Climatology and climate sensitivity of lowlevel clouds

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Climatology and climate sensitivity of low-level
clouds
Ping Zhu, Jim Hack
1. What are the major biases of GCM simulated
tropical and subtropical low-level cloud c
limatology compared with the satellite
measurements? Are these biases significant
enough to affect radiative budgets at TOA?
2. Can GCM simulated low-level clouds faithfull
y respond to a climate anomaly, such as EN
SO ? 3. How can we evaluate low cloud sensiti
vity in response to climate changes?
What are the mechanisms controlling low
cloud variability in a changing climate?
1. Satellite measurements ERBE, ERBS, CERES,
ISCCP, ISCCPDF 2. GCM simulations AMIP runs by
GFDL-AM2, NCAR-CAM3, CAM3-UW
Coupled CCSM runs, SOM runs.
CAM3-UW new moist turbulence scheme with an
explicit entrainment parameterization
new mass-fluxed based
shallow convection scheme with convection
triggering controlled by
TKE and CIN. Entrainment and detrainment rates
are determined by buoyancy
sorting and thermal penetrating mechanisms.
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• Observed and simulated low-level cloud climatology

Low clouds
3
0
3.6
23
4
Long Wave
Short Wave
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2. Change of low clouds in response to El Nin
o event
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(No Transcript)
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Thin 0
medium 3.6
thick 23
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9798 - ANN
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3. Low cloud sensitivity to climate changes
GFDL AM2-ML (2co2 CTRL)
NCAR CAM2 (Year 70 _at_1co2/yr CTRL)
2co2 CTRL suggests Negative feedback
?
El nino suggests Positive feedback
?
1. Which one is right? 2. Why
is that?
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1. It is not the local change of SST but the
spatial change pattern of SST that
determines the variability of low-level
clouds. 2. It is not an accurate statement to s
imply say that low-level clouds have a
negative or positive feedback in terms of
local SST change.
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?
?
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Conclusions 1. GCM simulated low clouds have a
serious bias compared with ISCCP observations
when breaking down in terms of optical
thickness, such a bias may have a
significant impact on radiative budgets at
TOA. 2. All models can reasonably reproduce t
he low cloud change in response to major
El Nino event. 3. ENSO events can be use
d to evaluate low cloud feedbacks in response to
climate changes when removing background SS
T increase. 4. Low cloud variability is contr
olled by spatial SST change in a changing climate.
Future
work 1. Identifying the contribution from each
cloud type to radiative budgets at TOA.
2. Understanding why climate sensitivity of low-
level clouds is not determined by
local SST change but by spatial change
pattern of SST. What role does dynamical
process play in this scenario?