Characteristics of Ground Flashes in Hefei Region

Characteristics of return strokes in Hefei Area measured by a VLF/VHF radiation recording system is presented. The number of strokes per flash does not show a exponential decay distribution. However, flashes whose stroke number are below 5 to occur almost equally to each other and single stroke flashes account for about 15% of total flashes. The average number of strokes per flash is 4.2 and the maximum number ever measured is 16. Normalized (to 100 km) strength of return strokes distributes log normally. For 169 first return strokes the geometric mean of initial electric field peak normalized to 100 km is 9.3 V·m -1 , the same value for 485 subsequent strokes is 4.5 V·m -1 and the most probable range is between 2 and 4 V·m -1 . On a statistical base the subsequent stroke is weaker than the first stroke, while it is interesting to note that about 18% of ground flashes have at least one subsequent stroke whose initial peak is greater than the peak of the first stroke. The interstroke interval distributes log normally with the minimum value of about 1.6 ms. About 30% intervals are larger than 100 ms and intervals between 40 ms and 100 ms account for about 50%. The ratio of the field peak of two successive return strokes varies systematically with the time interval between them: Intervals less than 40 ms are dominated by the fact that the larger stroke occurs first, while about 55% of intervals more than 100 ms are featured by the preceding return stroke being weaker than the successive one.