Raincloud Conditioning by Thunder

Abstract This article considers the role of thunder in the production of rain by gravitational processes. A previous numerical work has shown that a thunder event consisting of four consecutive thunderclaps can alone produce small but significant droplet growths in a lean cumulus cloud in less than 2 s. Here we consider, also numerically, the coalescence effects produced by both thunderclaps and gravity in a cumulus congestus cloud that had more than four times the liquid content of that lean cloud. Those effects are studied separately and in tandem, using the same set of assumptions. Therefore, the results presented here provide a basis for the comparison of the effectiveness of each type to produce droplet growth in thunderclouds. For thunder alone, these results show that a small number of thunderclaps can in less than 2 s produce mean size growths larger than 50%. For gravity alone, it is found that after 60 s, the longest time considered, gravitational coalescence increases the mean diameter of the original droplet size distribution by 20%. The tandem study considers the effects produced by gravitation on the droplet size distribution that resulted after the original distribution was modified by four or five thunderclaps. Significant increases are found in both cases. For four claps it was found that the mean size increased by 71% in 60 s. The corresponding growth for five claps was slightly larger than 100%. These substantial increases also show that the growths produced by the thunderclaps are not simply additive, but significantly accelerate those produced by gravitation. This acceleration implies that the droplet size growths produced by thunderclaps can substantially decrease the time required by gravitational coagulation to produce raindrops in rainclouds.