Atmospheric Chemistry and Physics Discussions www.atmos-chem-phys-discuss.net 1680-7367 1680-7375 6 1 2006 10.5194/acpd-6-1179-2006 http://www.atmos-chem-phys-discuss.net/6/1179/2006/ http://www.atmos-chem-phys-discuss.net/6/1179/2006/acpd-6-1179-2006.html http://www.atmos-chem-phys-discuss.net/6/1179/2006/acpd-6-1179-2006.pdf 1179 1198 2006-02-10 Switching cloud cover and dynamical regimes from open to closed Benard cells in response to the suppression of precipitation by aerosols D. Rosenfeld Y. J. Kaufman I. Koren 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 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.