11th Conference on Atmospheric Radiation and the 11th Conference on Cloud Physics

Wednesday, 5 June 2002: 11:45 AM
Impact of Mie-resonances on the atmospheric absorption of water clouds.
Warren Wiscombe, NASA/GSFC, Greenbelt, MD; and S. Kinne and H. Nussenzveig
Clouds strongly modulate radiative transfer processes in the Earth's atmosphere. Studies, which simulate bulk properties of clouds, such as absorption, require methods, that accurately account for multiple scattering among individual cloud particles. Good approximations for multiple scattering processes are provided by MIE-theory, if atmospheric particles have a spherical shape. This is a good assumption for water droplets. Thus, simulations for water clouds (especially for interaction with solar radiation) commonly apply readily available MIE-codes. These codes, however, miss contributions from densely distributed, sharp resonances. Despite their usually narrow width, integrated over the entire size-spectrum of a cloud droplet distribution, the impact of missed resonances may add up. The consideration of these resonances tends to increase cloud extinction and cloud absorption. This mechanism for a larger (than by MIE-methods predicted) solar absorption has the potential to explain observational evidence of larger than predicted cloud absorption at solar wavelengths. The presentation will address the absorption impact of added resonances for typical properties of water clouds (e.g. drop size distributions, drop concentrations and cloud geometry). Special attention will be given to scenarios with observational evidence of larger than simulated solar absorption; particularly if simultaneous measurements of cloud micro- and macrophysical properties are available.

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