Aerosol Indirect Effects in CAM
A. Gettelman (NCAR), F. Vitt (NCAR), P. Hess (Cornell)
H. Morrison (NCAR), P. R. Field (Met Office), S.J. Ghan (PNNL)
Outline
• Model Description
• Prescribed Aerosols
– Global Aerosol Indirect Effects (AIE)
– Global Sensitivity
– Regional Sensitivity (Mid Lat Cyclones)
• Prognostic Aerosols
– Plans & Progress
• Future Work
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Model Description
• Community Atmosphere Model (CAM)
• New 2-moment stratiform microphysics
– Morrison & Gettelman 2008, Gettelman et al 2008
• Aerosol activation forms cloud drops
– Abdul-Razzak & Ghan (1998,2000,2002)
• MOZART aerosol model
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Aerosols: Sulfur, Carbonaceous, Dust, Sea-salt
Mass representation, assumed size distributions
Off line (Prescribed) distributions (CAM3.5)
On line (Prognostic) experiments
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Prescribed Aerosols
• 1870, 1990 Conditions derived off-line
• Sensitivity tests with scaling aerosols, adding
a minimum number of activated aerosols
• 8 year runs with climatological SST & aerosol
• SST & GHGs same for both runs
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Global Maps (1990-1870)
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Global Sensitivity: AIE
IE= dln(re)/dln(ext) where ext=H*AOD
Feingold et al 2003 observations = 0.02-0.16
CAM Simulated = 0.05-0.11
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Global AIE Sensitivity
• LINSULF: different sulfate number
• SOA (Secondary Organic Aerosol) : increase
Hydrophilic Organic Carbon by factor of 100
• MIN: minimum Number conc of activated aerosol
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Regional Sensitivity
• Focus on mid-latitude cyclones
– Large radiative forcing from Aerosols
• Composite cyclones
– Based on location of surface pressure min
– Following Field & Wood 2007 (J. Clim)
– Look at storm structure, microphysics, RF
• Aerosols from 1990, 1870
• All other forcings (GHGs, SSTs) same
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Effects on Radiation
• N. Pacific Shortwave Cloud forcing
SWCF = Fsw - Fclearsw
• Cloud Forcing increases in present day
1990
1870
-80
-60
-120
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Effects on Particle Size
• N. Pacific maximum column Re (liquid)
• Particle sizes get smaller with more aerosols
1990
1870
13
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Effects on Precipitation
• N. Pacific Rain Rates (mm day-1)
• Rain intensity decreases in present day
1990
1870
>15
10
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Effects on Liquid Water Path
• N. Pacific LWP: peak similar
• +50% LWP Away from center (trailing)
1990
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Further Results: Cyclone AIE
• S. Hemisphere is the control case
– Little AOD change
– Do not see effects in the S. Hemisphere
• N. Atlantic can have different response
– Working on understanding this now
– May be due to different change in AOD
– Different wind speeds
– Need to look at cyclones by category
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Results: Prescribed Aero
• New Microphysics and Aerosol scheme
indicates largest AIE in Mid-lat storm tracks
• Sensitivities ‘reasonable’ relative to IE metrics
• Results are sensitive to background aerosols
– If more CCN available: sensitivity is reduced
• Mid-latitude cyclones do show effects:
– lower rain rates & smaller particles = more forcing
– LWP in far field different (lower precip)
• Note: Inconsistency with fixed aerosol
– Needs to be fixed with ‘relaxation’
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Off Line Aerosol Inconsistency
Clear
Precip
Clear
Current fixed aerosols
aerosol
Time
Desired (relaxation)
aerosol
Time
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Prognostic Aerosols
• Code runs with interactive (MZ) aerosols
• Status: sorting out inconsistency in
drivers for wet deposition
• Wet deposition rates are slow
– Aerosol lifetimes & burdens 2x over RK code
– Also affects Modal Aerosols
• Once resolved, continue tests
• Comments on testing?
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Aerosol Testing Issues
• What is reference/benchmark?
– MOZART and/or Modal Aerosols (present)
• How do we understand pre-industrial?
– Emissions?
• How do we constrain aerosol-cloud
interactions?
– Process level?
– Global/regional effects?
– Observations not sufficient to constrain AIE
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AIE Testing Plans
• Sort out Prognostic Aerosols
– Present day simulation
– Run same code for pre-industrial (AIE)
• Run prescribed aerosols from above
– AIE: Does on-line aerosol matter?
• May redo above with SOM
• Add Relaxation to prescribed aerosols
– Does this affect AIE?
• 20th Century run (repeat CAM3.5 run w/ MG)
– Prognostic
– Prescribed (from above) ‘Relaxed’
• Could also include Modal aerosol in tests
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