Transcript Wm -2 - CFMIP

Towards understanding the mechanisms
responsible for different cloud-climate
responses in GCMs.
Mark Webb, Adrian Lock (Met Office), Tomoo Ogura (NIES)
4th PAN-GCSS meeting: “Advances in modelling and observing
clouds and convection.” Toulouse , June 2008
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Acknowledgements
Sandrine Bony, Chris Bretherton, Gill Martin,
William Ingram, Bjorn Stevens, Joao Teixeira,
George Tselioudis, Mark Ringer, Keith Williams,
Rob Wood
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CFMIP-GCSS activities for better
understanding of cloud-climate feedback
processes
• Process studies based on the analysis of high-frequency
climate model output e.g. GPCI, ARM sites
• Sensitivity tests (in GCMs and SCMs)
• Idealised low cloud feedback studies with SCM/CRM/LES
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Standard deviation of cloud radiative
responses to CO2 doubling (W/m-2)
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SW cloud radiative response
TWP


GPCI
 
SE Pacific 
1
Wm-2

Triangles show ensemble standard deviations of local values
Squares show correlation of local and global values across ensemble
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LW cloud radiative response
TWP


GPCI
 
SE Pacific 
1
Wm-2-2
Wm

Triangles show ensemble standard deviations of local values
Squares show correlation of local and global values across ensemble
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CFMIP-2 Pilot experiments
HadGEM3 development version containing:
PC2 cloud scheme with prognostic cloud fraction, liquid and ice
Lock boundary layer scheme
Standard experiments (STD)
a/ 10 year atmos-only forced with SST/sea ice obs climatology
b/ CO2 forcing: fixed SST + 2xCO2 (Hansen method)
c/ Climate change: control + CMIP 1% patterned SST composite
d/ Climate change: control + 2K uniform SST
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GPCI
Control
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2CO2
Patterned SST
Uniform SST
SETP
Control
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2CO2
Patterned SST
Uniform SST
Could the SW responses be mainly due
to transitions between boundary layer
types?
Lock scheme uses following BL types:
1. Stable
2. Sc over stable
3. Well mixed
4. Decoupled Sc not over Cu
5. Decoupled Sc over Cu
6. Cumulus capped
decreasing cloud fraction/water
higher cloud top
positive SW response?
If so we would expect to see consistent changes in frequency
of BL types and the SW CRF response
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SETP
Control
2CO2
Patterned SST
Uniform SST
Control
2CO2
Patterned SST
Uniform SST
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SETP
Control
2CO2
Patterned SST
Uniform SST
Control
2CO2
Patterned SST
Uniform SST
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Are the SW responses mainly due to
transitions between BL types?
This idea is not supported for 2CO2, but is for the SST forcings:
3. Well mixed Sc
4. Decoupled Sc not over Cu
5. Decoupled Sc over Cu
6. Cumulus capped
SW CRF response
Pat +2K
+ve +ve
-ve
Transitions between BL types are larger in the SST forced
runs, so these may be dominating smaller changes in the SW
caused by cloud changes within the BL regimes
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How can we trace these responses
back to modelling assumptions?
In PC2 the largest source terms for clouds in this region are:
- condensation due to LW cooling (mainly at cloud top)
- condensate detrained from shallow convection
We can repeat the experiments removing these terms:
1/ liquid condensate formed through LW cooling falls out (No CTC)
2/ liquid condensate from convective detrainment falls out (No CD)
We might expect LW cooling to be more important nearer the
coast, and convective detrainment to be more important in the
centre of the section
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Control
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2CO2
Patterned SST
Uniform SST
Control
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2CO2
Patterned SST
Uniform SST
Control
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2CO2
Patterned SST
Uniform SST
Control
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2CO2
Patterned SST
Uniform SST
Summary
In HadGEM3/PC2, SETP SW responses to SST forcing are
largely consistent with transitions between BL types
Responses near the coast coincide with transitions between
non-convective BL types, and require condensation
from LW cooling but not convective detrainment.
Responses further west coincide mainly with transitions between
decoupled Sc over Cu and trade Cu, in opposite directions for
uniform and patterned SST perturbations.
These require convective detrainment, but are weaker without
LW cooling
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Future work
Caldwell and Bretherton (submitted) argue that reductions in LW
cooling due to increases in CO2 or water vapour can lead to
reduced cloud top entrainment and a shallower BL
We can change or remove the dependence of entrainment on
LW cooling to see if this mechanism explains the reductions in
cloud near BL top in our results
Why does shallow convective detrainment reduce? Does
reduced LW cooling of the BL force a reduction in latent heat
release from shallow convection?
We could test this by discarding the latent heating from shallow
convection.
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Concluding remarks
The South East Tropical Pacific is a convenient area to study
shallow cloud feedbacks in GCMs
Sensitivity tests in CFMIP-2 could help us to understand cloud
response mechanisms at play in other GCMs
We will be able to do many more such sensitivity tests if we can
develop relevant SCM forcing cases
Capturing the important shallow cloud feedbacks across models
may well require cases for well mixed, convective and transition
boundary layers
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