Atmos. Chem. Phys. Discuss., 13, 1717-1765, 2013
© Author(s) 2013. This work is distributed
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This discussion paper has been under review for the journal Atmospheric Chemistry and Physics (ACP). Please refer to the corresponding final paper in ACP.
Modelling the effects of gravity waves on stratocumulus clouds observed during VOCALS-UK
P. J. Connolly1, G. Vaughan1, P. Cook2, G. Allen1, H. Coe1, T. W. Choularton1, C. Dearden1, and A. Hill3
1School of Earth, Atmospheric and Environmental Sciences, The University of Manchester, UK
2University of East Anglia, UK
3Met. Office, UK

Abstract. During the VOCALS campaign spaceborne satellite observations showed that travelling gravity wave packets, generated by geostrophic adjustment, resulted in perturbations to marine boundary layer clouds over the south east Pacific ocean. Often, these perturbations were reversible in that passage of the wave resulted in the clouds becoming brighter (in the wave crest) then darker (in the wave trough) and subsequently recovering their properties after the passage of the wave. However, occasionally the wave packets triggered irreversible changes to the clouds, which transformed from closed mesoscale cellular convection to open form. In this paper we use large eddy simulation (LES) to examine the physical mechanisms that cause this transition. Specifically, we examine whether the clearing of the cloud is due to: (i) the wave causing additional cloud-top entrainment of warm, dry air or (ii) whether the additional condensation of liquid water onto the existing drops and the subsequent formation of drizzle are the important mechanisms. We find that although the wave does cause additional drizzle formation, this is not the reason for the persistent clearing of the cloud; rather it is the additional entrainment of warm, dry air into the cloud, although this only has a significant effect when the cloud is starting to de-couple from the boundary layer. The result in this case is a change from a stratocumulus to a more cumulus-like regime. For the simulations presented here, cloud condensation nuclei scavenging did not play an important role in the clearing of the cloud. The results have implications for understanding transitions between the different cellular regimes in marine boundary layer clouds.

Citation: Connolly, P. J., Vaughan, G., Cook, P., Allen, G., Coe, H., Choularton, T. W., Dearden, C., and Hill, A.: Modelling the effects of gravity waves on stratocumulus clouds observed during VOCALS-UK, Atmos. Chem. Phys. Discuss., 13, 1717-1765, doi:10.5194/acpd-13-1717-2013, 2013.
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