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Understanding Feedback Processes
Outline
• Definitions
• Magnitudes and uncertainties
• Geographic distributions and priorities
• Observational requirements
Climate Feedbacks
albedocloudwvPlanck
QT
• Climate sensitivity is the equilibrium change in global mean surface temperature (T) that results from a specified radiative forcing (Q).
• The sensitivity is a function of the feedback factors () .
Climate Feedbacks
albedocloudwvPlanck
QT
W/m2/K3.6
Planck (temperature+ lapse rate)
-1.6
Water Vapor(mixing ratio)
-0.4
Clouds(cloud amount,
water path,particle size)
-0.3
Albedo(snow, ice)
• Climate sensitivity is the equilibrium change in global mean surface temperature (T) that results from a specified radiative forcing (Q).
• The sensitivity is a function of the feedback factors () .
Radiative Forcing
4 W/m2
= 3 K
Range of Model Feedbacks
Colman (2003)
Neg
ativ
e Fe
edba
ckPo
siti
ve F
eedb
ack
Range of Model Feedbacks
+
Neg
ativ
e Fe
edba
ckPo
siti
ve F
eedb
ack
Colman (2003)
0 0.4 0.8 1.2 1.6 W/m2/K
The Importance of Water Vapor Feedback
Distribution of Water Vapor Feedback
Vertical ContributionsBoundary Layer Upper Trop(> 800 mb) (< 600 mb)~10% ~70%
Latitudinal ContributionsTropics Mid-Lat Polar ~65% ~30% ~5%
Stronger feedback
Weaker feedback
Water Vapor Feedback
Fractional Change in Water Vapor Concentration: GFDL GCM (SRES A1B)Pr
essu
re
Water Vapor Trends6.
7 m
Bri
ghtn
ess
Tem
pera
ture
Ano
mal
y (K
)
GCMNo Moistening
GCM
HIRS
Cloud Feedback
Change in Low Cloud Amount (%/K)
IPCC TAR Models
Summer 2002
NC
AR
GF
DL
2xC
O2 S
ensi
tivity
(K
)
Fall 2003
Cloud Feedback
NC
AR
GF
DL
2xC
O2 S
ensi
tivity
(K
)
Change in Low Cloud Amount (%/K)
Cloud Feedback
GFDL: GFDL: Strong Strong positive positive lowlow cloud cloud feedback.feedback.
NCAR: NCAR: Weak Weak negative negative lowlow cloud cloud feedback.feedback.
2x CO2ENSO
Response to Transient Forcings
Eruption of Mt. PinatuboJune 1991
Cloud and Radiation Trends
Wielicki et al. (2002)
Verification requires redundancy
Priorities
• Free tropospheric temperature •Tropical upper tropospheric water vapor (1%/dec, Z=0.5 km)
• Low cloud cover (0.5%/dec, Z=0.5 km)
• TOA radiative fluxes• Redundancy
• Climate OSSE to guide system design and requirements
Extra Slides
Calculation of Feedbacks: Control
SW attropopause
LW attropopause
RadiationModel
sfc albedo
temperature
mixing ratio
clouds
CO2
Calculation of Feedbacks: Perturbation
SW attropopause
LW attropopause
RadiationModel
sfc albedo
temperature
mixing ratio
clouds
CO2
Colman (2003)PRP Method
Cess et al. (1996)ΔCRF Method
1 of 10 models has negative cloud feedback
8 of 18 models have negative cloud forcing
Soden, B. J., A. J. Broccoli, and R. S. Hemler, 2004: On the use of cloud forcing to estimate cloud feedback, J. Climate, in
press.
CFMIP contribution from each feedback class
LW CRF
SW CRF
LW CRF response
SW CRF response
Mark Webb
Observable Radiative Damping Rates
Regional
Seasonal
Interannual
Cloud Changes in a Warmer Climate
AM2p10:Reduced tropical high cloud amount. This weakens LW feedback and changes sign of SW feedback in model.
AM2p5 – AM2p9:Increased tropical high cloud amount.