Alex Hall, Xin Qu, UCLA Dep

1
What controls the variability of net incoming
solar radiation?
Alex Hall, Xin Qu, UCLA Dept of Atmospheric and
Oceanic Sciences
standard deviation of planetary albedo () in the
ISCCP D2 data set broken down by season
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We assess the controls on planetary albedo
variability by examining the ISCCP D2 data set
(1983-2000). For both clear and all-sky cases,
the ISCCP data set (D2) contains (1) surface
radiation fluxes (2) TOA radiation fluxes These
were generated based on observations at 3
different channels (visible, near IR, and IR) and
a radiative transfer model. Rossow and Gardner
(1993a and b) J Clim. Rossow and Schiffer (1999)
BAMS.
3

• Planetary albedo variability can be divided into
  contributions from four components
• SURFACE the portion unambiguously related in
  linear fashion to surface albedo variability
• CLOUD The portion unambiguously related in
  linear fashion to cloud cover and optical depth
  variability
• RESIDUAL The portion that cannot be linearly
  related to either surface or cloud variability
• COVARIANCE The portion linearly related to
  surface and cloud variability but not
  unambiguously attributable to either.

(1) (2) (3)
(4)
SURFACE CLOUD RESIDUAL COVARIANCE
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• We defined six regions, guided by known
  differences in the behavior of surface albedo
  variability
• northern hemisphere snow-covered lands
• northern hemisphere sea ice zone
• southern hemisphere sea ice zone
• snow-free lands
• ice-free ocean
• Antarctica
• We averaged the contributions of the four
  components over each region for each season and
  normalized by the total planetary albedo
  variability. Note that the definition of the
  regions varies seasonally.

SURFACE CLOUD RESIDUAL COVARIANCE
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The surface contribution to planetary albedo
variability in ISCCP is significant everywhere
except for the ice-free oceans. It is dominant
in the SH sea ice zone year around, and in the
other cryosphere regions for most of the year.
SURFACE CLOUD RESIDUAL COVARIANCE
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Comparison to CCSM3 To allow for as direct a
comparison with the ISCCP data as possible, we
used a simulated time series with approximately
the same mix of internal variability and
externally-forced climate change --a CCSM3
scenario run was used (b30.030c and b30.040c time
series). --Data was taken from the same time
period as ISCCP (1983-2000).
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Controls on planetary albedo variability in CCSM3
SURFACE CLOUD RESIDUAL COVARIANCE
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CCSM3
ISCCP
A side-by-side comparison of CCSM3 and ISCCP
reveals much more contribution from the surface
in ISCCP to interannual planetary albedo
variability in all regions except for ice-free
oceans.
SURFACE CLOUD RESIDUAL COVARIANCE
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CCSM3
ISCCP
A prominent example of the difference between
CCSM3 and ISCCP is in the NH snow-covered land
areas during all seasons.
SURFACE CLOUD RESIDUAL COVARIANCE
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Why is the contribution of the surface so much
smaller in CCSM3? --Is it that clouds are more
variable, increasing the relative contribution of
clouds? --Or is surface albedo itself less
variable in CCSM3? --Or is it that the CCSM3
atmosphere is more opaque to solar radiation,
attenuating the effect of surface albedo
anomalies?
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ISCCP
Clear-sky surface albedo standard deviation ()
in ISCCP and CCSM3, broken down by season. ISCCP
has consistently more surface albedo variability.
CCSM3
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ISCCP
Clear-sky surface albedo standard deviation ()
in ISCCP and CCSM3, broken down by season. The
larger surface albedo variability in ISCCP is
particularly apparent in the interior of the
northern hemisphere snowpack. (e.g. DJF, MAM)
CCSM3
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CONCLUSIONS --The surface contribution to
planetary albedo variability in ISCCP is
significant everywhere except for the ice-free
oceans. It is dominant in the SH sea ice zone
year around, and in the other cryosphere regions
for most of the year. --CCSM3 has substantially
less surface albedo variability than ISCCP,
resulting in a significantly smaller contribution
of the surface to planetary albedo variability.