Wednesday, 12 February 2003: 4:45 PM
Sensitivity of cloud-radiation interactions to cloud microphysics
Sam F. Iacobellis, SIO/Univ. of California, La Jolla, CA; and R. C. J. Somerville, G. M. McFarquhar, and D. Mitchell
Poster PDF
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Unrealistic parameterizations are the Achilles' heel of climate models.
When a single-column model (SCM), which consists of one isolated column of a
global atmospheric model, is forced with observational estimates of
horizontal advection terms, the parameterizations within the SCM produce
time-dependent fields which can be compared directly with measurements. In
the case of cloud microphysical schemes, these fields include cloud
altitude, cloud amount, liquid and ice content, particle size spectra, and
radiative fluxes at the surface and the top of the atmosphere. Comparisons
with data from the Atmospheric Radiation Measurement (ARM) Program show
conclusively that prognostic cloud algorithms with detailed microphysics are
far more realistic than simpler diagnostic approaches. Long-term
comparisons of quantities strongly modulated by clouds, such as monthly mean
downwelling surface shortwave radiation, clearly demonstrate the superiority
of parameterizations based on comprehensive treatments of cloud microphysics
and radiative interactions. These results also demonstrate the critical
need for more and better in situ observations of cloud microphysical
variables.
We have used an SCM to examine the sensitivity of fundamental quantities
such as atmospheric radiative heating rates and surface and
top-of-atmosphere radiative fluxes to various parameterizations of clouds
and cloud microphysics. The single-column model was run at the ARM
Southern Great Plains, Tropical Western Pacific, and North Slope of Alaska
sites using forcing data derived from operational numerical weather
prediction. Our results indicate that atmospheric radiative fluxes are
sensitive to the scheme used to specify the ice particle effective radius by
up to 30 W m-2 on a daily time scale and up to 5 W m-2 on a seasonal time
scale. We also found that the inclusion of ice particle fallout can have a
significant effect on the amount and location of high cirrus clouds. On a
seasonal time scale, atmospheric fluxes were sensitive to the inclusion of ice
particle fallout by 8 W m-2. An unexpected finding was that the variance
of the modeled ice particle effective radius at a given level is
considerably smaller than that suggested by ARM cloud radar measurements.
Our results indicate that this
theoretical underestimate of the ice particle effective radius variance can
have effects on modeled radiative fluxes comparable in magnitude to those
cited above for sensitivity to the mean values of ice particle effective
radius.
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