Atmospheric Chemistry and Physics www.atmos-chem-phys.net 1680-7316 1680-7324 8 6 2008 10.5194/acp-8-1661-2008 http://www.atmos-chem-phys.net/8/1661/2008/ http://www.atmos-chem-phys.net/8/1661/2008/acp-8-1661-2008.html http://www.atmos-chem-phys.net/8/1661/2008/acp-8-1661-2008.pdf 1661 1675 2008-03-18 Robust relations between CCN and the vertical evolution of cloud drop size distribution in deep convective clouds E. Freud eyal.freud@mail.huji.ac.il D. Rosenfeld M. O. Andreae A. A. Costa P. Artaxo Institute of Earth Sciences, The Hebrew University of Jerusalem, Jerusalem, Israel Department of Applied Environmental Sciences, Stockholm University, Stockholm, Sweden Biogeochemistry Department, Max Planck Institute for Chemistry, Mainz, Germany Department of Geology and Geophysics, Yale University, CT, USA Institute of Physics, Sao Paulo University, Brazil In-situ measurements in convective clouds (up to the freezing level) over the Amazon basin show that smoke from deforestation fires prevents clouds from precipitating until they acquire a vertical development of at least 4 km, compared to only 1–2 km in clean clouds. The average cloud depth required for the onset of warm rain increased by ~350 m for each additional 100 cloud condensation nuclei per cm3 at a super-saturation of 0.5% (CCN0.5%). In polluted clouds, the diameter of modal liquid water content grows much slower with cloud depth (at least by a factor of ~2), due to the large number of droplets that compete for available water and to the suppressed coalescence processes. Contrary to what other studies have suggested, we did not observe this effect to reach saturation at 3000 or more accumulation mode particles per cm3. The CCN0.5% concentration was found to be a very good predictor for the cloud depth required for the onset of warm precipitation and other microphysical factors, leaving only a secondary role for the updraft velocities in determining the cloud drop size distributions. The effective radius of the cloud droplets (re) was found to be a quite robust parameter for a given environment and cloud depth, showing only a small effect of partial droplet evaporation from the cloud's mixing with its drier environment. 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