Aerosol Indirect Effects in CAM and MIRAGE Steve Ghan Pacific Northwest National Laboratory Jean-Francois Lamarque, Peter Hess, and Francis Vitt, NCAR

Indirect Effects Physics N k = droplet number mixing ratio in layer k A k = droplet loss by autoconversion of droplets C k = droplet loss by collection by precipitation E k = droplet loss by evaporation S k = droplet nucleation source in layer k

Indirect Effects Physics N k = droplet number mixing ratio in layer k A k = droplet loss by autoconversion of droplets C k = droplet loss by collection by precipitation E k = droplet loss by evaporation S k = droplet nucleation source in layer k f = cloud fraction w = updraft velocity N n = number nucleated (parameterized in terms of w and aerosol) p ( w ) = probability density function of w w * = σ w = characteristic updraft velocity in growing part of cloud

Indirect Effects Physics N k = droplet number mixing ratio in layer k A k = droplet loss by autoconversion of droplets C k = droplet loss by collection by precipitation E k = droplet loss by evaporation S k = droplet nucleation source in layer k f = cloud fraction w = updraft velocity N n = number nucleated (parameterized in terms of w and aerosol) p ( w ) = probability density function of w w * = σ w = characteristic updraft velocity in growing part of cloud 2nd IE: Autoconversion connected to droplet number. 1st IE:

CAM and MIRAGE

CAM Aerosol Properties

Estimating Direct and Indirect Effects Two simulations: 1.All aerosol sources 2.All sources except anthropogenic sulfate Each simulation calculates radiative fluxes with (F aer ) and without aerosol (F noaer ).

Estimating Direct and Indirect Effects Two simulations: 1.All aerosol sources 2.All sources except anthropogenic sulfate Each simulation calculates radiative fluxes with (F aer ) and without aerosol (F noaer ). Direct effect of all aerosol in a simulation is F direct = F aer - F noaer.

Estimating Direct and Indirect Effects Two simulations: 1.All aerosol sources 2.All sources except anthropogenic sulfate Each simulation calculates radiative fluxes with (F aer ) and without aerosol (F noaer ). Direct effect of all aerosol in a simulation is F direct = F aer - F noaer. Difference between simulations is . Then  F direct =  F aer -  F noaer

Estimating Direct and Indirect Effects Two simulations: 1.All aerosol sources 2.All sources except anthropogenic sulfate Each simulation calculates radiative fluxes with (F aer ) and without aerosol (F noaer ). Direct effect of all aerosol in a simulation is F direct = F aer - F noaer. Difference between simulations is . Then  F direct =  F aer -  F noaer  F indirect =  F aer -  F direct =  F noaer

IE, DE with 2nd IE CAM tau=0, MIRAGE nudge

No 2nd indirect effect

No nudging

Change LWP w/, w/o nudging  lwp cam no2ndindir, mirage no2ndindir nudge, mirage no2ndindir nonudge

Zonal mean IE

Dlwp vs indirect effect

Ndrop cam, mirage Ndrop cam progaer no2ndindir tau=0 Mirage prognaer no2ndindir nudge Anthro, noanthro

Ccn3 cam, mirage, anthro, noanthro CCN3 cam progaer no2ndindir tau=0 Mirage prognaer no2ndindir nudge Anthro, noanthro

Ndrop cam updraft spectrum mirage updraft spectrum

DE, IE cam updraft spectrum

Background aerosol

Noanthro ccn3 cam tau=0,0.01, 0.02 Noanthro [CCN3] cam progaer no2ndindir tau=0, 0.01, 0.02 Mirage prognaer no2ndindir nonudge

IE, DE cam progaer 2ndindir tau=0.02, cam prescribe_aer 2ndindir tau=0.02

Sensitivity to size r=0.05 for oc, bc, volcanic

Conclusions The much larger indirect effect produced by CAM has not been completely explained. The much larger feedback of liquid water path explains at least part of larger indirect effect. The larger relative sensitivity of CCN to emissions in CAM may also contribute. The CAM CCN and IE are insensitive to the size distribution of OC and volcanic. A background aerosol reduces the IE from CAM, but cannot be justified.

Future Work Resolve differences between CAM and MIRAGE: insert monthly mean aerosol from each model into simulations by the other. Add detrainment of droplet number from cumulus. Integrate with UW turbulence and shallow cumulus schemes. Couple with MIRAGE treatment of aerosol dynamics and mixing state. Add nucleation scavenging and size-dependent impaction scavenging. Size and composition dependent optical properties. Add primary and secondary marine organic emissions.