Impact of aerosol on droplet number at cloud base and on the altitude of freezing in convective clouds

Abstract

We have performed cloud and aerosol measurements in and below growing convective clouds above the Amazonian basin during die ACRIDICON-CHUVA mission in 2014. The measurements were performed during 14 flights with the German research aircraft HALO stationed in Manaus equipped with a large set of novel cloud and aerosol instruments to measure size distributions, liquid and ice water content (IWC) as well as habit of the cloud particles up to a ceiling altitude of 14 km. The measurements obtained during cloud profiling in growing convective cumulus show a clear dependence of cloud particle number concentration on cloud condensation nuclei number below cloud base. Further the depth of the cloud (altitude from cloud base until the level where the ice initiation takes place) is correlated to the number of cloud particles at cloud base. Warm rain formation was suppressed under heavily polluted conditions. Instead a larger number of small ice particles were generated at higher altitudes in the cloud than under moderately polluted conditions or convection over clean marine environment. A linear relationship of cloud depths and droplet number concentration at cloud base is found. The limited IWC measurements obtained in the anvil of the clouds show moderate values but demand further research on the dependence of IWC in continental vs marine convection. The implications of our results on IWC and mass median diameter in continental polluted convection is discussed with respect to the impact on aviation and the forecast on ice water content near Mega-Cities or regions influence by wildfires or dust events.