Radiative Effect of Clouds on Tropospheric Chemistry: Sensitivity to Cloud Vertical Distributions and Optical Properties

HR: 0800h
AN: A51C-0062 [Abstracts]
TI: Radiative Effect of Clouds on Tropospheric Chemistry: Sensitivity to Cloud Vertical Distributions and Optical Properties
AU: * Liu, H
EM: hyl@nianet.org
AF: National Institute of Aerospace, 100 Exploration Way, Hampton, VA 23662-6147 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: Pierce, R B
EM: r.b.pierce@larc.nasa.gov
AF: NASA Langley Research Center, Mail Stop 401B, Hampton, VA 23681 United States
AU: Considine, D B
EM: david.b.considine@nasa.gov
AF: NASA Langley Research Center, Mail Stop 401B, Hampton, VA 23681 United States
AU: Logan, J A
EM: jal@io.harvard.edu
AF: Harvard University, 29 Oxford Street, Cambridge, MA 02138 United States
AU: Duncan, B N
EM: duncan@hyperion.gsfc.nasa.gov
AF: NASA Goddard Space Flight Center, Code 613.3, Greenbelt, MD 20771 United States
AU: Duncan, B N
EM: duncan@hyperion.gsfc.nasa.gov
AF: University of Maryland at Baltimore County, 1000 Hilltop Circle, Baltimore, MD 21250 United States
AU: Norris, P
EM: pnorris@gmao.gsfc.nasa.gov
AF: NASA Goddard Space Flight Center, Code 613.3, Greenbelt, MD 20771 United States
AU: Norris, P
EM: pnorris@gmao.gsfc.nasa.gov
AF: University of Maryland at Baltimore County, 1000 Hilltop Circle, Baltimore, MD 21250 United States
AU: Platnick, S E
EM: steven.e.platnick@nasa.gov
AF: NASA Goddard Space Flight Center, Code 613.3, Greenbelt, MD 20771 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: Yantosca, R M
EM: bmy@io.harvard.edu
AF: Harvard University, 29 Oxford Street, Cambridge, MA 02138 United States
AU: Evans, M J
EM: mat@env.leeds.ac.uk
AF: Harvard University, 29 Oxford Street, Cambridge, MA 02138 United States
AU: Evans, M J
EM: mat@env.leeds.ac.uk
AF: University of Leeds, School of Earth and the Environment, Leeds, LS2 9JT United Kingdom
AB: Representation of clouds in global models poses a significant challenge since most cloud processes occur on sub-grid scales and must be parameterized. Uncertainties in cloud distributions and optical properties are therefore a limiting factor in model assessments of the radiative effect of clouds on global tropospheric chemistry. We present an analysis of the sensitivity of the radiative effect of clouds to cloud vertical distributions and optical properties with the use of the GEOS-CHEM global 3-D chemistry transport model coupled with the Fast-J radiative transfer algorithm. GEOS-CHEM was driven with a series of meteorological archives (GEOS1-STRAT, GEOS-3, and GEOS-4) generated by the Goddard Earth Observing System data assimilation system (GEOS DAS) at the NASA global Modeling and Assimilation Office (GMAO), which have significantly different cloud optical depths and vertical distributions. The column cloud optical depths in GEOS-3 generally agree with the satellite retrieval products from the Moderate Resolution Imaging Spectroradiometer (MODIS) and the International Satellite Cloud Climatology Project (ISCCP) within ±10%, while those in GEOS1-STRAT and GEOS-4 are too low by factors of about 5 and 2, respectively. With respect to vertical distribution, clouds in GEOS-4 are optically much thinner in the tropical upper troposphere compared to those in GEOS1-STRAT and GEOS-3. Assuming linear scaling of cloud optical depth with cloud fraction in a grid-box, our model calculations indicate that the changes in global mean hydroxyl radical (OH) due to the radiative effect of clouds in June are about -1% (GEOS1-STRAT), 1% (GEOS-3), and 14% (GEOS-4), respectively. The effects on global mean OH are similar for GEOS1-STRAT and GEOS-3 due to similar vertical distributions of clouds, even though the column cloud optical depths in the two archives differ by a factor of about 5. Clouds in GEOS-4 have a much larger impact on global mean OH because more solar radiation is able to penetrate through the optically thin clouds in the upper troposphere, increasing backscattering from low-level clouds. These illustrate that the radiative impact of clouds on global tropospheric chemistry is more sensitive to cloud vertical distribution than to the magnitude of column cloud optical depth. Model simulations with each of the three cloud distributions all show that tropical upper tropospheric ozone (O₃) is much less sensitive to the radiative effect of clouds than previously reported by Tie et al. [2003] using MOZART-2 (~5% versus ~20-30%), suggesting that differing cloud vertical distributions cannot explain the majority of the discrepancies between the two models. Sensitivities to progressively adjusted cloud optical depths in GEOS-3 and to cloud absorption assumptions will also be discussed.
UR: http://research.nianet.org/~hyl
DE: 0360 Radiation: transmission and scattering
DE: 0365 Troposphere: composition and chemistry
DE: 3359 Radiative processes
SC: Atmospheric Sciences [A]
MN: Fall Meeting 2005