Understanding Fast Positive Breakdown Processes Based on a Macro‐Physical Model

Fast positive breakdown (FPB) process is a recently identified in‐cloud discharge that produces most narrow bipolar events (NBEs). Existing models cannot fully explain observations of FPBs due to their unresolved discharge physics. Here we develop a macro‐physical NBE model and apply it to investigate FPB processes. The NBE is assumed as a transient streamer‐to‐arc discharge initiated by a weak corona discharge under a locally enhanced electric field. Based on a time domain electric field integral equation, the model self‐consistently simulates NBE electrodynamics such as charge transfer, nonlinear conductance variation, and current evolution. Simulations reveal that the initial ambient electric field is the key parameter governing the discharge. Under specific fields, the model reproduces observational characteristics of FPB, including ground‐based electric field responses, spatial development, and propagation speeds. Results show that FPBs can occur without extremely intense electric fields, requiring average strengths of ∼300 kV/m and local peaks reaching ∼900 kV/m. Such a field profile can arise from a configuration where a compact, high‐density spherical positive charge region above a larger, lower‐density spherical negative charge region. The model also supports that the FPB propagates bidirectionally from the initiation point. The upward negative breakdown is difficult to detect due to rapid decay, while a downward positive breakdown persists longer and dominates the radiation.