Transcript Slide 1

Impact of convection schemes on
cloud feedbacks in the tropics
Mark Ringer
CFMIP/GCSS, Vancouver, June 8-12th 2009.
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Aims and outline
• Initial study of structural uncertainty & cloud
feedbacks – in this case dependence on
convection scheme
• Compare with parameter-perturbed (QUMP,
climateprediction.net) & multi-model ensembles
• Focus on tropical response to 2 × CO2 in Met
Office slab model, HadSM3
• Three experiments, using Gregory-Rowntree
(“Standard”, STD), Emanuel (EMA), Betts-Miller
(B-M) convection schemes
• “Usual” diagnostics (cloud radiative forcing,
ISCCP clouds) plus analysis methods from three
recent papers
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Surface temperature change (K/K)
HadSM3 is dominated by minima in
warming over Pacific & Atlantic
STD
B-M
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EMA
ΔT(K) STD
EMA B-M
Mean
2.69
3.47
3.37
Ocean 2.18
3.02
2.89
Land
4.15
4.79
4.76
Global 3.47
4.03
3.77
This is also a feature of the coupled
model’s response to warming
STD
EMA
STD: Coupled: A1B:2090s-2000s
B-M
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Change in cloud forcing (Wm-2/K)
Minimum in the warming is clearly
associated with local cloud changes
STD
EMA
B-M
Large negative cloud feedback
in SE Pacific in STD, weaker in
EMA, opposite sign in B-M.
(Similar in Atlantic)
Larger positive feedback over
land, particularly over South
America, in STD.
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The cloud forcing changes are also
evident in the coupled model
STD
EMA
STD: Coupled: A1B:2090s-2000s
B-M
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Change in ISCCP high cloud (%/K)
Large responses in STD & EMA,
weak response in B-M
Thin
STD
EMA
B-M
Thick
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Change in ISCCP low cloud (%/K)
Large response in STD, weakens in
EMA, changes sign in B-M
All
STD
EMA
B-M
Thick
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500 hPa vertical velocity (hPa/day/K)
STD has a strong large-scale
dynamical response
STD
B-M
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EMA
This gives a good indication of
the changes to the large-scale
circulation – the major areas of
increasing/decreasing ascent &
descent.
Precipitation change relative to
control (%/K)
Strongly related to vertical velocity
STD
EMA
B-M
In general, increased ascent
corresponds to increased
precipitation and vice versa.
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“Dynamic” precipitation (%/K)
Large areas in STD where dynamic
changes lead to increased precipitation
STD
EMA
B-M
Difference between the
precipitation change and the rate
of moistening from the ClausiusClapeyron relation (i.e. ~ 7% K-1
of warming)
(following Vecchi & Soden, 2007)
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B-M more closely resembles AR4
coupled models
(This pattern is also seen in AR4 slab models)
STD
EMA
B-M
(Figure 8c from Vecchi & Soden, 2007)
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Links to SST variability in key regions
(following Toniazzo et al. 2007)
Nino 3.4
SE Pacific
7.5-17.5ºS
80-125ºW
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Nino 3
In STD, increased low-level
cloud in SETP associated
with increased SST
variability locally and over
the Equator.
This behaviour is
suppressed with EMA and
B-M schemes.
Relationship between lower and upper
level moisture over tropical ocean
High correlation with all three schemes
STD
EMA
Joint pdfs binned
into intervals of
lower and upper
tropospheric
moisture
(PWC – UTH)
B-M
r(UTH, PWC)
OBS 0.85
STD
0.79
EMA 0.89
B-M
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0.84
(following
Bennhold &
Sherwood 2008)
Relation between changes in lower
and upper level moisture is much
weaker with EMA and B-M schemes
STD
B-M
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EMA
r(ΔUTH, ΔPWC)
STD 0.84
EMA 0.32
B-M 0.44
pdfs of changes
in UTH & PWC
under 2×CO2
Summary – 1
• Changing the convection scheme in HadSM3 has a
significant impact on the tropical surface
temperature response to CO2 doubling which is
linked to different responses of low-level cloud
With the standard Gregory-Rowntree scheme (STD):
• The increased low cloud in the SE Tropical Pacific
=> increased SST variability
=> large-scale pattern of cloud/precipitation changes
• Circulation changes act to increase precipitation
changes in many areas
• Strong connection between moisture changes in the
lower and upper troposphere
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Summary – 2
• These relationships are suppressed with the EMA
and B-M schemes
• With B-M low cloud is reduced in SETP and
circulation changes act to reduce the precipitation
over most of the tropics – this is more consistent
with (the ensemble mean of) AR4 models
• Weaker relationship between changes in lower and
upper tropospheric moisture with B-M and EMA
With the standard Gregory-Rowntree scheme the
slab model responses of surface temperature and
low cloud, and the hypothesized link between low
cloud in the SETP, SST variability and circulation are
consistent with the behaviour of the coupled model
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ΔTS in the CFMIP1 models (K/K)
HadSM3 is not unique but has the clearest
example of SE Pacific warming minimum
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Cloud forcing response to 2 × CO2
The low cloud change in HadSM3 (STD)
is a ‘fast’ response
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