Indirect Effects: Aerosol and Cloud
Microphysics
Ulrike Lohmann
Institute for Atmospheric andClimate Science, Swiss Federal
Institute of Technological (ETH),
Zurich, Switzerland

IPCC Expert Meeting on Aerosols,
Geneva, May 2, 2005

Outline• What are aerosol indirect effects?
– Twomey effect– Cloud lifetime effect– Semi-direct effect• Evidence of the different indirect aerosol effects
from field studies• Estimates of global mean indirect aerosol effects• Conclusions

Waterclouds

Cloud condensation
nuclei (CCN)
Aerosol removal
by precipitation

Oxidation,
Nucleation

SO2, NOx,
OrganicsIndustrySoot,
organics

Soil dust
saltation

Automobiles Biomass burning Arid area

Sea salt

Ocean sprayIndirectaerosol
effects onclimate

Penner et al., IPCC, 2001

Different aerosol effects on water clouds• Cloud albedo effect (pure forcing)– for a constant cloud water content, more aerosols lead tomore and smaller cloud droplets Æ larger cross sectional
area Æ more reflection of solar radiation• Cloud lifetime effect (involves feedbacks)– the more and smaller cloud droplets will not collide as
efficiently Æ decrease drizzle formation Æ increase cloud
lifetime Æ more reflection of solar radiation• Semi-direct effect (involves feedbacks)– absorption of solar radiation by black carbon within a cloudincreases the temperature Æ decreases relative humidity Æ
evaporation of cloud droplets Æ more absorption of solar
radiation (opposite sign)Cloud evolution in a clean and polluted atmosphere

Kaufmanet al.
2002

Shiptracks
off the coastofWashington

Durkee et al., 2000

Evidence for the cloud albedo effect

Durkee et al.,JAS, 2000

Top-of-the-atmosphere global-mean radiative
forcing (W m-2) for 2000 relative to 1750 [IPCC, 2001]

Estimate of the total radiative (aerosol and nonaerosol) forcing since pre-industrial times

Boucher andHaywood, Clim.
Dyn., 2001

Summary of aerosol forcing estimates[Anderson et al., Science, 2003]

Example of an inverse simulation [Knutti et al., 2002]
Simulated relation between
climate sensitivity (∆T/2xCO2) and atmosphericand oceanic warming:a) global ocean heat uptake
from 1955-1995 in the upper
3 kmb) atmospheric temperature from 1900-2000
Probability density functionsof the global mean indirecteffect:d) not constrainede) contrained by observed T
recordf) also constrained by the IPCC
climate sensitivity

How are aerosol effects on clouds simulated inclimate models?
• Predict aerosol mass concentrations:– sources (aerosol emissions of the major aerosol species:
sulfate, black carbon, organic carbon, sea salt, dust)– transformation (dry and wet deposition, chemicaltransformation and transport)• Need a good description of cloud properties:– precipitation formation (collision/coalescence of cloud
droplets and ice crystals, riming of snow flakes)• Need to parameterize aerosol-cloud interactions:– cloud droplet nucleation (activation of hygroscopic aerosolparticles)– ice crystal formation (contact and immersion freezing,
homogeneous freezing in cirrus clouds)

Global annual mean aerosol emissions(representative for 1985)
100
90
80
70
60
50
40
30
20
100318Tg S/C per year
NaturalAnthropogenic

Anthropogenic
Emissionsfrom
Biomass burning

SulfurSootOC

Dust Sea-salt< 1 mu < 1 muandFossil fuel use

Aerosol mass resulting from human activity

Cloud microphysical processes in a climate model

Aerosols

Sulfate, sea salt, organics, dust, sootEvaporation

nCo

ql

Vapor

SubDe
pos itHomo/Hetero. Freezing

NdAutoconversion
Accretion

l im
ati

Nucleation
onionqi
Melting
R iming

Depositional GrowthAggregation
Accretion

Freezing

Rainfall

MeltingNi

SnowfallSelfcollection

on
it
ora
on
ipt
ans a
Eved

Nucleation

Sublimation

Aerosol - cloud droplet relationships

Penner etal., IPCC,2001

Temporal
evolution ofsulphuremissionand direct
and indirect
radiative
forcing ofsulfate
aerosols

Boucher andPham, GRL, 2002

Top panel:
Direct effectof sulphateaerosols (–0.4W/m2)

Lower panel:
Indirect cloud
albedo effect(–1.o W/m2)
Boucher andPham, GRL, 2002

Cloud lifetime effect calculations
The autoconversion rate (precipitation formation rate in clouds
with no ice) in climate models depends on the cloud watercontent ql and the number concentration of cloud droplets N:
Qaut ~ qla Nbwith a=2-5
b=-1 to -3.3

Æ more cloud
droplets decrease
drizzle formation

Aerosol effects
on cloud watercontent
between preindustrial andpresent-daytimes

Indirect aerosol effectDifference between two 5-year simulations one with pre-industrial and one withpresent-day aerosol emissions[Global mean change in top-of-the-atmosphere net radiation: -1.4 W/m2]

Peng and Lohmann, GRL, 2003

Semi-direct
effectAckerman et al., Science, 2000

Semi-direct
effectChange in liquid water
path with black carbon
for the experiments
DIRECT, INDIRECT andALL

Lohmann and Feichter,
GRL, 2001

DIRECT

INDIRECTALL

Global mean indirect aerosol effect (Twomey vs.
lifetime) from different climate models

SulfateSoot (BC) andsulfate
Organic aerosols(OC) and sulfate
BC, OC and sulfate

Lohmann andFeichter, ACP,
2005

Summary• Aerosol effects on the radiative balance aresignificant. At the top-of-the atmosphere, thecooling effect from sulfate and organic aerosols ispartly offset by the warming by black carbon.
• All aerosols cause a reduction of solar radiation at
the Earth surface.
• In addition aerosols significantly influence air
quality and the hydrological cycle.
• We will know more about each individual aerosol
species, including their effects on ice clouds, by thetime the Forth IPCC Assessment report is published.