Transcript Aerosol Indirect Effects in CAM A. Gettelman (NCAR), F. Vitt (NCAR), P.

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
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Effects on Precipitation
• N. Pacific Rain Rates (mm day-1)
• Rain intensity decreases in present day
1990
1870
>15
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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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