Atmospheric Chemistry and Physics
www.atmos-chem-phys.net
1680-7316
1680-7324
6
9
2006
10.5194/acp-6-2503-2006
http://www.atmos-chem-phys.net/6/2503/2006/
http://www.atmos-chem-phys.net/6/2503/2006/acp-6-2503-2006.html
http://www.atmos-chem-phys.net/6/2503/2006/acp-6-2503-2006.pdf
2503
2511
2006-06-29
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
deceased
The dynamic structure of the weakly sheared atmospheric marine boundary
layer (MBL) supports three distinct states of cloud cover, which are
associated with the concentrations of cloud condensation nuclei (CCN)
aerosols in the MBL: (i) CCN rich MBL with closed Benard cellular convection
that forms nearly full cloud cover; (ii) CCN depleted MBL with open cellular
convection that forms <40% cloud cover; and, (iii) CCN starved MBL
where clouds cannot form due to insufficient CCN, with near zero cloud
cover. Here we propose a mechanism for the transition between these three
states that involves the aerosol impacts on precipitation and the feedbacks
on the dynamics of the clouds and on the aerosols deposition. By suppressing
precipitation aerosols can reverse the direction of the airflow, converting
the cloud structure from open to closed cells and more than doubling the
cloud cover. The three 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 between the closed and open cells
occur at near pristine background level of aerosols, creating a large
sensitivity of cloud radiative forcing to very small changes in aerosols at
the MBL. The third state of super clean air can occur as the more efficient
precipitation in cleaner air deposits the aerosols ever faster in a runaway
positive feedback process. The proposed mechanism suggests that very small
changes in the aerosols input to the MBL can have large impacts on the
oceanic cloud cover and likely in turn on the global temperature, in ways
that are not yet accounted for in the climate models.
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