1School of Earth, Atmospheric and Environmental Sciences, The University of Manchester, Manchester, UK
2Department of Meteorology, University of Reading, Reading, UK
3Met Office, Exeter, UK
Received: 18 Dec 2012 – Discussion started: 15 Jan 2013
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 (MBL) clouds over the south-east Pacific Ocean (SEP). 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 followed by a reduction in longwave cooling, although this only has a significant effect when the cloud is starting to decouple from the boundary layer. The result in this case is a change from a stratocumulus to a more patchy cloud regime. For the simulations presented here, cloud condensation nuclei (CCN) 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 (MBL) clouds.
Revised: 08 Jun 2013 – Accepted: 24 Jun 2013 – Published: 29 Jul 2013
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., 13, 7133-7152, doi:10.5194/acp-13-7133-2013, 2013.