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© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 04 Jun 2019

Submitted as: research article | 04 Jun 2019

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This discussion paper is a preprint. A revision of the manuscript is under review for the journal Atmospheric Chemistry and Physics (ACP).

Diurnal cycle of the semi–direct effect over marine stratocumulus in large–eddy simulations

Ross J. Herbert1, Nicolas Bellouin1, Ellie J. Highwood1, and Adrian A. Hill2 Ross J. Herbert et al.
  • 1Department of Meteorology, University of Reading, Reading, RG6 6BB, UK
  • 2Met Office, Fitzroy Road, Exeter, EX1 3PB, UK

Abstract. The rapid adjustment, or semi–direct effect, of marine stratocumulus clouds to elevated layers of absorbing aerosols may enhance or dampen the radiative effect of aerosol–radiation interactions. Here we use large eddy simulations to investigate the sensitivity of stratocumulus clouds to the properties of an absorbing aerosol layer located above the inversion layer. The sign of the daily mean semi–direct effect depends on the properties of the aerosol layer, the properties of the boundary layer, and the model setup. Diurnal variations in the cloud response mean that an instantaneous semi–direct effect is unrepresentative of the daily mean, and that observational studies may under– or over–estimate semi–direct effects depending on the observed time of day. The observed role of the distance between the cloud top and the absorbing layer in modulating the strength of the cloud and radiative response is reproduced by the large eddy simulations. Both cloud response and semi–direct effect increase for thinner, denser, layers of absorbing aerosol located nearer the cloud layer. The cloud response is particularly sensitive to the mixing state of the boundary layer: well-mixed boundary layers generally result in a negative daily mean semi–direct effect, and poorly mixed boundary layers result in a positive daily mean semi–direct effect. Properties of the boundary layer and model setup, particularly the sea surface temperature, precipitation, and properties of the air entrained from the free troposphere, also impact the magnitude of the semi–direct effect and the timescale of adjustment. These results suggest that the semi–direct effect simulated by coarse-resolution models may be erroneous because the cloud response is sensitive to small-scale processes, especially the sources and sinks of buoyancy.

Ross J. Herbert et al.
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Ross J. Herbert et al.
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Publications Copernicus
Short summary
Marine stratocumulus clouds cover large regions of the ocean and act to cool the climate. We use high resolution simulations to understand how observed layers of elevated smoke impact the stratocumulus via solar heating that occurs within the smoke layers. We find the cloud response is strongest for thin, dense, layers of smoke close to the cloud, and that properties of the boundary layer play an important role. Generally, the smoke acts to thicken the clouds and enhance their cooling effect.
Marine stratocumulus clouds cover large regions of the ocean and act to cool the climate. We use...