the relationship between intense precipitation and vigorous lightning flashes using the Weather Research and Forecasting model coupled with an atmospheric electricity module WRF_ELEC. The WRF_ELEC module can describe basic discharging process, but it cannot identify intra-cloud and cloud-to-ground flashes. This study improved the discharging algorithm of WRF_ELEC by applying the region-labeling method, which provides more detailed information on the electrical properties and geometry of lightning flashes. Simulation results show that the proposed method can significantly improve the lightning flash frequency. Also, it is able to reveal the polarity of lighting flash associated with the microphysical structure of thunderstorm. Positive cloud-to-ground flash is active at initial stage of thunderstorm when graupel formation becomes significant. Intra-cloud flash is active at the mature stage of thunderstorm when the updraft is strong enough to reach high levels. Negative cloud-to-ground flash is active during the dissipating stage of thunderstorm when precipitation results mainly from the outflow stratiform region. These important indicators are valuable for the nowcasting of heavy precipitation.