American Geophysical Union Become an AGU Member
Subscribe to AGU Journals		 AGU Home AGU Publications

Read Full Article (file size: 175541 bytes)    Cited by

GEOPHYSICAL RESEARCH LETTERS, VOL. 34, L03813, doi:10.1029/2006GL027648, 2007

Cloud droplet sedimentation, entrainment efficiency, and subtropical stratocumulus albedo

C. S. Bretherton

Department of Atmospheric Sciences, University of Washington, Seattle, Washington, USA


P. N. Blossey

Department of Atmospheric Sciences, University of Washington, Seattle, Washington, USA


J. Uchida

Department of Applied Mathematics, University of Washington, Seattle, Washington, USA


Abstract

The effect of cloud droplet sedimentation on the entrainment rate and liquid water path of a nocturnal nondrizzling stratocumulus layer is examined using large-eddy simulations (LES) with bulk microphysics. In agreement with a prior study by Ackerman et al. (2004), sedimentation is found to decrease entrainment rate and thereby increase liquid water path. They suggested this is due to reduction of boundary-layer turbulence. Our simulations suggest otherwise. Instead, sedimentation reduces entrainment by removing liquid water from the entrainment zone. This inhibits two mechanisms that promote the sinking of entrained air into the cloud layer–entrainment-induced evaporative cooling and longwave radiative cooling. A sensitivity study shows that the radiative effect is less important than the reduced evaporation. A possible parameterization of the effect of sedimentation on entrainment rate in a mixed layer model is proposed and tested. Since the droplet sedimentation rate is inversely related to cloud droplet (and presumably aerosol) concentration and nearly nondrizzling marine stratocumulus are widespread, sedimentation impacts on stratocumulus entrainment efficiency should be considered in climate model simulations of the aerosol indirect effect.

Received 27 July 2006; accepted 19 December 2006; published 9 February 2007.

Keywords: HULIS; activation; hygroscopicity.

Index Terms: 3310 Atmospheric Processes: Clouds and cloud feedbacks; 3311 Atmospheric Processes: Clouds and aerosols; 3307 Atmospheric Processes: Boundary layer processes; 0321 Atmospheric Composition and Structure: Cloud/radiation interaction.

Read Full Article (file size: 175541 bytes)    Cited by

Citation: Bretherton, C. S., P. N. Blossey, and J. Uchida (2007), Cloud droplet sedimentation, entrainment efficiency, and subtropical stratocumulus albedo, Geophys. Res. Lett., 34, L03813, doi:10.1029/2006GL027648.