Aerosol concentrations determine 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 Grulich101Centro 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
Received: 22 Dec 2016 – Accepted for review: 05 Feb 2017 – Discussion started: 09 Feb 2017
Abstract. We have investigated how pollution 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 (High Altitude and Long Range Research 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). When Nd exceeded values of about 1000 cm−3, Dr became greater than 5000 m, and particles of precipitation size were initiated as ice hydrometeors. Therefore, no liquid water raindrops were observed within growing convective cumulus during polluted conditions. Furthermore, also the formation of ice particles 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 almost completely inhomogeneous. Secondary nucleation of droplets 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.
Braga, R. C., Rosenfeld, D., Weigel, R., Jurkat, T., Andreae, M. O., Wendisch, M., Pöschl, U., Voigt, C., Mahnke, C., Borrmann, S., Albrecht, R. I., Molleker, S., Vila, D. A., Machado, L. A. T., and Grulich, L.: Aerosol concentrations determine the height of warm rain and ice initiation in convective clouds over the Amazon basin, Atmos. Chem. Phys. Discuss., doi:10.5194/acp-2016-1155, in review, 2017.