HR: 09:30h
AN: A41C-06    [Abstracts]
TI: Radiative Effect of Clouds on Tropospheric Chemistry in a Global Three-Dimensional Chemical Transport Model
AU: * Liu, H
EM: hyl@nianet.org
AF: National Institute of Aerospace, 144 Research Drive, Hampton, VA 23666 United States
AU: Crawford, J H
EM: j.h.crawford@larc.nasa.gov
AF: NASA Langley Research Center, Mail Stop 401B, Hampton, VA 23681 United States
AU: Chen, G
EM: g.chen@larc.nasa.gov
AF: NASA Langley Research Center, Mail Stop 401B, Hampton, VA 23681 United States
AU: Pierce, R B
EM: r.b.pierce@larc.nasa.gov
AF: NASA Langley Research Center, Mail Stop 401B, Hampton, VA 23681 United States
AU: Norris, P
EM: pnorris@gmao.gsfc.nasa.gov
AF: NASA Goddard Space Flight Center, Code 913, Greenbelt, MD 20771 United States
AU: Norris, P
EM: pnorris@gmao.gsfc.nasa.gov
AF: University of Maryland, Baltimore County, Baltimore, MD 21250 United States
AU: Platnick, S E
EM: steven.e.platnick@nasa.gov
AF: NASA Goddard Space Flight Center, Code 913, Greenbelt, MD 20771 United States
AU: Kittaka, C
EM: fn.c.kittaka@larc.nasa.gov
AF: Science Applications International Corporation, Suite 120, One Enterprise Parkway, Hampton, VA 23666 United States
AU: Yantosca, R M
EM: bmy@io.harvard.edu
AF: Harvard University, 29 Oxford Street, Cambridge, VA 02138 United States
AB: Clouds exert an important influence on tropospheric photochemistry through modification of solar radiation which determines photolysis rates (J-values). We assess the radiative effect of clouds on photolysis rates and key oxidants in the troposphere with a global three-dimensional (3-D) chemical transport model (GEOS-CHEM) driven by assimilated meteorological observations from the Goddard Earth Observing System data assimilation system (GEOS DAS) at the NASA Global Modeling and Assimilation Office (GMAO). We focus on the year of 2001 with the GEOS-3 meteorological observations. Photolysis rates are calculated using the Fast-J algorithm of Wild et al. [2000], which uses a seven-wavelength quadrature scheme and accounts accurately for Mie scattering by clouds. The GEOS-3 global 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). The impact of clouds on radiative transfer is assessed using three different methods: 1) the uniform cloud distribution method (most commonly used), 2) the random overlap scheme, and 3) the maximum-random overlap scheme. Results using the uniform cloud distribution method, which assumes linear scaling of cloud optical depth with cloud fraction in a grid-box, show global monthly mean OH concentrations generally increase by less than 5% due to the radiative effect of clouds. The OH distribution shows much larger changes, reflecting the opposite effects of enhanced (weakened) photochemistry above (below) clouds. Nevertheless, the change in the calculated methylchloroform lifetime is insignificant. The global monthly mean photolysis rates for J[O$_{3}$] and J[NO$_{2}$] in the troposphere are enhanced by less than 10% due to clouds; global mean O$_{3}$ concentrations in the troposphere are increased by less than 5%. Even though the uniform cloud distribution method presumably overestimates the backscattering of solar radiation above the clouds, our results emphasize that dominant effect of clouds is to influence the vertical redistribution of the intensity of photochemical activity while the global average effect remains modest, contrasting with previous studies. Using the random overlap scheme or the maximum-random overlap scheme reduces the impact of clouds on photochemistry, but does not change our results qualitatively. The results from sensitivity experiments where the model cloud optical depth is adjusted progressively will also be discussed.
DE: 3337 Numerical modeling and data assimilation
DE: 3359 Radiative processes
DE: 0300 ATMOSPHERIC COMPOSITION AND STRUCTURE
DE: 0317 Chemical kinetic and photochemical properties
DE: 0365 Troposphere--composition and chemistry
SC: Atmospheric Sciences [A]
MN: 2004 AGU Fall Meeting