Upward leader inception caused by a sudden change of cloud electric field

摘要

Discharge processes such as glow, streamer, and leader inception among others take place before an upward leader can be launched from a grounded structure during thunderstorms. Electrostatic fields below the thundercloud could lead to the formation of glow charge from grounded objects. if the electric field is high enough and ionization keeps expanding into the gap, streamers can be incepted. Depending on the available charge and the thermodynamic properties of the gas, there is a possibility to incept or not a positive upward leader towards the cloud. Usually, the inception of positive upward leaders is directly related with the appearance of a downward coming leader from cloud towards the grounded object. Such a downward leader will intensify the electric field in such a way that the streamer discharges could thermalize and produce an unstable upward leader channel. However, experimental observations have indicated the inception of upward leaders from grounded structures without registering connecting downward leaders towards the structure. The present paper intends to explain the inception of positive upward leaders from the top of a rod, whenever the electric field produced by the cloud suddenly changes e.g. due to intra-cloud discharges or distance cloud to ground flash. A two dimensional model based on the gas-dynamic equations, the main processes responsible for gas heating such as vibrational excitation and transfer of energy into electronic, rotational and translational excitation, coupled with Poisson equation is presented in this paper. Rods of different lengths under thundercloud electric field were studied. Simulation results indicate that positive upward leaders can be incepted from long rods under certain conditions of thundercloud electric field without the need of a coming downward leader. However, for rods of tenths of meters the thundercloud electric field is not enough to incept positive upward leaders and an intensification of the electric fiel- is required in order to incept a positive upward leader from the structure, e.g., a coming downward leader.