The Fixed Anvil Temperature (FAT) Hypothesis: The Tropics and Climate Sensitivity

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The Fixed Anvil Temperature (FAT) HypothesisThe
Tropics and Climate Sensitivity

• Dennis L. Hartmann and Kristin Larson
• Department of Atmospheric Sciences
• University of Washington
• Seattle, Washington
• AGU A21D-05

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GMS-5 IR image
Cloud Feedback
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High Cloud (plt440mb) in the tropics is most
common over warmest SST, or over land.
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t gt 1
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Finally, the FAT Hypothesis, Fixed Anvil
Temperatures for All Climates.
I want to argue that tropical anvil clouds appear
at a fixed temperature given by fundamental
considerations of
Clausius-Clapeyron definition of saturation
vapor pressure dependence on temperature.
Dependence of emissivity of rotational lines of
water vapor on vapor pressure.
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Rotational Lines of Water Vapor and
Upper- Tropospheric Cooling
Total Beyond 18.5mm --gt
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Fact The radiatively-driven divergence in the
clear regions is related to the decrease of
water vapor with temperature following the
Clausius-Clapeyron relation and the consequent
low emissivity of water vapor at those low
temperatures.
Hypothesis 200 hPa Convective outflow
and associated large-scale divergence near 200
hPa are both associated by radiatively-driven
divergence in clear skies.
Further Conjecture The temperature at which
the radiatively-driven divergence occurs will
always remain the same, and so will the
temperature of the cloud anvil tops.
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Testing the FAT Hypothesis in a model.
Larson and Hartmann (2002a,b) Model Study
MM5 in doubly periodic domain
a) 16x16 box with uniform SST (297, 299, 301,
303K)
b) 16x160 box with sinusoidal SST
Clouds and circulation are predicted
Cloud interact with radiation
Basically a radiative-convective model in which
the large-scale circulation is allowed to play a
role by dividing the domain into cloudy (rising)
and clear (sinking) regions.
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Radiative Cooling in non-convective region for
SSTs ranging from 297K to 303K. From Larson
Hartmann (2002a).
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The temperature of the 200 hPa surface
increases about 13K, while the surface
temperature rises 6K.
The temperature at which the radiative cooling
reaches -0.5 K/day remains constant at about
212K.
The temperature at which the visible optical
depth of upper cloud reaches 0.1 remains
constant at about 200K.
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Kuan-Man Xu, Personal Communication
Dr. Kuan-Man Xu NASA Langley Research Center
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Conclusions
The favored temperature for tropical anvil cloud
tops should remain approximately constant during
climate changes of reasonable magnitude. FAT
Hypothesis. Hartmann and Larson, GRL, 2002.
The emission temperature of the rotational lines
of water vapor should also remain approximately
constant during climate change. Hartmann and
Larson, GRL, 2002. -
These assertions imply relatively strong water
vapor and IR cloud feedback.
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Remaining Questions
To what extent is what I just said correct?
Will the area occupied by tropical convection
change with climate? If so, how?
Will the area, or optical properties of boundary
layer clouds change with climate?
What will happen at the tropical tropopause?
Will it get warmer or colder and what will this
mean for climate?
Fin