Black Carbon+Organic carbon

Sulphate

Sea saltSoil dust

Takemura et al. 2005 JGRMore in NH than SH

First, aerosols scatter and can absorb solar radiation. – direct aerosol effect

Second, they can scatter, absorb and emit thermal radiation. – direct aerosol effect

Third, aerosol particles act as cloud condensation nuclei (CCN) and ice nuclei (IN) thereby changing the microphysical and optical properties of cloud droplets. – indirect aerosol effectTakemura et al. 2005

OUTLINE

Introduction Indirect aerosol effects

Aerosol effects On warm clouds On mixed phase clouds On ice clouds On vertical structure

Consequence of AIE on surface budget Indirect aerosols effect – forcing or response? Feedbacks of clouds on aerosols Summary

with constant LWC

Even though there are different effects, most climate model simulations usually ignore indirect aerosol effects beyond the Twomey effect(Roeckner et al. 1999)

Aerosol effects on warm clouds The three effects that act on the warm

liquid clouds are the Twomey, cloud-lifetime and semi-direct effects

Cloud-lifetime effect is believed to be of comparable magnitude as Twomey effect

Observations show that cloud droplets are smaller in polluted clouds than in clean clouds

…and that polluted clouds are thinner as they originate over the continents, which cause them to be drier than their counterpart marine clean clouds

(Lohmann and Lesins, 2003)

Brenguier et al., 2000

In GCMs, AIE is made by conducting a present-day simulation and a pre-industrial simulation in which the anthropogenic emissions are set to zero.

The difference in the TOA radiation budget is then taken as anthropogenic AIE where then the aerosol parameters are related to CDNC empirically or by using physically-based parameterization.

Warm clouds form precipitation-size particles by the collision/coalescence process.

In GCMs, this is divided into Autoconversion – collisions and coalescence among

cloud droplets Accretion of rain drops with cloud droplets.

Wood, 2005

Twomey Effect Sulfate sulfate + OC sulfate + BC +

OC

Definition is not unique Change in SW

flux @TOA – Chuang; Rot.-Liu; Quaas

Change in net cloud readiative forcing @TOA -- Menon

In general, sulphate seems to be causing more cooling in NH than SH. Rem. the first

plot?

SO4 + SS + OCSea salt plays a minor role in SH, though there are large quantities

Twomey vs Lifetime

Sulfate Sulfate + BC Sulfate + OC Sulfate + BC +

OC

Reason for the different estimates: Emperical

treatment of aerosol mass & CDNC

Background aerosol concentration – i.e starting conc.

Different microphysical schemes used, esp. in autoconversion

Land vs. Ocean

Sulfate Sulfate + BC Sulfate + OC Sulfate + BC + OC GCM + POLDER

In general, continental clouds are less susceptible to the effect of anthropogenic increase in CCN, because more natural CCN over land than over ocean..

Aerosol effects on mixed-phase clouds

In addition to the effects mentioned earlier, mixed-phase clouds are also affected by: thermodynamic, glaciation and riming indirect effects.

Lohmann (2002) showed that if, in addition to mineral dust, a fraction of the hydrophilic soot aerosol particles is assumed to act as contact ice nuclei at temperatures between 0C & −35C, then increases in aerosol concentration from pre-industrial times to present-day pose a new indirect effect, a “glaciation indirect effect”

… and this effect partly offsets the cooling by cloud-lifetime effect

Land surface

Warm indirect effect

Glaciation indirect effect

Lohmann (2002)

solid line – BC10%dot-dashed line – BC1% dotted line – BC0%

Riming/snowfall indirect effect

Observations by Borys et al. (2003) in midlatitude orographic clouds show that for a given supercooled liquid water content, both the riming and the snowfall rates are smaller if the supercooled cloud has more cloud droplets as, for example, caused by anthropogenic pollution

…but this may not be completely true!!

Examination of this effect in global climate model simulations with pre-industrial and present-day aerosol concentrations showed that while the riming rate in stratiform clouds has indeed decreased due to the smaller cloud droplets in polluted clouds, the snowfall rate has actually increased

Thermodynamic effect Increase in (pollution)

aerosols delay precipitation by decreasing cloud droplet size

Andreae et al. (2004) reported a delayed onset of precipitation from 1.5 km above cloud base in pristine clouds to more than 5 km in polluted clouds, and to more than 7 km in pyro-clouds for forest fire in the Amazon basin

They suggested that elevating the onset of precipitation released latent heat higher in the atmosphere and allowed invigoration of the updrafts, causing intense thunderstorms and large hail

Andreae et al. (2004)

Thermodynamic effect

For deep convective clouds, Khain et al. (2004) postulated that smaller cloud droplets, such as originating from anthropogenic activity, would reduce the production of drizzle drops.

When these droplets freeze, the associated latent heat release results in more vigorous convection.

In a clean cloud, on the other hand, drizzle would have left the cloud so that less latent heat is released when the cloud glaciates resulting in less vigorous convection

Therefore, no squall line is formed with maritime aerosol concentrations, but the squall line arises under continental aerosol concentrations

Aerosol effect on ice cloud Increase in freezing nuclei in upper troposphere

leads to increase in cirrus cloudiness. (Boucher 1999)

In fact, an increase of 2% per decade has been reported for northern America and Atlantic as well as over Europe (Zerefos et al. 2003; Stordel et al, 2004) from analysis of ISCCP data

However, this effect on a large scale is small (Hendricks et al. 2004)

What if… several ice nuclei types with different freezing threshold compete during freezing process?

AIE on vertical structure

Aerosol cooling extends up to the tropopause

Max cooling Mid & High lat. –

BL Tropics -- UT

More cooling in NH then in SH

40oS-40oN; 40oN–90oN; 40oS–90oS

Temp. change

AIE on vertical structure +ve semi-direct effect

if absorb. aerosol is within BL destabilizes the

atmosphere as a result of heating within the BL

Lead to enhanced vertical motion

-ve effect if absorb. aerosol is above the BL Lead to increased

stability

40oS-40oN; 40oN–90oN; 40oS–90oS

Temp. change

Consequence of AIE on surface budget

Increasing aerosol and cloud optical depth, human emissions of aerosols cause a reduction of solar radiation at the surface (“solar dimming”)

Due to this effect solar radiation at the surface of NH has reduced by 1.3% per decade from 1961-1990

Liepert 2002

For surface energy to reach a new equilibrium state, there has to be some adjustment: Fsw = Flw + Fl + Fs + Fcond

Fsw => FL

This mechanism could explain the observations of decreased pan evaporation over the last 50 years reported by Roderick and Farquhar (2002).

However…

Wild et al., 2004 reported that this decline disapered in the 1990s.

Hence, the increasing greenhouse effect may no longer be masked by an aerosol induced decline in solar radiation, resulting in the enhanced warming observed since the 1990s.

All-sky

Clear-sky

AIE – forcing or response? Most AIE involve fast feedback

processes and are therefore not consider “forcing” in the “classical” sense, but they have huge impact in hydrological cycle

For an climatic system – Climate sensitivity parameter is defined as λ = ΔTsfc/Fi

Where Fi is the instantaneous radiative forcing at the TOA that needs to be used to estimate ΔTsfc in transient models

AIE – forcing or response?

This approach is a problem for estimating AIE: e.g absorbing aerosol like black carbon has very small TOA forcing but because it absorbs large amount of solar radiation, it has a significant surface temp. change.

AIE – forcing or response?

For adjusted system: IPCC defined Fa as “The radiative forcing of the surface-troposphere system due to the perturbation in or the introduction of an agent is the change in net irradiance at the tropopause after allowing for stratospheric temperatures to readjust to radiative equilibrium, but with the surface and tropospheric temperatures and state held fixed at the unperturbed values”

AIE – forcing or response? The new

approach is to fix SST and allow the system to respond.

This helps to separate between forcing that changes the atmospheric parameters and those that invoke surface temperature changes e.g forcing due to semidirect effect

AIE – forcing or response? If we define efficacy as the ratio of the climate sensitivity

parameter λ for a given forcing agent to that for a given change in CO2. i,e E = λ/λCO2 and then effective forcing will be: Fe = F*E

AIE – forcing or response? One question: is forcing really ADDICTIVE???

Answer: NO!!!

Even in global hydrological sensitivity…

Hence there is an embedded non-linearity between forcing and response! Which may be caused by the feedback mechanism in aerosol

cloud interaction, e.g. cloud-lifetime effect. AND/OR Saturation effect within AIE – i.e. sublinear increase with

aerosol number concentration (Boucher and Pham, 2002)

However, if we argue that cloud-lifetime effect is NOT forcing but encompass feedback, then the forcing part may be ADDICTIVE!!!

Feedback and Response of AIE cannot be separated!!!

Feedbacks of clouds on aerosols Positive –

More aerosols => decrease in precipitation formation rate => increase lifetime of aerosols => more long-range transport to remote regions => where wet removal is less effective =>may lead to more aerosols.

Example, Lohmann and Feichter(1997) => 50% increase of sulphate burden

Negative – More aerosols(black carbon) => more contact ice

nuclei => more precip. via ice phase => removes aerosols from atmosphere => less aerosol in the atmosphere

Example, Lohmann, 2002 => 38-58% decrease in black carbon

Summary