What%20Controls%20Planetary%20Albedo%20and%20its%20Interannual%20Variability%20over%20the%20Cryosphere?

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What Controls Planetary Albedo and its
Interannual Variability over the Cryosphere?

• Xin Qu and Alex Hall
• Department of Atmospheric and Oceanic Sciences,
  UCLA
• 85th AMS Annual Meeting
• San Diego, CA
• 9-13 January 2005
• January 13, 2005

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Q1 Is the surface contribution to climatological
planetary albedo over the cryosphere regions
larger than the atmospheric contribution?
Motivations

• On a global-mean basis, a large portion of
  upwelling solar photons at the top of atmosphere
  are reflected by the atmosphere rather than the
  surface. However, it is unclear whether this is
  true over the cryosphere regions. Large surface
  albedo there may increase the surface
  contribution considerably.

• Surface albedo in the cryosphere regions changes
  from year to year due to fluctuations in sea ice
  and snow. On the other hand, fluctuations in
  atmospheric constituents, such as clouds, also
  result in variability in atmospheric albedo. It
  is unclear which component contributes more to
  planetary albedo variability over the cryosphere
  regions.

Q2 Is the surface contribution to planetary
albedo variability over the cryosphere regions
dominant over the atmospheric contribution?
The answer to this question can be used to
examine the effectiveness of surface albedo
feedback in the climate. For example, if the
surfaces contribution to planetary albedo
variability is small, then the contribution of
surface albedo feedback in the climate would be
negligible.
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Climatological case
?p ?a ?s Te
A simple equation for planetary albedo
(1) (2) (3)

1. Atmospheric albedo
2. Surface albedo
3. Atmospheric effective transmissivity

• The atmosphere attenuates the surfaces
  contribution to planetary albedo in two ways
• The atmosphere absorbs and scatters incoming
  solar radiation, reducing the number of photons
  ultimately reaching the surface.
• The atmosphere absorbs and scatters solar
  radiation reflected by the surface, preventing
  these photons from reaching the top of atmosphere.

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Climatological case
Surface Vs atmosphere
ISCCP D-series flux data set, containing
surface and TOA radiation fluxes (1983-2000).
These were generated based on ISCCP D-series
cloud data set (1983-2000) and a radiative
transfer model.
The cryosphere is segregated into 4 regions NH
snow-covered lands, NH sea ice zone, SH sea ice
zone and Antarctica. We also include
non-cryosphere regions in our analysis for sake
of comparison.
The surface contribution is still considerably
smaller compared to atmospheric contribution in
most of the cryosphere regions. However, in
Antarctica, the surface accounts for more of
planetary albedo than the atmosphere.
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Climatological case
Springtime snow-free lands, NH snow-covered lands
and Antarctica
Surface albedo Atmospheric albedo Atmospheric effective transmissivity
Snow-free lands 0.18 0.22 0.45
NH snow-covered lands 0.32 0.30 0.36
Antarctica 0.77 0.26 0.52
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Variability case
Planetary albedo variability can be divided into
surface and cloud contributions
(?p)2 ?2 (?s)2 (?pc)2 2?(?s, ?pc)
(?r)2
(1) (2)
(3) (4)

• 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
• Covariance The portion linearly related to
  surface and cloud variability but not
  unambiguously attributable to either.
• Residual The portion that cannot be linearly
  related to either surface or cloud variability

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Variability case
Four components to planetary albedo variability
In nearly all cryosphere regions, surface
contribution is dominant over cloud contribution,
accounting for more than 50 of total variability.
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Variability case
Springtime NH snow-covered lands, NH sea ice and
SH sea ice zones
(?s)2 (?pc)2 ?2
NH Snow-covered lands 2.610-3 2.4 10-4 0.14
NH Sea ice zone 3.010-3 1.2 10-4 0.12
SH Sea ice zone 6.310-3 2.2 10-4 0.14
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Conclusions

• Atmospheric albedo accounts for much more of
  climatological planetary albedo than surface
  albedo in most of the cryosphere regions. This is
  because the attenuation effect of atmosphere on
  the surfaces contribution to climatological
  planetary albedo is significantly large.
• In all cryosphere regions, surface contribution
  to interannual variability in planetary albedo is
  dominant over cloud contribution at nearly any
  time of year. This is because the surface albedo
  variability associated with snow and ice
  fluctuations in the cryosphere regions is much
  larger than atmospheric albedo variability due to
  clouds. Even damped by the atmosphere, the
  surface contribution is still dominant over the
  cloud contribution.

Qu X. and Hall A. (2005) What controls planetary
albedo and its interannual variability? Accepted
by J. Climate.