ACPD Atmospheric Chemistry and Physics Discussions ACPD 1680-7375 Copernicus GmbH Göttingen, Germany 10.5194/acpd-10-25287-2010 Dependence of aerosol-precipitation interactions on humidity in a multiple-cloud system Lee S. S. 1 2 NOAA Earth System Research Laboratory, Chemical Sciences Division, Boulder Colorado, USA Cooperative Institute for Research in Environmental Sciences (CIRES), University of Colorado, Boulder Colorado, USA 29 10 2010 10 10 25287 25327 This is an open-access article ditributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. This article is available from http://www.atmos-chem-phys-discuss.net/10/25287/2010/acpd-10-25287-2010.html The full text article is available as a PDF file from http://www.atmos-chem-phys-discuss.net/10/25287/2010/acpd-10-25287-2010.pdf

This study examines the dependence of aerosol-precipitation interactions on environmental humidity in a mesoscale cloud ensemble (MCE) driven by deep convective clouds. It is found that increases in aerosol enhance evaporative cooling, which raises not only the intensity of vorticity and entrainment but also that of downdrafts and low-level convergence or gustiness. The increase in vorticity tends to suppress precipitation. The increase in low-level convergence tends to enhance precipitation by generating more secondary clouds in a muptiple-cloud system simulated here. <br><br> At high humidity, the effect of increased vorticity on cloud-liquid mass and, thus, precipitation is outweighed by that of increased low-level convergence. This leads to aerosol-induced precipitation enhancement. When humidity lowers to mid humidity, the effect of aerosol on low-level convergence still dominates that on entrainment, leading to precipitation enhancement with increased aerosol. With the lowest humidity in the current work, the effect of aerosol on entrainment dominates that on low-level convergence, leading to precipitation suppression with increased aerosol. Hence, there is not only a competition between the effect of evaporation on vorticity and that on low-level convergence but also the variation of the competition with humidity. This competition and variation are absent in a single-cloud system where the effect of low-level convergence on secondary clouds is absent. This exemplifies a difference in the mechanism which controls aerosol-precipitation interactions between a single cloud and a multiple-cloud system.

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