1. Robert Wood University of Washington
Re-examining links between aerosols, cloud droplet concentration, cloud cover and precipitation using satellite observations 
Robert Wood
University of Washington
With contributions from Eowyn Baughman, Dan Grosvenor, Daniel McCoy, Chris Terai, Matt Lebsock, Dave Leon, Jeff Snider, Tony Clarke, Duli Chand

2. Motivation
Correlations between aerosols and clouds in the current climate are used to infer aerosol indirect effects (e.g. Lebsock, Quaas, IPCC….)
But aerosols and clouds both respond to meteorological variability
In this presentation, we explore three ways in which meteorology complicates the relationship between aerosols and clouds

3. Message 1: Precipitation variations drive changes in cloud droplet concentration in marine low cloud
High droplet
concentration
Low droplet
concentration
Photograph: Tony Clarke, VOCALS REx flight RF07
Image: Pablo Saide

4. What controls CCN variability in the marine boundary layer
What controls CCN variability in the marine boundary layer? A simple CCN budget for the PBL
Entrainment
Surf. Source Precip. Sink
Assume nucleation/secondary processes unimportant
Dry deposition is negligible (Georgi 1990)
Sea-spray formulation (e.g. Clarke et al. 2006)
Ignore advection
Precipitation sink primarily from accretion process
Equivalency of CCN and cloud drop conc. Nd
Wood et al. (J. Geophys. Res. 2012)

5. Steady-state CCN budget
Sea-Spray Production
Free Tropospheric CCN
Precip. Sink
Concentration relaxes to FT concentration NFT + wind speed dependent surface contribution dependent upon subsidence rate (D zi)
Precipitation sink controlled by precipitation rate at cloud base PCB. Use expression from Wood (2006).
Wood et al. (J. Geophys. Res. 2012)

6. Precipitation is primary control of Nd away from coastal zones
Without aerosol source variations, model reproduces significant amount of variance in Nd over oceans  implications for interpretation of AOD vs precipitation relationships
Wood et al. (J. Geophys. Res. 2012)

7. Consequences Cloud LWP Cloud LWP CCN CCN Possible causal relationship
Observable relationship
Cloud LWP
Cloud LWP
CCN
CCN
Aerosol-driven precipitation suppression
or precipitation-driven aerosol suppression?

8. Message 2: Hygroscopic growth of aerosols in the vicinity of clouds leads to correlations between AOD and cloud cover that have little to do with aerosol indirect effects
Chand, D., R. Wood, S. J. Ghan, M. Wang, M. Ovchinnikov, P. J. Rasch, S. Miller, B. Schichtel, and T. Moore (2012), Aerosol optical depth increase in partly cloudy conditions, J. Geophys. Res., 117, D17207.

9. Retrieved AOD is greater when it is cloudier
Relative AOD enhancement (=/CF=0.15) shows no clear pattern with aerosol sources and is almost hemispherically symmetrical

10. Enhancement of AOD captured with simple statistical model
Relate cloud fraction to RH pdf
Relate hygroscopic aerosol growth in clear portion of box to RH pdf
Make assumptions about vertical structure
Can explain correlation between AOD and cloud cover without recourse to AIEs

11. Consequences Cloud cover, albedo Cloud cover, albedo AOD AOD
Possible causal relationship
Observable relationship
Cloud cover, albedo
Cloud cover, albedo
AOD
AOD
Aerosol effects on clouds
or cloud effects on aerosols?

12. Message 3: Using aerosol-cloud correlations in the current climate to infer aerosol-driven cloud changes can be misleading
WRF-Chem, indirect effect of biomass burning aerosols on cloud
Two simulations: one with BB aerosols on; one without
BBon - BBoff
Baughman et al. [2014]

13. CCN composited on an offshore flow index
CCN anomaly
[cm-3]
t=-5 day
time
t=0
t=+5 day

14. LWP composited as a function of offshore flow
LWP anomaly
[g m-2]
BBon
LWP lower when high CCN loading offshore….

15. LWP composited as a function of offshore flow
LWP anomaly
[g m-2]
BBoff
Response very similar even without BB aerosols

16. Take-home messages
Precipitation variations drive variations in cloud droplet concentration on a range of timescales
Hygroscopic growth of aerosols in the vicinity of clouds leads to correlations between AOD and cloud cover that have little to do with aerosol indirect effects
Using aerosol-cloud correlations in the current climate to infer aerosol-driven cloud changes can be misleading

18. What controls CCN variability in the marine boundary layer
What controls CCN variability in the marine boundary layer? A simple CCN budget for the PBL
Entrainment Nucleation/Secondary Dry Dep.
Surf. Source Precip. Sink Advection
Assume nucleation/secondary processes unimportant
Dry deposition is negligible (Georgi 1990)
Sea-spray formulation (e.g. Clarke et al. 2006)
Ignore advection
Precipitation sink primarily from accretion process
Equivalency of CCN and cloud drop conc. Nd
Wood et al. (J. Geophys. Res. 2012)

19. Retrieved AOD is greater when it is cloudier
Difference () in AOD (MODIS, 550 nm) between 1x1o boxes with 85% cloud cover and those with 15% cloud cover

20. Consequences True sensitivity to CCN Observable ‘sensitivity’ to CCN
Using aerosol-cloud correlations in the current climate to infer aerosol-driven cloud changes can be misleading