1. Integration of models and observations of aerosol-cloud interactions
Robert Wood
University of Washington

2. Radiative “forcing” components
Cloud effects
tcld  Nd1/3 LWP5/6
(First AIE/Twomey)
Nd   Precip. 
(Second AIE/Albrecht)
Direct Twomey Mixed ph.
Semi-dir nd AIE
Isaksen et al. (Atmos. Env. 2009)

3. State of play
IPCC 2007
Isaksen et al. (Atmos. Env., 2009)

4. Model estimates of the two major aerosol indirect effects (AIEs)
Pincus and Baker (1994)
– 1st and 2nd AIEs comparable
GCMs (Lohmann and Feichter 2005)
1st AIE: to -1.9 W m-2
2nd AIE: -0.3 to -1.4 W m-2
Aerosol particles induce changes in cloud macrophysical properties.
The Twomey effect is insufficient

5. Shiptrack surprises!
3.7 m
Liquid water content in shiptracks is typically reduced compared with surrounding cloud
Clear refutation of Albrecht’s hypothesis
courtesy Jim Coakley, see Coakley and Walsh (2002)

6. LES results LWP [g m-2] P0 [mm d-1] we [cm s-1] Ackerman et al. (2004)
Impact of aerosols simulated by varying Nd
Increased Nd  Reduced precipitation  increased TKE  increased entrainment we
Changes in we can sometimes result in cloud thinning (reduced LWP)
Also noted by Jiang et al. (2002)
LWP [g m-2]
P0 [mm d-1]
we [cm s-1]
Cloud droplet concentration [cm-3]

7. Transient response of an equilibrated mixed layer PBL model to Nd increases
Ratio of Albrecht to Twomey effect RIE (right) is a strong function of cloud base height
More elevated cloud base heights zcb lead to Albrecht effects which partly cancel those due to Twomey effect
Elevated zcb associated with dry FT and less surface drizzle, consistent with LES results, but with a far less sophisticated model  hope for the representation in climate models
Wood (J. Atmos. Sci., 2007)

8. Sedimentation of cloud droplets
Cloud droplet sedimentation removes water from the (10 m thick) entrainment interface, lowers LWC there, reduces evaporative cooling, and suppresses entrainment, resulting in thicker clouds
Since increased Nd reduces sedimentation  pollution can lead to thinner clouds
Bretherton, Blossey and Uchida, GRL, 2007

9. Effects of drizzle vs effects of sedimentation of cloud droplets GCSS DYCOMS-2 RF02 drizzling Sc case study
Effect of drizzle Effect of sedimentation
With drizzle, without sedimentation
With drizzle and sedimentation
.....in this case, sedimentation dominates over drizzle impact
on cloud LWP
Ackerman et al. (MWR, 2009)

10. Microphysically-driven supersaturation differences, can drive LWC differences
Height [km]
Steady-state supersaturation inversely proportional to N and mean radius
More polluted clouds have more active turbulence and (in this case) more cloud water
Also microphysically-limited evaporation rate (Feingold inter alia.)
Kogan and Martin, Kogan et al. (JAS, 1994, 1995)

11. Necessary conditions for AIEs in warm clouds
Aerosols must result in increases in cloud droplet concentration
Present day geographical variability of cloud droplet concentration should be simulated by GCMs

12. January
MODIS
Use method of Boers and Mitchell (1996), applied by Bennartz (2007)
Screen to remove heterogeneous clouds by insisting on CFliq>0.6 in daily L3
CAM-5
Cloud top droplet concentration in warm clouds from CAM-5

13. July MODIS CAM-5 CAM-5 broadly captures land-ocean contrasts in Nd
Opposite sign of seasonal cycle
over NH land (MODIS>CAM in winter; MODIS<CAM in summer)
Clear evidence of S. African and S. American biomass burning in MODIS and CAM
CAM-5

14. Determine weak-link parameterizations
Effect of varying autoconversion schemes (in CAM 5) on second AIE
Second AIE varies by a factor of 5 or more
Chuang et al. (2011)

15. Autoconversion in the real worldz*
Accretion, not autoconversion is the dominant precipitation production mechanism .....even in weakly-precipitating clouds
cloud top z*
cloud base
Composite of aircraft data in stratocumulus from Wood (JAS, 2005)

16. Precipitation susceptibility
Construct from Feingold and Siebert (2009) can be used to examine aerosol influences on precipitation in both models and observations
S = -(dlnRCB/dlnNa)LWP,h
S decreases strongly with cloud thickness
Consistent with increasing importance of accretion in thicker clouds
Consistent with results from A-Train (Kubar et al. 2009, Wood et al. 2009)
Data from stratocumulus over the SE Pacific, Terai and Wood (Geophys. Res. Lett., 2011)

17. What controls Nd? Simple budget model for CCN/Nd in the MBL:
Assume aerosol sources constant (here represented by FT concentration “buffer”)
Model pattern almost entirely driven by precipitation sinks
Can reproduce significant amount of variance in Nd over oceans  implications for significance of AOD vs re relation ships
Wood (2011)

18. Conclusions
Most ways (and these are numerous) in which aerosols impact warm clouds are mediated via cloud droplet concentration
Singling out Twomey effect is introducing a biased effect, and is pointless
Cloud droplet concentration can be estimated from space but need to establish credibility of estimates, especially away from Sc regions
CloudSat and A-Train providing ways of establishing sensitivity of warm rain to aerosols...and vice versa

19. A proposal
A limited area perturbation experiment to critically test hypotheses related to aerosol indirect effects
Cost $30M

21. Stevens and Feingold (Nature, 2009)