Articles | Volume 17, issue 23
Atmos. Chem. Phys., 17, 14433–14456, 2017

Special issue: The ACRIDICON-CHUVA campaign to study deep convective clouds...

Special issue: BACCHUS – Impact of Biogenic versus Anthropogenic emissions...

Atmos. Chem. Phys., 17, 14433–14456, 2017
Research article
05 Dec 2017
Research article | 05 Dec 2017

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

Ramon Campos Braga1, Daniel Rosenfeld2, Ralf Weigel3, Tina Jurkat4, Meinrat O. Andreae5,9, Manfred Wendisch6, Ulrich Pöschl5, Christiane Voigt3,4, Christoph Mahnke3,8, Stephan Borrmann3,8, Rachel I. Albrecht7, Sergej Molleker8, Daniel A. Vila1, Luiz A. T. Machado1, and Lucas Grulich10 Ramon Campos Braga et al.
  • 1Centro de Previsão de Tempo e Estudos Climáticos, Instituto Nacional de Pesquisas Espaciais, Cachoeira Paulista, Brasil
  • 2Institute of Earth Sciences, The Hebrew University of Jerusalem, Israel
  • 3Institut für Physik der Atmosphäre, Johannes Gutenberg-Universität, Mainz, Germany
  • 4Institut für Physik der Atmosphäre, Deutsches Zentrum für Luft- und Raumfahrt (DLR), Oberpfaffenhofen, Germany
  • 5Multiphase Chemistry and Biogeochemistry Departments, Max Planck Institute for Chemistry, 55020 Mainz, Germany
  • 6Leipziger Institut für Meteorologie (LIM), Universität Leipzig, Stephanstr. 3, 04103 Leipzig, Germany
  • 7Instituto de Astronomia, Geofísica e Ciências Atmosféricas, Universidade de São Paulo, Sao Paulo, Brazil
  • 8Particle Chemistry Department, Max Planck Institute for Chemistry, 55020 Mainz, Germany
  • 9Scripps Institution of Oceanography, University of California San Diego, La Jolla, CA 92098, USA
  • 10Institut für Informatik, Johannes Gutenberg-Universität, Mainz, Germany

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.

Final-revised paper