black carbon+organic carbon sulphate sea saltsoil dust takemura et al. 2005 jgr more in nh than sh
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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
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