HR: 17:15h
AN: A34A-06    [Abstracts]
TI: Are There Aerosol Indirect Effects on Buoyancy and Entrainment in Shallow Cumulus Clouds?
AU: * Small, J D
EM: jsmall@es.ucsc.edu
AF: University of California Santa Cruz, Earth and Planetary Sciences 1156 High Street, Santa Cruz, CA 95064, United States
AU: Chuang, P Y
EM: pchuang@es.ucsc.edu
AF: University of California Santa Cruz, Earth and Planetary Sciences 1156 High Street, Santa Cruz, CA 95064, United States
AU: Feingold, G
EM: graham.feingold@noaa.gov
AF: NOAA Earth System Research Laboratory, Chemical Sciences Division 325 Broadway, Boulder, CO 80305, United States
AU: Jiang, H
EM: hongli.jiang@noaa.gov
AF: NOAA Earth System Research Laboratory, Chemical Sciences Division 325 Broadway, Boulder, CO 80305, United States
AB: During the Gulf of Mexico Atmospheric Composition and Climate Study (GoMACCS) project, the microphysical structure of non-precipitating shallow cumulus was observed using the Artium Flight Phase Doppler Interferometer (F/PDI) under a range of aerosol conditions during 11 research flights. We examine here the nature of buoyancy and entrainment in such clouds, with the goal of understanding whether aerosol affects these processes, and, therefore, the radiative impacts of such clouds. Jiang et al. (GRL 2006) report model calculations showing vertical buoyancy profiles that vary with aerosol concentration. Modeled polluted clouds exhibit greater negative and positive buoyancy, which in turn enhances entrainment and therefore reduces cloud lifetime. We utilize GoMACCS F/PDI measurements to examine whether such an effect can be observed. If so, how do these observations compare with the model results? What are the implications for cloud entrainment/detrainment and, therefore, cloud lifetime? The nature of entrainment in clouds, and specifically its impact on cloud microphysical properties, has been the subject of much interest. Two end-member entrainment models, homogeneous and inhomogeneous, have been proposed, where real entrainment falls somewhere in the continuum in between. Where an actual entrainment event falls on this continuum depends on two time scales: (a) the time scale of mixing of saturated and unsaturated air and (b) the time scale for drop evaporation. Because the latter is drop size-dependent, one might expect that any changes to the cloud drop size distribution, such as that caused by changes in aerosol concentration, can ultimately change the nature of entrainment. We examine this question using the GoMACCS F/PDI data set. Implications of any observed aerosol indirect effect on entrainment for the cloud radiative properties are also explored.
DE: 0320 Cloud physics and chemistry
DE: 3310 Clouds and cloud feedbacks
DE: 3311 Clouds and aerosols
SC: Atmospheric Sciences [A]
MN: 2007 Fall Meeting