R. T. WETHERRALD and S. MANABE

1

• R. T. WETHERRALD and S. MANABE
• JOURNAL OF THE ATMOSPHERIC SCIENCES
• Vol. 45 No. 8 (1988)

• ?????
• ???
• 2006/05/22

2
Outline

• Introduction
• Description of the model
• Result
• Sensitivity experiment
• Change in cloud cover
• Feedback analysis
• Comparison with previous results
• Summary and conclusion

3
Introduction

• The change in the distribution of cloud cover can
  have a large effect upon the sensitivity of
  climate.
• (Cess 1996, Hartmann and Short 1980 )
• Authors (1980) indicate the cloud feedback
  process in their
• model has very little effect upon the
  sensitivity of climate to a forcring. (ex. solar
  constant,CO2)
• Hansen et al. (1984) suggests that doubling the
  concentration of CO2 enhances the surface
  warming.
• The features of cloud cover changed.
  (Washington and Meehl 1984)

4

• So it appears worthwhile to conduct an in-depth
  analysis of the results from a climate
  sensitivity and identify some of the basic
  mechanisms which controls the CO2-induced change
  of cloud cover and its effect upon the
  sensitivity of climate.

5
Description of the model

• ( a) Structure
• Atmospheric GCM (Gorden and Stern 1982)
  coupled with a static mixed layer ocean model
  (Manabe and Stouffer 1980).

6
CO2 constant 300 ppm
Vertical vorticity, horizontal divergence,
temperature, surface pressure, moisture
RH gt 99 ? clouds
Dynamic equation
Continunity equation
Prognostic equation
Radiation and Cloud
Thermodynamic equation
precipitation
Isolation is seasonal
T lt 0? snow
TOA
Heat transport in ocean

• M

Snow fall, sublimation, snowmelt (surface heat
balance)
7
The fractional absorption and reflection of solar
radiation by variation types of cloud (Rodgers
1967)
8
The simplicity of the model

• Model doesnt compute cloud condensed water, so
  the effect of varying optical depth of clouds not
  taken into consideration.
• Its assume all condensed water vapor immediately
  precipitates out of the model.

9
Description of the model

• ( a) Standard simulation
• compare with the observation

10
Annually zonal mean surface temperature

• observation values
• Crutcher and Meserve 1970
• Taljaard et al. 1969
• quit good for most latitudes

warmer
11
Seasonal zonal mean surface T anomaly
Observed
Model
Too warm
12
Latitude-time variation of zonal mean total cloud
amount
Well-simulated
13
Latitude-time distribution of zonal mean
planetary albedo
45-55N / JA
14
Result I

CO2 X 2
15
Latitude-height distribution of zonal mean
temperature
GVC
1.25 times
GPC
Additional CO2 is unaffected by the cloud
feedback mechanism. Since the vertical heat
exchange between the stratosphere and troposphere
is much smaller then the exchange in the
troposphere itself.
16
Result II change in cloud cover

• Annual mean change
• Seasonal mean change

17
Zonal mean cloud amount (CO2X2) GVC

(3)An increase of cloud amount in the stable
region near the model surface in higher
latitudes.

• A reduction of cloud amount in the upper
  troposphere in middle and low latitude.

(2)An increase of cloud amount around the
tropopause for all latitudes.(high)
18
Zonal mean total cloud amount
19

• The altitude of this high cloud layer decreases
  with increasing latitudes in a manner similar to
  the height of the tropopause.
• Assumption
• The tropopause and the upper tropospheric
  layer beneath it serve as a lid to the upper
  motion.

20

• Moist static stability (derivative of equivalent
  potential temperature)
• (rs saturation
  mixing ratio of water vapor)

21
? The altitude of the high cloud in the upper
model troposphere increases in response to the
doubling of atmospheric CO2.
Moist static stability
Zonal mean cloudiness

• Vertical profile at 50N
• (a) A rise if the cloudy layer of the upper
  troposphere where the moist static stability
  sharply increase altitude and saturated.
• (b) The upward shift of cloud layer. (350 mb
  level)

• Vertical profile at 50N
• (a) A rise if the cloudy layer of the upper
  troposphere where the moist static stability
  sharply increase altitude and saturated.
• (b) The upward shift of cloud layer. (350 mb
  level)

22
In the model tropics where the static stability
of the lower and middle troposphere is moist
adiabatically and unstable and the Coriolis force
is small.

• Vertical profile at equator
• Vertical cloudiness profile is altered.
• The cloudiness is reduced over a very thick layer
  of the upper troposphere.

23
But the physical mechanism is still unknown.
24
Seasonal variation of zonal mean total
cloudiness (CO2 x2)

• In middle and high latitude total cloud amount
• Summer ?
• Winter ?

25
Feedback analysis

• Formulation
• Feedback parameters

26
Formulation
(5.1)
(5.2)
(5.3)
(5.4)
(5.5)
X CO2 T Temperature r mixing ratio of
water vapor A surface albedo C cloud amount
27
(5.7)
(5.8)
28
?T
?0 The mean T feedback parameter ?LR The
lapse rate feedback parameter
29
?Alt 0 positive feedback effect of snow and sea
ice ?clt 0 positive feedback at the TOA ?LR
changes a lot, increasing the feedback
parameter ? (GVC) lt ? (GPC) cloud feedback
enhances the sensitivity
30
Zonal mean of the rate of radiation T change
Radiative cooling is reduced due to the
CO2-induced increase of cloud amount.
31
Zonal mean of the rate of radiation T
change(GVC-GPC)

• The increase of the static stability of the
  troposphere accompanying a unit increase of
  global mean SAT is larger in the GVC.
• (about 3.2)

32
Cloud-induced changes in radiative forcing
33
A amount H height
Cloud forcing
34
Small positive value
Reduce the cloud amount
35
Comparison with previous results

• This result shows the interaction between cloud
  cover and radiation enhances the CO2-induced
  warming of climate and is a positive feedback.
• The authors in 1980 suggested the cloud process
  has a very little influence upon the sensitivity.
• In both, the cloud cover increased around the
  tropopause and in the surface layer in the high
  latitude. But this study is larger then before.

36
Comparison with previous results(cont.)

• From several models constructed by different
  groups (Hansen et al.1984, Washington and Meehel
  1984), there is a tendency for cloud amount to
  increase around the tropopause poleward.
• In the experiment of Hensen et al., an increases
  of humidity is indicated around the tropical
  tropopause.
• Wilson et al.(1986) also indicate an increase of
  cloud amount around the tropical tropopause.

37
Summary and conclusions

• The reduction of cloud amount in the upper
  troposphere.
• An increasing of cloud amount around the
  tropopause.
• An increasing of cloud amount near the earths
  surface in high latitudes.

38
Summary and conclusions (cont.)

• It is found that the interaction between cloud
  cover and radiation is a positive feedback
  process enhancing the CO2-induced warming of the
  model troposphere.
• Moist convection process may greatly affect the
  changes of temperature and cloud amount and have
  a significant effect upon the sensitivity of a
  climate model.

39
THE ENDThanks !!