HR: 0800h
AN: A31A-0822    [Abstracts]
TI: Indirect Effect of Anthropogenic Aerosols on Cirrus Clouds: Satellite Observations and Climate Forcing
AU: * Dubey, M
EM: dubey@lanl.gov
AF: Los Alamos National Laboratory, MS B244, LANL, Los Alamos, NM 87545 United States
AU: Chylek, P
EM: chylek@lanl.gov
AF: Los Alamos National Laboratory, MS B244, LANL, Los Alamos, NM 87545 United States
AU: Lohmann, U
EM: ulrike.lohmann@env.ethz.ch
AF: Institute for Atmospheric and Climate Science, ETH, Universitätstrasse 16 ETH-Zentrum, CHN O 11, Zurich, 8092 Switzerland
AB: Our analysis of satellite observations shows that the effective radius of cloud droplets decreases over the seas adjacent to the Indian sub-continent during periods of increased anthropogenic pollution. In contrast, the effective radius of ice crystals increases in these regions during the same time. The decrease of water droplet sizes (due to aerosol pollution) has been observed and is expected as a result of a larger number of aerosol particles that serve as cloud condensation nuclei. A possibility of aerosol pollution leading to larger ice crystals by heterogeneous ice nucleation has been suggested, and recently observed in a few airborne measurements. Laboratory measurements have also identified mineral dust and some specific organics as efficient heterogeneous ice nuclei. We provide here the first observational evidence that the changes in cloud particle sizes proceeds in opposite directions (smaller water droplets and larger ice crystals) during the pollution periods over vast regions. We make the case that vertical transport and cloud processing of aerosols in the polluted Indian Ocean boundary layer could supply heterogeneous ice nuclei to the mid to upper troposphere. In spite of the opposite effects of aerosols on the size of ice crystals and cloud water droplets, both of these effects (smaller water droplets and larger ice crystals) lead to an aerosol induced regional cooling, thereby providing a larger masking of the warming by greenhouse gases. We present global climate simulations with heterogeneous ice nucleation that also show an increase in cirrus ice crystal size over the polluted Indian Ocean with a substantial negative regional climate forcing. Our results underscore the need for focused field measurents of cirrus clouds over this region to establish that heterogeneous nucleation causes the observed ice particle growth.
UR: http://aerosols.lanl.gov
DE: 0305 Aerosols and particles (0345, 4801, 4906)
DE: 0320 Cloud physics and chemistry
DE: 0321 Cloud/radiation interaction
SC: Atmospheric Sciences [A]
MN: Fall Meeting 2005