Correlation Between the First Return Stroke of Negative CG Lightning and Its Preceding Discharge Processes

摘要

Using the negative cloud-to-ground lightning cases well located by the Fast Antenna Lightning Mapping Array, we have examined the general characteristics of the preliminary breakdown (PB) and stepped leader (SL) processes, and further studied the correlation between the first return stroke (RS1st) intensity and its preceding discharge processes. The RS1st intensity is measured from the range-normalized radiation field. The parameters used to characterize PB/SL include PB altitude, PB/SL vertical speed, PB/SL pulse rate, and SL duration. The arithmetic mean PB altitude is about 5.6 km and exhibits a weak inverse correlation with the RS1st intensity. As the PB develops into the SL, the arithmetic mean vertical speed decreases from 4.7 × 10~5 to 1.7 × 10~5 m/s and the arithmetic mean pulse rate reduces from 5.1 to 2.4 ms~(-1). In the PB/SL processes, the average vertical speed and pulse rate have positive correlations with the RS1st intensity while the corresponding Spearman's correlation coefficient for SL is considerably larger than that for PB. The average SL duration is 34 ms and has a negative relation to the RS1st intensity, with the coefficient of -0.71. These results demonstrate that on average, lower initiation altitude, faster PB/SL vertical speed, higher PB/SL pulse rate, and shorter SL duration tend to be followed by stronger RS_(1st). According to the above correlations, we speculate that the combination of a middle negative charge region with a uniform and wide horizontal distribution and a small lower positive charge region is a favorable condition for an intense RS_(1st).