ACPD Atmospheric Chemistry and Physics Discussions ACPD 1680-7375 Copernicus GmbH Göttingen, Germany 10.5194/acpd-6-1179-2006 Switching cloud cover and dynamical regimes from open to closed Benard cells in response to the suppression of precipitation by aerosols Rosenfeld D. 1 Kaufman Y. J. 2 Koren I. 3 Institute of Earth Sciences, The Hebrew University, Jerusalem 91904, Israel NASA/Goddard Space Flight Center, Greenbelt, MD 20771, USA Department of Environmental Sciences, Weizmann Institute, Rehovot 76100, Israel 10 02 2006 6 1 1179 1198 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/6/1179/2006/acpd-6-1179-2006.html The full text article is available as a PDF file from http://www.atmos-chem-phys-discuss.net/6/1179/2006/acpd-6-1179-2006.pdf

The dynamic structure of the atmospheric marine boundary layer (MBL) supports two distinct states of cloud cover: closed and open Benard cellular convection. Closed cells are nearly fully cloud covered, while the open cells have <40% cloud cover. Here we show that aerosols have a greater than expected impact on the cloud cover by changing the mode of cellular convection. By suppressing precipitation aerosols can reverse the direction of the airflow, converting the cloud structure from open to closed cells and doubling the cloud cover. The two states possess positive feedbacks for self maintenance, so that small changes of the conditions can lead to bifurcation of the MBL cloud regime. The transition occurs at near pristine background level of aerosols, creating a large sensitivity of cloud radiative forcing to very small changes in aerosols at the MBL. This can have a major impact on global temperatures.