The Spectral Radiative Effects of Inhomogeneous
Clouds and Aerosols
S. Schmidt, B. Kindel, & P. Pilewskie
Laboratory for Atmospheric and Space Physics
University of Colorado
SORCE Science Meeting, 13-16 Sep. 2011
Clouds & Aerosols
Schmidt page 1
Wavelength-dependent deposition of solar irradiance in atmosphere
atmospheric heating rates
SORCE Science Meeting, 13-16 Sep. 2011
• This example shows where in the
atmosphere solar irradiance is
depleted for a clear-sky scene.
• This is important for dynamics and
shortwave radiative energy budget.
• This presentation: What happens if
clouds and aerosols are added to
the picture.
• Most of the time, the spatial
structure of cloud-aerosol fields is
complex, which leads to formerly
unknown spectral effects.
Clouds & Aerosols
Schmidt page 2
The contribution of aerosols to atmospheric optical thickness
SORCE Science Meeting, 13-16 Sep. 2011
Clouds & Aerosols
Schmidt page 3
Aerosol Optical Properties…
Bergstrom et al., 2003; 2007; Pilewskie et al., 2005; 2010; Russell et al. 2010
Cloud Optical Properties
optical thickness & effective radius …
=7
re = 27 μm
• …can be determined from
two wavelengths for
homogeneous scenes.
• The cloud radiative effects
(forcing, absorption) can then
be calculated from passive
imager observations + scenedependent angular
distribution of radiance
Kindel et al., 2010
SORCE Science Meeting, 13-16 Sep. 2011
Clouds & Aerosols
Schmidt page 5
For heterogeneous clouds…
 = 53
re = 30 μm
• …the spectral dependence of
albedo (reflectance) can no
longer be re-calculated from
the two retrieved
parameters, optical thickness
and effective radius.
• Consequently, spectral and
broadband forcing and
absorption cannot be based
on these parameters alone.
Kindel et al., 2010
SORCE Science Meeting, 13-16 Sep. 2011
Clouds & Aerosols
Schmidt page 6
A new method for deriving spectral shortwave cloud
absorption from aircraft …
SORCE Science Meeting, 13-16 Sep. 2011
Kindel
, Pilewskie,
et al., 2011
Clouds &
Aerosols
Schmidt page 7
… that agrees well with modeled absorption
Model
2% residual
SORCE Science Meeting, 13-16 Sep. 2011
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Contributions to Absorbed Irradiance In Cloud
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Kindel, Pilewskie, et al.,
2011; Schmidt & Pilewskie, 2011
Clouds & Aerosols
SORCE Science Meeting, 13-16 Sep. 2011
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Schmidt page 9
Heterogeneous Clouds
Need to understand the relationship between spatial structure and
spectral radiative cloud effects.
Aircraft data (TC4)  3D radiative transfer model
SORCE Science Meeting, 13-16 Sep. 2011
Clouds & Aerosols
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Heterogeneous Clouds Reproduce Apparent Absorption
“colored” horizontal flux divergence
SORCE Science Meeting, 13-16 Sep. 2011
Schmidt et al., 2010
Clouds & Aerosols
Schmidt page 11
3-D Clouds and Aerosols
What is the combined radiative effect of clouds and aerosols?
Can the radiative forcing of a cloud-aerosol field be calculated
from that of aerosols and clouds individually?
F, F
SORCE Science Meeting, 13-16 Sep. 2011
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3-D Clouds and Aerosols
"CLD"
SSFR 16:30 0.20
LES runs 19:48
cloud+aerosol
cloud only
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Extreme cloud edge effect
(individual cloud)
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"GAP"
PDF
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Downward Irradiance [W m nm ]
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3-D Clouds and Aerosols
Aerosols decrease direct
irradiance / increase
diffuse irradiance
clouds enhance irradiance
3-D Clouds and Aerosols
These observations lead to the conclusion (Schmidt et al., GRL,
2009) that for heterogeneous scenes:
fcld-aer(λ) ≠ fcld(λ) + faer(λ)
In summary:
• Inhomogeneous clouds introduce spectral biases that need to be considered in
the deposition of solar irradiance in the atmosphere and at the surface.
Optical thickness and effective radius alone are not sufficient to determine
radiative effects.
• In heterogeneous scenes, aerosols superimpose their radiative signature, and
the cloud-aerosol radiative properties cannot be obtained from the sum of
cloud and aerosol effects.
 None of these effects have been validated on a global scale.
 Need full spectral imagery from space.
More detail: Schmidt and Pilewskie, 2011: Airborne Measurements of Spectral
Shortwave Radiation in Cloud and Aerosol Remote Sensing and Energy Budget Studies,
Light Scattering Reviews, 6, A. Kokhanovsky (ed.), Springer
ftp://laspftp.colorado.edu/pub/schmidt/LSR/
SORCE Science Meeting, 13-16 Sep. 2011
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Schmidt page 15