Monday, 3 June 2002: 9:30 AM
Effects of clouds on direct radiative forcing by absorptive aerosols
We derive new global estimates of annual-mean shortwave absorption by
carbonaceous aerosols under all-sky conditions. The objective is to
calculate the absorption using cloud and aerosol data sets consistent
with global satellite observations. The estimates are based upon two
improvements relative to previous studies: cloud properties are
obtained from the 3-hourly ISCCP data, and the aerosol distributions
are produced by an aerosol assimilation model constrained by satellite
retrievals. Our calculation is based upon a new, fast
independent-column approximation (ICA) developed for the NCAR climate
models. The estimates are computed for 1998 conditions. The clouds
input to the ICA code are obtained from the 3-hourly global ISCCP
data. The aerosols input to the ICA code are obtained from a global
chemical transport model integrated using global NCEP meteorological
analysis fields. The chemical model is adjusted by
assimilation of satellite retrievals of aerosol optical depth. We
obtain several estimates of the absorption assuming that the aerosols
and cloud droplets are either internally or externally mixed. For the
internal mixtures, the hydrophobic and elemental carbonaceous aerosols
are treated as inclusions.
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