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Informal Seminar by Hongyu Liu |
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Date: August 3, 2005
Time: 12:00pm
Location: NIA, Rm 137
Radiative Effect of Clouds on Tropospheric Chemistry and Sensitivity to Cloud Vertical Distribution and Optical Properties
Hongyu Liu, NIA
Clouds play critical roles in influencing not only the earth's climate through modulation of the earth's energy and hydrological cycles but also tropospheric photochemistry through modification of solar radiation that determines photolysis rates. The radiative effect of clouds on tropospheric photochemistry and the associated uncertainty, however, have been paid little attention in the literature. A quantitative understanding of this effect is required for using global models to assess anthropogenic perturbations to the Earth system.
We present an assessment of the radiative effect of clouds on photolysis rates and key oxidants in the troposphere with a global three-dimensional chemical transport model (GEOS-CHEM) coupled with a fast radiative transfer model (Fast-J). The model (GEOS-3) cloud optical depth and cloud fraction are evaluated with the satellite retrieval products from the Moderate Resolution Imaging Spectroradiometer (MODIS) and the International Satellite Cloud Climatology Project (ISCCP). Our results contrast with previous studies, in particular, in finding that the dominant effect of clouds is to influence the vertical redistribution of the intensity of photochemical activity while the global average effect remains modest. The sensitivities to cloud overlap assumptions, cloud vertical distribution, and cloud optical properties will be discussed.
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