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Marine boundary layer cloud feedbacks in a constant relative humidity atmosphere

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Rieck,  Malte
Hans Ertel Research Group Clouds and Convection, The Atmosphere in the Earth System, MPI for Meteorology, Max Planck Society;
IMPRS on Earth System Modelling, MPI for Meteorology, Max Planck Society;

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Nuijens,  Luise
Observations and Process Studies, The Atmosphere in the Earth System, MPI for Meteorology, Max Planck Society;

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Stevens,  Bjorn
Director’s Research Group AES, The Atmosphere in the Earth System, MPI for Meteorology, Max Planck Society;

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Citation

Rieck, M., Nuijens, L., & Stevens, B. (2012). Marine boundary layer cloud feedbacks in a constant relative humidity atmosphere. Journal of the Atmospheric Sciences, 69, 2538-2550. doi:10.1175/JAS-D-11-0203.1.


Cite as: http://hdl.handle.net/11858/00-001M-0000-000F-EC5B-2
Abstract
The mechanisms that govern the response of shallow cumulus, such as found in the trade wind regions, to a warming of the atmosphere in which large-scale atmospheric processes act to keep relative humidity constant are explored. Two robust effects are identified. First, and as is well known, the liquid water lapse rate increases with temperature and tends to increase the amount of water in clouds, making clouds more reflective of solar radiation. Second, and less well appreciated, the surface fluxes increase with the saturation specific humidity, which itself is a strong function of temperature. Using large-eddy simulations it is shown that the liquid water lapse rate acts as a negative feedback: a positive temperature increase driven by radiative forcing is reduced by the increase in cloud water and hence cloud albedo. However, this effect is more than compensated by a reduction of cloudiness associated with the deepening and relative drying of the boundary layer, driven by larger surface moisture fluxes. Because they are so robust, these effects are thought to underlie changes in the structure of the marine boundary layer as a result of global warming.