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J. ATMOS. SCI., 65, 1979-1990, 2008.
Shortwave radiative impacts from aerosol effects on marine shallow cumuli.
P. Zuidema, H. Xue, and G. Feingold
Abstract
The net shortwave radiative impact of aerosol on simulations of two shallow marine cloud cases is
investigated using a Monte Carlo radiative transfer model. For a shallow cumulus case, increased aerosol
concentrations are associated not only with smaller droplet sizes but also reduced cloud fractions and cloud
dimensions, a result of evaporation-induced mixing and a lack of precipitation. Three-dimensional radiative
transfer (3DRT) effects alter the fluxes by 10%–20% from values calculated using the independent column
approximation for these simulations. The first (Twomey) aerosol indirect effect is dominant but the de-
creased cloud fraction reduces the magnitude of the shortwave cloud forcing substantially. The 3DRT
effects slightly decrease the sensitivity of the cloud albedo to changes in droplet size under an overhead sun
for the two ranges of cloud liquid water paths examined, but not strongly so. A popular two-stream radiative
transfer approximation to the cloud susceptibility overestimates the more directly calculated values for the
low liquid-water-path clouds within pristine aerosol conditions by a factor of 2 despite performing well
otherwise, suggesting caution in its application to the cloud albedos within broken cloud fields. An evalu-
ation of the influence of cloud susceptibility and cloud fraction changes to a “domain” area-weighted cloud
susceptibility found that the domain cloud albedo is more likely to increase under aerosol loading at interme-
diate aerosol concentrations than under the most pristine conditions, contrary to traditional expectations.
The second simulation (cumulus penetrating into stratus) is characterized by higher cloud fractions and
more precipitation. This case has two regimes: a clean, precipitating regime where cloud fraction increases
with increasing aerosol, and a more polluted regime where cloud fraction decreases with increasing aerosol.
For this case the domain-mean cloud albedo increases steadily with aerosol loading under clean conditions,
but increases only slightly after the cloud coverage decreases. Three-dimensional radiative transfer effects
are mostly negligible for this case. Both sets of simulations suggest that aerosol-induced cloud fraction
changes must be considered in tandem with the Twomey effect for clouds of small dimensions when
assessing the net radiative impact, because both effects are drop size dependent and radiatively significant.
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