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

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

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

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: -0.5 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

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)

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]

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)

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

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)

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)

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

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

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

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)

Autoconversion in the real world z* 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)

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)

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)

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

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

Stevens and Feingold (Nature, 2009)