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

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

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

What controls CCN variability in the marine boundary layer? A simple CCN budget for the PBL E NTRAINMENT S URF. S OURCE P RECIP. S INK 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. N d Wood et al. (J. Geophys. Res. 2012)

Steady-state CCN budget Concentration relaxes to FT concentration N FT + wind speed dependent surface contribution dependent upon subsidence rate (D z i ) Precipitation sink controlled by precipitation rate at cloud base P CB. Use expression from Wood (2006). F REE T ROPOSPHERIC CCN P RECIP. S INK S EA -S PRAY P RODUCTION Wood et al. (J. Geophys. Res. 2012)

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

Consequences CCN Cloud LWP CCN Cloud LWP Aerosol-driven precipitation suppression or precipitation-driven aerosol suppression? Possible causal relationshipObservable relationship

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.

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

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

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

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 Baughman et al. [2014] BBon - BBoff

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

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

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

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

What controls CCN variability in the marine boundary layer? A simple CCN budget for the PBL 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. N d E NTRAINMENT N UCLEATION /S ECONDARY D RY D EP. S URF. S OURCE P RECIP. S INK A DVECTION Wood et al. (J. Geophys. Res. 2012)

Difference (  ) in AOD (MODIS, 550 nm) between 1x1 o boxes with 85% cloud cover and those with 15% cloud cover Retrieved AOD is greater when it is cloudier

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