HR: 17:35h
AN: A14B-07    [Abstracts]
TI: Evidence for Enhanced Aerosol Indirect Effects in Low-Level Arctic Mixed-Phase Clouds
AU: * Fridlind, A M
EM: ann.fridlind@nasa.gov
AF: NASA Goddard Institute for Space Studies, 2880 Broadway, New York, NY 10025, United States
AU: Ackerman, A S
EM: andrew.ackerman@nasa.gov
AF: NASA Goddard Institute for Space Studies, 2880 Broadway, New York, NY 10025, United States
AB: Several decades of research has greatly broadened our understanding of aerosol indirect effects on cloud radiative forcings regionally and globally, especially for low-level marine clouds. However, little work has yet focused on aerosol indirect effects on radiative forcing by low-level mixed-phase clouds. Here we present evidence that glaciation enhances aerosol indirect effects on cloud radiative forcing in the Arctic, based on our analysis of large-eddy simulations with mixed-phase size-resolved microphysics, using data from three field campaigns---the 1984 Beaufort Arctic Storms Experiment (BASE), the 1998 Surface Heat Budget of the Arctic (SHEBA) experiment, and the 2004 Mixed-Phase Arctic Cloud Experiment (MPACE). Where the aerosol indirect effect is primarily positive (associated with surface warming), owing to unique regional conditions of low- emissivity cloud cover over sea ice at low solar zenith angles, the tendency of increasing aerosol numbers to decrease glaciation (in addition to increasing cloud emissivity) leads to enhanced surface warming. Similarly, where the aerosol indirect effect is primarily negative (associated with surface cooling), owing to high-emissivity cloud cover over open ocean at higher solar zenith angles, the tendency of increasing aerosols to decrease glaciation (in addition to increasing cloud optical depth) leads to enhanced surface cooling. While ice formation mechanisms remain highly uncertain in the moderately supercooled clouds that occurred during all three field experiments and are typical of many Arctic environments, we use field data to constrain our model performance under three cases: clean conditions over ice with low-emissivity clouds (during BASE), polluted conditions over ice with low-emissivity clouds (during SHEBA), and clean conditions over open ocean with high-emissivity clouds (during M-PACE). We then perform sensitivity tests in which aerosol conditions are switched for each field experiment in order to evaluate the strength of the indirect effect. In low-emissivity clouds under polluted conditions, we find that reduced glaciation dominates the total aerosol indirect effect because liquid water path and cloud emissivity are significantly increased when desiccation by precipitating ice is reduced.
DE: 0305 Aerosols and particles (0345, 4801, 4906)
DE: 0320 Cloud physics and chemistry
DE: 0321 Cloud/radiation interaction
DE: 3310 Clouds and cloud feedbacks
DE: 3311 Clouds and aerosols
SC: Atmospheric Sciences [A]
MN: 2007 Fall Meeting