Transcript Qu and Hall (2007)

Snow albedo feedback:
a comparison between climate models
and new observational data
Chris Fletcher
Collaborators: Paul Kushner,
Richard Fernandes, Hongxu Zhao
January 19, 2010
SAF Fundamentals
 Climate feedbacks amplify the simple effect of CO2 doubling
 The dominant feedbacks involve water phase changes:
• vapour to liquid/solid: changes to cloud amount & altitude
• liquid to vapour: abs. humidity increases exponentially with T
• solid to liquid/vapour: solar absorption increases after melting
snow & ice
 SAF amplifies global warming by 10-20%, but is worth
studying because it:
• has potentially large regional impacts on climate and hydrology
• is well constrained theoretically
• offers the prospect of more accurate measurement in nature
and therefore the opportunity to constrain models (challenging with
other feedbacks).
Understanding SAF using models
Normal SAF
Suppressed SAF
In a 2xCO2 simulation with a coupled climate
model, SAF contributes an extra 50-75% to the
warming response over northern high latitudes.
Hall [2004]
Summertime surface circulation response to
climate change associated with SAF
tas
soil moisture
psl
wind
All results are for
JJA means from 17
models
Fletcher et al. [2009]
Predictions of SAF under climate change vary
significantly among current models.
Qu and Hall (2007)
Predictions of SAF under climate change vary
significantly among current models.
TOTAL SAF ~
Qu ic kTi me™ a nd a
de co mpre ss or
are n ee de d to s ee th is pi ctu re .
TOTAL SAF = Snow Cover Changes
+ Snow Metamorphosis
Qu and Hall (2007)
Present
Snow Cover
Changes
Future?
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decompressor
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MODIS: Feb 6, 2005
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Metamorphosis
MODIS: Mar 14, 2005
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are nee ded to s ee this picture.
Key result 1: Snow cover component
is most important
Qu and Hall (2007)
Key result 2: SAF from present day seasonal cycle
is a good predictor of SAF under climate change
Hall and Qu (2006)
So, what do the observations show?
TOTAL
Snow Cover
Metamorphosis
OBS
MODELS
Data
• Obs modified from
Fernandes et al. (2009)
• Seasonal Cycle
• 1900-99 (20c3m)
• MAM avg: (MA, AM, MJ)
• 17-model mean
Fletcher et al. (in prep.)
Zonal averages
TOTAL
Snow Cover
Metamorphosis
MAX SPREAD
MAX SPREAD
OBS
17-model mean
Model std dev
Fletcher et al. (in prep.)
Metamorphosis component shows largest
uncertainty: it is method dependent
Fletcher et al. (in prep.)
TOTAL
Snow Cover
Metamorphosis
Does the seasonal
cycle argument hold
for SAF subcomponents?
Fletcher et al. (in prep.)