Cleansing Beijing Air Pollution by Extensive Precipitation Leads to Marine-Like Clouds With Warm Rain

Warm rain prevails in clean marine clouds and is one of the most abundant types of precipitation in the world. However, warm rain is rarely observed in urban polluted atmospheric environments due to high number concentrations of aerosols generally suppress the occurrence of warm rain. Three aircraft observation of clouds were used during summer in Beijing area. Here, we documented warm rain processes similar to shallow marine clouds in the urban (Beijing) atmospheric environment after cleansing air pollution by extensive precipitation observed on 11 September 2019. Our results showed that, compared with aircraft observations of similar cumulus clouds under polluted conditions, large-scale precipitation can efficiently scavenge air pollution in Beijing and lead to marine-like clouds with warm rain. The average aerosol concentration (Na) near the cloud base in these clean cases was only 225 cm⁻³ with cloud droplet number concentration (N_d) of 36 cm⁻³, (maximum of 142 cm⁻³) which led to enhanced coalescence. The warm rain was initiated at 600 m above the cloud base. Our results highlight the significance of pollution scavenging by precipitation in warm rain production in urban environment.