The Fixed Anvil Temperature
(FAT) Hypothesis:
The Tropics and Climate Sensitivity
Dennis L. Hartmann and Kristin Larson
Department of Atmospheric Sciences
University of Washington
Seattle, Washington
AGU A21D-05
GMS-5
IR image
Cloud Feedback
High Cloud (p<440mb)
in the tropics
is most common over
warmest SST,
or over land.
t>1
9%
22%
10%
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.
Rotational Lines
of Water Vapor
and UpperTropospheric
Cooling
Total Beyond 18.5mm -->
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.
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.
Radiative Cooling
in non-convective
region for SST’s
ranging from
297K to 303K.
From Larson &
Hartmann (2002a).
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.
Kuan-Man Xu, Personal Communication
Dr. Kuan-Man Xu
NASA Langley Research Center
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.
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