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

€

∂Nk

∂t= −(V • ∇N )k −

∂ρ ′ w ′ N

ρ∂z

⎛

⎝ ⎜

⎞

⎠ ⎟k

+ Sk − Ak −Ck − Ek

Nk = droplet number mixing ratio in layer k

Ak = droplet loss by autoconversion of droplets

Ck = droplet loss by collection by precipitation

Ek = droplet loss by evaporation

Sk = droplet nucleation source in layer k

Indirect Effects Physics

€

∂Nk

∂t= −(V • ∇N )k −

∂ρ ′ w ′ N

ρ∂z

⎛

⎝ ⎜

⎞

⎠ ⎟k

+ Sk − Ak −Ck − Ek

Nk = droplet number mixing ratio in layer k

Ak = droplet loss by autoconversion of droplets

Ck = droplet loss by collection by precipitation

Ek = droplet loss by evaporation

Sk = droplet nucleation source in layer k

f = cloud fraction

w = updraft velocity

Nn = number nucleated (parameterized in terms of w and aerosol)

p(w) = probability density function of w

w* = σw= characteristic updraft velocity in growing part of cloud

€

=max( fk − fk−1,0)

zk+ 1

2− z

k−12

wNnwmin

∞

∫ (w)p(w)dw +∂fk

∂tNn(w*)

Indirect Effects Physics

€

∂Nk

∂t= −(V • ∇N )k −

∂ρ ′ w ′ N

ρ∂z

⎛

⎝ ⎜

⎞

⎠ ⎟k

+ Sk − Ak −Ck − Ek

Nk = droplet number mixing ratio in layer k

Ak = droplet loss by autoconversion of droplets

Ck = droplet loss by collection by precipitation

Ek = droplet loss by evaporation

Sk = droplet nucleation source in layer k

f = cloud fraction

w = updraft velocity

Nn = number nucleated (parameterized in terms of w and aerosol)

p(w) = probability density function of w

w* = σw= characteristic updraft velocity in growing part of cloud

€

reff = β3ql

4πρ wN

⎛

⎝ ⎜

⎞

⎠ ⎟

13

2nd IE: Autoconversion connected to droplet number.

€

=max( fk − fk−1,0)

zk+ 1

2− z

k−12

wNnwmin

∞

∫ (w)p(w)dw +∂fk

∂tNn(w*)

1st IE:

CAM and MIRAGE

model version aerosol mass

aerosol number

size distribution

species mixing state

winds

CAM cam3.3.11.tropmz12

prescribed or predicted

diagnosed from mass

prescribed SO4, 2xOC, 2xBC,4xSS, 4xdust, background

external free

MIRAGE CAM2 predicted predicted or diagnosed

variable mode radius

SO4, OC, BC, SS, dust 4 modes

internal within each mode

nudged or free

CAM Aerosol Properties

Aerosol Specie Mode Radius ( m) Geometric Standard Deviation

Hygroscopicity

SO4 0.05 2 0.51 OC 0.02 2.24 0.14, 10-10 BC 0.01 2 10-10 background 0.05 2 0.51 volcanic 0.375 1.25 0.44 sea salt 0.5, 2, 5, 15 1.6, 1.37, 1.4, 1.22 1.3 soil dust 0.2, 3, 15, 30 1.94, 1.3, 1.22, 1.22 0.14

Estimating Direct and Indirect Effects

Two simulations:

1. All aerosol sources

2. All sources except anthropogenic sulfate

Each simulation calculates radiative fluxes with (Faer) and without aerosol (Fnoaer).

Estimating Direct and Indirect Effects

Two simulations:

1. All aerosol sources

2. All sources except anthropogenic sulfate

Each simulation calculates radiative fluxes with (Faer) and without aerosol (Fnoaer).

Direct effect of all aerosol in a simulation is

Fdirect = Faer -Fnoaer.

Estimating Direct and Indirect Effects

Two simulations:

1. All aerosol sources

2. All sources except anthropogenic sulfate

Each simulation calculates radiative fluxes with (Faer) and without aerosol (Fnoaer).

Direct effect of all aerosol in a simulation is

Fdirect = Faer -Fnoaer.

Difference between simulations is . Then

Fdirect = Faer -Fnoaer

Estimating Direct and Indirect Effects

Two simulations:

1. All aerosol sources

2. All sources except anthropogenic sulfate

Each simulation calculates radiative fluxes with (Faer) and without aerosol (Fnoaer).

Direct effect of all aerosol in a simulation is

Fdirect = Faer -Fnoaer.

Difference between simulations is . Then

Fdirect = Faer -Fnoaer

Findirect = Faer -Fdirect

= Fnoaer

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

First and Second Indirect Effect

-3.5 -3 -2.5 -2 -1.5 -1 -0.5 0

CAM

MIRAGE

Radiative Forcing (W/m2)

Indirect Effect

Direct Effect

No 2nd indirect effectNo Second Indirect Effect

-3.5 -3 -2.5 -2 -1.5 -1 -0.5 0

CAM

MIRAGE

Radiative Forcing (W/m2)

Indirect Effect

Direct Effect

No nudging

No nudging

-3.5 -3 -2.5 -2 -1.5 -1 -0.5 0

CAM

MIRAGE

Radiative Forcing (W/m2)

Indirect Effect

Direct Effect

Change LWP w/, w/o nudging

lwp cam no2ndindir, mirage no2ndindir nudge, mirage no2ndindir

nonudge

Zonal mean IE

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

CAM Single vs Spectrum of Updrafts

-4 -3.5 -3 -2.5 -2 -1.5 -1 -0.5 0

single updraft

spectrum

Radiative Forcing (W/m2)

Indirect Effect

Direct Effect

Background aerosol

Background Aerosol

-3.5 -3 -2.5 -2 -1.5 -1 -0.5 0

CAM tau=0

CAM tau=0.01

CAM tau=0.02

MIRAGE

Radiative Forcing (W/m2)

Indirect Effect

Direct Effect

Noanthro ccn3 cam tau=0,0.01, 0.02

Noanthro [CCN3] cam progaer no2ndindir tau=0, 0.01, 0.02

Mirage prognaer no2ndindir nonudge

Sensitivity to size

r=0.05 for oc, bc, volcanic

CAM Sensitivity to Size Distribution

-4 -3.5 -3 -2.5 -2 -1.5 -1 -0.5 0

baseline

rad=0.05

Radiative Forcing (W/m2)

Indirect Effect

Direct Effect

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.