Issues in modeling the aerosol direct effects on climate

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Issues in modeling theaerosol direct effectson
climate
Chul Eddy Chung
Center for Cloud, Chemistry and Climate
(C4) Scripps Institution of Oceanography La
Jolla, California, USA
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(IPCC report 2001)
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INDOEX (Indian Ocean EXperiment) Aerosol
Radiative Forcing (W m-2) (Jan - March, 1999 0
- 20N)
-7.01
-2.02
-52.5
18.03

16.02
10.5
-232
-203
-63
Direct (Clear Sky)
Direct (Cloudy Sky)
First Indirect
(Ramanathan et al. 2001a)
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Why do the surface forcing and atmosphere forcing
oppose strongly in South Asia and the Indian
Ocean?
(BC SSA 0.2)
(sulfate SSA 0.99)
(Ramanathan et al. 2001a)
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Global mean vs. local impact
AA
(Ramanathan et al. 2001b)
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Global anthropogenic aerosol forcing
estimate (2001-03) (Chung, Ramanathan, Kim and
Podgorny 2005)
Methodology 1) Integrate satellite and ground
based aerosol observations with GOCART model
outputs 2) Bring cloud observation from the
ISCCP and 2) Insert integrated global AOD,
SSA and asymmetry parameter into the MACR
(Monte-Carlo Aerosol Cloud Radiation) model.
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Global anthropogenic aerosol forcing estimate for
the period 2001-03 (Chung, Ramanathan, Kim and
Podgorny 2005)
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Other issues?
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Vertical profile of aerosols and convective
precipitation
From Chung and Zhang (2004)
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Typical PBL profile
Typical lifted profile
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5
3 / 25 / 1999
2 / 16/ 1999
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4
. C-130 (5.7N, 73.3E) Lidar
(4.2N,73.5E)
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3
Ca
Altitude (km)
Cs
2
2
1
1
C-130 (4.2N, 73.5E)
0
0
50
100
150
0
-10
10
30
50
70
90
110
Cs, Ca (Mm-1)
Extinction Coefficient (Mm-1)
Idealized profiles for this study
670
700
Lifted profile
Altitude (hPa)
800
Uniform profile
850
PBL profile
Ps
0.7
Prescribed aerosol forcing (K/day)
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Numerical experiment designs (Imposed
January-March South-Asian haze forcing)
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January-March Ts change (CCM3)
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Precipitation change (CCM3)
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Understanding precipitation change CAPE
CAPE variation consists of two parts
contributions from the boundary layer (parcels)
changes and contributions from the free
tropospheric (parcels environment) changes
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Low-level aerosol heating and dCAPE/dCAPEe
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Spatial and seasonal variation of aerosol
radiative forcing
From Ramanathan, Chung et al. (2005), and Chung
and Ramanathan (2005)
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An idealized S. Asian haze experiment with PCM
(Parallel Climate Model)
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An improved S. Asian haze experiment with PCM
(Regional and temporal average from 1995 to 1999)
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Latitudinal gradient (Longitudinal and temporal
average from 1995 to 1999)
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ABC effects in 1985-2000 (60-100E streamline)
In winter, F(A) outweighs F(S).
In summer, F(S) outweighs F(A).
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Ramanathan, Chung et al. (2005)
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Drying Sahel!
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1985-2002 observed trend
1951-2002 observed trend
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Connection between Indian summer monsoon andN.
African summer monsoon
Monsoon dynamics explained by Webster and
Fascullo (2003)
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AOD
SST (K)
Surface aerosol forcing Ramanathan et al. (2005)
FS (W/m2)
SST (K)
2001-02 mean
SST (K)
Hadley SST
1930-50 mean
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Numerical experiments with the NCAR/CCM3
N
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Precipitation change
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SST gradient change vs. haze heating
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500-300hPa vertical motion and surface
streamline (June-September)
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Vertical motion and streamline at 10-20N
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Greenhouse gas effects
1951-2002 observed trend
S. Asian haze effects
S. Asian haze effects
1985-2002 observed trend
1951-2002 observed trend
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Conclusions

1. Observations show that SSTs in the equatorial
   Indian Ocean have warmed by about 0.6 to 0.8 K
   since the 1950s, accompanied by very little
   warming or even a slight cooling trend over the
   northern Indian Ocean. The SST meridional
   gradient in N. Indian has been weakened in
   summer.
2. The weakening of the meridional SST gradient in
   N. Indian Ocean alone leads to a large decrease
   in Indian rainfall during summer months, ranging
   from 2 to 3 mm/day (CCM3 experiments). The SST
   weakening also enhances rainfall in sub-Saharan
   Africa.
3. The SST gradient change in this basin is likely
   due to anthropogenic aerosols in South Asia and
   the Indian Ocean.
4. The overall S. Asian haze effects (SST gradient
   change aerosol radiative forcing) in CCM3 still
   produce drought in Indian and excess rainfall in
   Sahel.
5. It is thus implicated that the South Asian haze
   has mitigated the Sahel desiccation considerably.

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Sub-monthly fluctuations of aerosol radiative
forcing
From Chung (2005)
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Issues

1. Absorbing aerosols are another atmospheric
   diabatic heating source, and their distribution
   and amounts fluctuate as circulation and
   precipitation change.
2. In modeling the climatic effects of aerosols,
   aerosols are either simulated or prescribed.
3. When aerosols are simulated (i.e., coupling
   approach), the simulated aerosols inevitably
   differ from the observed due to the model
   deficiencies.
4. In case of prescribed aerosols (off-line
   approach), aerosols do not affect climate on fine
   time scales.

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Is it acceptable to use monthly aerosol
observations and prescribe them into a climate
model?
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Methodology

1. A tracer is added in the NCAR/CCM3. Aerosol
   emission at the surface was used for the source
   for the added tracer. Two cases are chosen
   Chinese haze and Indian haze.
2. The aerosol wet deposition code by Rasch et al.
   (1997) was linked to the CCM3, as the sink for
   the added tracer.
3. The enhancement of the atmospheric solar
   radiation by the added tracer was accounted for
   in the CCM3 solar radiation module.

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Indian haze and CCM3 precipitation climatology
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Chinese haze and CCM3 precipitation climatology
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Interactive Indian haze
Interactive Chinese haze
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Analysis of the Indian haze
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Average forcing 0.31 K/day
interactive
steady
Average forcing 0.65 K/day
steady
interactive
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Conclusions

• Using monthly haze-induced diabatic heating does
  not produce sizable errors related to ignoring
  the sub-monthly fluctuations in the case of the
  Chinese haze. However, ignoring such sub-monthly
  scales leads to overestimation of the impacts of
  the haze heating on precipitation around India.
• The Indian haze heating has 23 times higher
  precipitation increase efficiency than the
  Chinese haze heating.
• Precipitation increase within the Chinese haze is
  totally irrelevant to the climatological
  precipitation

Implication
The climatic effects of tropical absorbing haze
need to be handled more carefully than those of
extratropical absorbing haze.