Evolution of radar parameters and lightning activity during thunderstorm cells merging

The merging of thunderstorm cells not only plays a very important role in the growth, intensification and maintenance of convective systems, but also has a close relationship with the development of severe weather. This paper analyzes the merging processes of a hailstorm event and a heavy rainstorm case respectively and summarizes the features of multiple samples. In total 23 samples are used. Detections of Doppler radar and SAFIR3000 lightning location system are selected for the present study. The method of radar parameters reconstruction is implemented to verify the phenomenon of "dump", which is the phenomenon that the volume of a convective cell with more than 40 dBz above 6 km often experiences a sudden decrease in the process of merging, and to reveal the lighting characteristics during the "dump" process. The results indicate that: (1) At the initiation of merging, most of the samples are below 5 km, which account for 86% of the total. From the perspective of time-consuming (from the time when the 30 dBz begins to converge to the moment when the strongest echoes merge into one), all the samples fall within the range of 6-36 min. The merging time is less than 12 min for 56.5% of the samples, while no more than 16% of the samples take 30 min for merging. After the beginning of merging, radar parameters of V₄₀ (total volume above 40 dBz) and V_40up-6 (total volume with more than 40 dBz over 6 km) experience increasing for 97% of the samples, and V₄₀ increases by 7%-590% and V_40up-6 increases by 3%-638%; 60.1% of the samples have the maximum value of V_40up-6 appearing between 12-18 min, and 34.8% of the samples have the maximum value appearing between 24-36 min after the initiation of merging. (2) 87% of the samples demonstrate the feature of "dump" during the merging process, which occurs within 6-18 min after the initiation of merging for 77% of the samples. (3) In the 10 lightning samples, the lighting frequencies of nine samples experience "jump" or even reach the peak after the initiation of merging. The height of the main positive charge region of all samples decreases to some extent following the "dump" with the decreases within the range of 1-4 km, but the frequency of lightning remains almost unchanged.