Numerical Simulation Of Thunderstorm Electrification Mechanism

Recent studies show that noninductive charging is one of the dominant charging mechanism inside clouds and the charging rate is affected by factors such as particle sizes,relative terminal velocity, cloud temperature and liquid water content. But the sign and magnitude of charges transferred per rebounding collision is set fixed in the three-dimensional dynamics and electrification coupled model. In order to simulate electric field development more reasonably, the noninductive charging mechanism was improved by considering the affection of supercooled cloud water content, particle diameters and relative terminal velocity according to related laboratory results,. A Comparative study of prelightning thunderstorm electrification with three different noninductive charging mechanisms was conducted using the hailstorm case in Changchun of the June 10th,2005 .The numerical results show that the scheme with sign of charge transfer depends on cloud temperature and cloud water content;magnitude of charge transfer depends on ice crystal size, relative terminal velocity and cloud water content, and temperature of charge reversal between-10℃and-20℃is considered better.The process of charging accumulation and electric field proceeding is different inside various convective clouds due to the differences in microphysical processes. Therefore,comparative studies were conducted referring to two convective clouds with different degree of convection and wind shear in Changchun, and two convective clouds in Changchun and Maqu. Results show that updraft is an important parameter to reflect the development of electric field. And moderate updraft is favorable for the electric field to reach the breakdown value at relatively shorter time by promoting charging accumulation at the middle part of clouds. However, electric field reaches breakdown value at the falling stage of updraft velocity in different cases.Transportive forcing plays an important role to the distribution of charges. For the main charging zone of graupel, the magnitudes of vertical transport is an order smaller than that of NI charging rate, and they have the same order of magnitudes after about 10 min of electrification.Besides, the lasting of lower positive or negative charge center is greatly dependent on the sedimentation of graupel and frozen drops at higher level, and positive charges on ice above 8 km are mainly from the transport of updrafts.Besides, rich supercooled rain water is favorable for the formation of frozen drops, and sedimentation and melting of them contributes to the lower negative charge zone at the early stage of precipitation.