Further evidence for CCN aerosol concentrations determining the height of warm rain and ice initiation in convective clouds over the Amazon basin
Year: 2017
Type: article
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 (Dr, in units of meters above cloud base) is linearly correlated with the number concentration of droplets (Nd in cm−3) nucleated at cloud base (Dr ≈ 5 ⋅ Nd). Additional cloud processes associated with Dr, such as GCCN, cloud, and mixing with ambient air and other processes, produce deviations of ∼ 21 % in the linear relationship, but it does not mask the clear relationship between Dr and Nd, which was also found at different regions around the globe (e.g., Israel and India). When Nd exceeded values of about 1000 cm−3, Dr became greater than 5000 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.
Institutions: Instituto Nacional de Pesquisas Espaciais, Hebrew University of Jerusalem, Johannes Gutenberg University Mainz, Deutsches Zentrum für Luft- und Raumfahrt e. V. (DLR), Scripps Institution of Oceanography, University of California San Diego, Max Planck Institute for Chemistry, Leipzig University, Universidade de São Paulo
Language: English
Subfield: Global and Planetary Change
Field: Environmental Science
Domain: Physical Sciences
SDG: Climate action
Open Access status: gold
Estimated APC paid: $1,725
Funders: European Commission, Deutsche Forschungsgemeinschaft, Fundação de Amparo à Pesquisa do Estado de São Paulo, Coordenação de Aperfeiçoamento de Pessoal de Nível Superior, Bundesministerium für Bildung und Forschung, Deutsches Zentrum für Luft- und Raumfahrt, Conselho Nacional de Desenvolvimento Científico e Tecnológico, Max-Planck-Gesellschaft
Awards: G135623947, Rolle der mittleren Atmosphäre bezogen auf das Klima (ROMIC): Untersuchung der Quellen und Prozesse des stratosphärischen Aerosols (SPITFIRE) - Teilvorhaben 1: Aerosolmikrophysikalische in-situ Feldmessungen und Verbundkoordination, G459682394, G5562498387, Impact of Biogenic versus Anthropogenic emissions on Clouds and Climate: towards a Holistic UnderStanding
OpenAlex ID: W2772880772