Further evidence for CCN aerosol concentrations determining the height of warm rain and ice initiation in convective clouds over the Amazon basin

Abstract

We have investigated how aerosols affect the height above cloud base of rain and ice hydrometeor initiation and the subsequent vertical evolution of cloud droplet size and number concentrations in growing convective cumulus. For this purpose we used in situ data of hydrometeor size distributions measured with instruments mounted on HALO aircraft during the ACRIDICON-CHUVA campaign over the Amazon during September 2014. The results show that the height of rain initiation by collision and coalescence processes (D r, in units of meters above cloud base) is linearly correlated with the number concentration of droplets (Nd in cm-33) nucleated at cloud base (Dr∼5 N d). Additional cloud processes associated with D r, such as GCCN, cloud, and mixing with ambient air and other processes, produce deviations of ĝ1/4 ĝ€21ĝ€% in the linear relationship, but it does not mask the clear relationship between Drand N d, which was also found at different regions around the globe (e.g., Israel and India). When N d exceeded values of about 1000-3, Drbecame greater than 5000ĝ€m, and the first observed precipitation particles were ice hydrometeors. Therefore, no liquid water raindrops were observed within growing convective cumulus during polluted conditions. Furthermore, the formation of ice particles also took place at higher altitudes in the clouds in polluted conditions because the resulting smaller cloud droplets froze at colder temperatures compared to the larger drops in the unpolluted cases. The measured vertical profiles of droplet effective radius (/re) were close to those estimated by assuming adiabatic conditions (/rea), supporting the hypothesis that the entrainment and mixing of air into convective clouds is nearly inhomogeneous. Additional CCN activation on aerosol particles from biomass burning and air pollution reduced /re below rea which further inhibited the formation of raindrops and ice particles and resulted in even higher altitudes for rain and ice initiation.