Toward understanding the process-level impacts of aerosols on microphysical properties of shallow cumulus cloud using aircraft observations

Abstract

Representation of clouds remains as the largest uncertainty in future climate predictions. Numerous studies have found and investigated the impacts of aerosols on cloud microphysical properties. However, few studies have investigated the process-level impact of aerosols on cloud microphysical properties, particularly over heavy polluted north China plain region. Using the aircraft observations, this study investigates the variation of cloud droplet size distribution (DSD) with aerosol concentration and vertical velocity (VV) in a shallow cumulus cloud with sufficient liquid water content (LWC).Strong variation in both cloud droplet number concentration (N) and LWC exists, with values from a few cm−3 to >1200 cm−3, and from 0 to 3.0 g/m3, respectively. In general, the total cloud N (effective radius re) for cases with weak VV is slightly less (smaller) than that for cases with high VV for this cumulus cloud with high LWC. Potential mechanisms about the impacts of aerosols (or VV) on the cumulus microphysical properties are proposed for both high and low LWC conditions. Simply said, the change of cloud N mainly depends on the amount of aerosols, and the change of cloud droplet re depends on both the supply of water content and the amount of aerosols: if LWC is high and aerosol amount is not too large, both cloud N and re increase with increasing aerosols; if LWC is low or if LWC is high but aerosol amount is too large, cloud N increase but re decrease with increasing aerosols. Note that for two cases with very strong downdraft, the cloud droplets seem less and smaller than strong VV cases. The most likely reason is that very strong downdraft along with the adiabatic cooling makes dry air above the cloud enter into clouds and causes evaporation of droplets, resulting in smaller and less cloud droplets.