On the impact of thunder on cloud ice crystals and droplets

Abstract. Calculations are presented on the impact of thunder on cloud particles. The results show that the creation of a shockwave front near the lightning channel results in shattering of ice crystals, droplets, and dust aerosols, the former being a yet unidentified mechanism for secondary ice production in clouds. At low altitudes shattering is more efficient. At the distance where the shockwave front decays to audio wave, it can cause agglomeration of particles. The cloud particles’ characteristics appear not very suitable for extensive acoustic agglomeration if the Sound Pressure Level (SPL) is below 120 dB. Nevertheless, even for SPL<120 dB, some agglomeration will occur. Agglomeration will occur readily if SPL>135 dB at sound frequencies 10–200 Hz. Agglomeration efficiency increases with height. More agglomeration will occur in pyroclouds, due to their large particle number densities. These results show that the electrical environment in clouds has, through thunder, effects on the size distribution and number density of ice particles and droplets, will hence influence thundercloud radiative properties, and it may be a significant driver of secondary ice production. As global warming may influence the occurrence rate of lightning, the mechanisms discussed here may induce a climate feedback.