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Abstract

Large-eddy simulation is used to study the sensitivity of trade wind cumulus clouds to perturbations in cloud droplet number concentrations. We find that the trade wind cumulus system approaches a radiative-convective equilibrium state, modified by net warming and drying from imposed large-scale advective forcing. The system requires several days to reach equilibrium when cooling rates are specified but much less time, and with less sensitivity to cloud droplet number density, when radiation depends realistically on the vertical distribution of water vapor. The transient behavior and the properties of the near-equilibrium cloud field depend on the microphysical state and therefore on the cloud droplet number density, here taken as a proxy for the ambient aerosol. The primary response of the cloud field to changes in the cloud droplet number density is deepening of the cloud layer. This deepening leads to a decrease in relative humidity and a faster evaporation of small clouds and cloud remnants constituting a negative lifetime effect. In the near-equilibrium regime, the decrease in cloud cover compensates much of the Twomey effect, i.e., the brightening of the clouds, and the overall aerosol effect on the albedo of the organized precipitating cumulus cloud field is small.