1Key Laboratory of Cloud-Precipitation Physics and Severe Storms (LACS), Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100029, China
2Department of Atmospheric and Oceanic Sciences, McGill University, Montreal, Quebec H3A 0B9, Canada
3Department of Chemistry, McGill University, Montreal, Quebec H3A 2K6, Canada
Received: 23 Jun 2012 – Discussion started: 18 Jul 2012
Abstract. Cumulus clouds have long been recognized as being the results of ascending moist air from below the cloud base. Cloud droplet nucleation is understood to take place near the cloud base and inside accelerating rising cloudy air. Here we describe circumstances under which cloud droplet nucleation takes place at the interface of ascending cloudy air and clear air. Evaporation is normally expected to occur at this interface. However, continuity of moving air requires cloud-free air above the boundary of rising cloudy air to move upwards in response to the gradient force of perturbation pressure. We used a one and half dimensional non-hydrostatic cloud model and the Weather Research and Forecast model to investigate the impacts of this force on the evolution of cloud spectra. Our study shows that expansion and cooling of ascending moist air above the cloud top causes it to become supersaturated with condensation rather than evaporation occurring at the interface. We also confirm that Eulerian models can describe the cloud droplet activation and prohibit spurious activation at this interface. The continuous feeding of newly activated cloud droplets at the cloud summit may accelerate warm rain formation.
Revised: 17 Dec 2012 – Accepted: 19 Dec 2012 – Published: 21 Dec 2012
Sun, J., Leighton, H., Yau, M. K., and Ariya, P.: Numerical evidence for cloud droplet nucleation at the cloud-environment interface, Atmos. Chem. Phys., 12, 12155-12164, doi:10.5194/acp-12-12155-2012, 2012.