Characteristic pulse trains of preliminary breakdown in four isolated small thunderstorms

Using a low‐frequency six‐station local network, preliminary breakdown (PB) pulses not followed or followed by negative return stroke (RS), which are defined as PB‐type and PB cloud‐to‐ground (PBCG)‐type flashes, are analyzed based on four isolated small thunderstorms for the first time. On the basis of 22 PB‐type flashes out of totally 2155 flashes, it indicates that the number of PB‐type flashes is very small. At the early stage, PB‐type flashes are observed in all four thunderstorms. At the active stage, PB‐type flashes still can occur; meanwhile, there are few or no negative cloud‐to‐ground (CG) flashes. However, at the final stage no PB‐type flashes occur. At the stage of distinct cell merging or splitting, PB‐type flashes are also observed. Based on the 123 PBCG‐type flashes, we discuss the percentage of PBCG‐type flashes and also analyze the relationship between the electric field (E‐field) amplitude of the largest pulse in the PB pulse train normalized to 100 km (PBA), the E‐field amplitude of the first return stroke normalized to 100 km (RSA), the time interval between PBA and RSA (PB‐RS interval), and the ratio between PBA and RSA (PB‐RS ratio). We find that the percentage of PBCG‐type flashes is not always dependent on PBA or PB‐RS ratio; the type of thunderstorms may also have an impact on this percentage. None of the PB‐RS intervals is less than 20 ms; we speculate that such long PB‐RS interval is the feature of isolated small thunderstorms, but more observations are needed to further investigate this question.