1. 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)

2. Gettelman-AIEAMWG2008 2 Outline Model Description Prescribed Aerosols –Global Aerosol Indirect Effects (AIE) –Global Sensitivity –Regional Sensitivity (Mid Lat Cyclones) Prognostic Aerosols –Plans & Progress Future Work

3. Gettelman-AIEAMWG2008 3 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 –Aerosols: Sulfur, Carbonaceous, Dust, Sea-salt –Mass representation, assumed size distributions –Off line (Prescribed) distributions (CAM3.5) –On line (Prognostic) experiments

4. Gettelman-AIEAMWG2008 4 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

5. Gettelman-AIEAMWG2008 5 Global Maps (1990-1870)

6. Gettelman-AIEAMWG2008 6 Global Sensitivity: AIE IE= dln(r e )/dln(ext) where ext=H*AOD Feingold et al 2003 observations = 0.02-0.16 CAM Simulated = 0.05-0.11

7. Gettelman-AIEAMWG2008 7 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

8. Gettelman-AIEAMWG2008 8 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

9. Gettelman-AIEAMWG2008 9 Effects on Radiation N. Pacific Shortwave Cloud forcing SWCF = F sw - Fclear sw Cloud Forcing increases in present day 19901870 -110 -120 -60-80

10. Gettelman-AIEAMWG2008 10 Effects on Particle Size N. Pacific maximum column R e (liquid) Particle sizes get smaller with more aerosols 18701990 13 15

11. Gettelman-AIEAMWG2008 11 Effects on Precipitation N. Pacific Rain Rates (mm day -1 ) Rain intensity decreases in present day 1990 1870 >15 10

12. Gettelman-AIEAMWG2008 12 Effects on Liquid Water Path N. Pacific LWP: peak similar +50% LWP Away from center (trailing) 19901870

13. Gettelman-AIEAMWG2008 13 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

14. Gettelman-AIEAMWG2008 14 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’

15. Gettelman-AIEAMWG2008 15 Off Line Aerosol Inconsistency Current fixed aerosols Desired (relaxation) ClearPrecipClear  aerosol Time  aerosol Time

16. Gettelman-AIEAMWG2008 16 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?

17. Gettelman-AIEAMWG2008 17 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

18. Gettelman-AIEAMWG2008 18 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