| How lightning initiate in thundercloud electric fields is still the focus on lightning research,which is helpful to understand and explain the mechanism of lightning.Coupling with other macro-meteorological conditions,related research can help to achieve lightning protection and disaster reduction.However,the mechanism how lightning frequently occurs in thundercloud electric fields that are much lower than the conventional breakdown threshold field of air is still unclear so far.One theory is that streamer discharges can be initiated from enhanced local electric field around hydrometeors in thundercloud.Because of electric polarization of hydrometeors in ambient electric field,field in the vicinity of hydrometeors will be significantly enhanced.Electric breakdown may be caused by the enhanced field.Therefore,Studying the physical mechanism and properties of streamer discharge initiated from a hydrometeor in thundercloud electric fields is critical to understand the formation and propagation of lightning and the entire process of discharge.In this work,an isolated dielectric column with rigid interface is used to simulate the physical mechanism of streamer initiation and evolution from a real hydrometeor in cloud at weak ambient field.At 7 km altitude,the ambient field of 0.5_kE is adopted in our simulations.A model with two equipotential conducting spheres is built to calculate the electric fields of a hydrometeor with two hemispherical ends.In this,potentials and electric fields can be calculated by multiple image method.The simulation model includes electron impact ionization,two-body and three-body electron attachments,electron-positive ion and negative-positive ion recombination,drift and diffusion of electrons.In addition,the photoionization is considered during the process of discharges.Modeling results show that the electric field of the polarized column hydrometeors at ambient field of 0.5E_k in thundercloud,electric field is significantly enhanced around the tips of the hydrometeor.A positive streamer can be initiated from the positive tip of the hydrometeor with a rigid interface in an ambient electric field of 0.5E_k,and no negative streamer is generated at the other tip.Although the main features of the positive streamer are similar with those reported previously,significant differences exist because the present model has a rigid surface as real hydrometeor.The peak electric field decreases slightly before it begins to increase rapidly to the maximum value of the entire simulation.After it reaches the maximum value,it decreases rapidly first and then gradually as the streamer propagates.Both the peak electric field and electron density do not approach constant values and there is no fluctuation happening during our simulation period.It is also found that charge accumulates on the hydrometeor increases with time.The accumulation rate does not change monotonically,but it increases approximately linearly with time after the streamer is fully formed.Analysis shows that the electron flux on the surface of hydrometeor undergoes a complex process.The results demonstrate that the electric field of the polarized hydrometeors at a lower ambient field in thundercloud will be significantly enhanced in their vicinity,a positive streamer can be initiated and stably evolves from a hydrometeor with a rigid interface in the ambient electric field of 0.5_kE.This works helps clear the physical mechanism and nature of the lightning process. |
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