2. Black Carbon+Organic carbon Sulphate
More in NH than SH
Takemura et al JGR
Black Carbon+Organic carbon
Sulphate
Soil dust
Sea salt
This shows the different components of aerosol and the global distribution.
The radiative effects of these different aerosol can be divided into direct and indirect effects
Takemura et al. (2005), Simulation of climate response to aerosol direct and indirect effects with aerosol transport-radiation model, J. Geophys. Res.

3. First, aerosols scatter and can absorb solar radiation
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 effect
Aerosol efect is divided into direct and indirect effect
Magnitude and spatial Spatial distribution of the two types is diferent
Takemura et al. 2005

4. OUTLINE Introduction Aerosol effects
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

5. with constant LWC

6. Even though there are different effects, most climate model simulations usually ignore indirect aerosol effects beyond the Twomey effect(Roeckner et al. 1999)
A point to note is ->ß

7. 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
The twomey effect refers to the enhanced reflection of solar radiation due to the more but smaller cloud droplets in a cloud whose liquid water content remains constant.
Cloud lifetime effect refers to the enhanced reflection of solar radiation due to more but smaller cloud droplets which reduces precipitation effiency and hence cloud lifetime. – Also known as second indirect aerosol effect
Absorption of solar radiation by aerosols leads to a heating of the air, which can result in an evaporation of cloud droplets. It is referred to as semi-direct effect – This maybe warming/cooling.

8. …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)
W = LWC ; N = CDNC; dv = volume diameter from Brenguier et al., 2000
Brenguier et al., 2000

9. 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.
Taking a global average of TOA process will miss out from different spatial configuration that accompanies this effect

10. In GCMs, this is divided into
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

11. Twomey Effect SO4 + SS + OC
Sea salt plays a minor role in SH, though there are large quantities
Twomey Effect
Sulfate
sulfate + OC
sulfate + BC + OC
Definition is not unique
Change in SW – Chuang; Rot.-Liu; Quaas
Change in net cloud readiative -- Menon
In general, sulphate seems to be causing more cooling in NH than SH.
Rem. the first plot?
Small value for Quaas may be due to the emperical fomulation used.
But this is partly expected becos since sea salt are large aerosol, they get to the air and condense out pretty quickly.

12. 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
One reason for the large aerosol indirect effects obtained by Menon et al. (2002a) could be due to their empirical treatment between the aerosol mass and the cloud droplet number
Another reason for the discrepancy between models could be the dependence of the indirect aerosol effect on the background aerosol concentration.
Likewise differences in the cloud microphysics scheme, especially in the autoconversion rate

13. 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..
The parameterization of Rotstayn (which ise sulphate alone) caused the clouds over ocean to be more subscetible to CCN

14. 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
Glaciation effects brings warming
Note that contact nucleation is most efficient at the temperature between 0 &-35C
Ways of ice-initiation bet aerosol
Contact freezing -- most efficient for slight supercool
Immersion or condensation freezing – maybe more prevalent at more lower temperature
Acting as deposition nuclei – least effiecient becos higher energy is needed to sublime vapour to ice

15. Glaciation indirect effect
Land surface
Warm indirect effect
Glaciation indirect effect
Here increases in contact ice nuclei in the present-day climate result in more frequent glaciation of supercooled clouds and increase the amount of precipitation via the ice phase. This reduces the cloud cover and the cloud optical depth of mid-level clouds in mid- and high latitudes of the Northern Hemisphere and results in more absorption of solar radiation within the Earth-atmosphere system.

16. solid line – BC10% dot-dashed line – BC1% dotted line – BC0%
Figure 3. Zonal annual mean changes between present-day and pre-industrial times for the experiments BC10% (solid line), BC1% (dot-dashed line) and BC0% (dotted line).
The ice water content increases in both BC1% and BC10% are much smaller than the liquid water content decreases because the precipitation formation via the ice phase is more efficient.
Lohmann (2002)

17. 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!!

18. 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
This is caused by the pollution induced increase in aerosol and cloud optical thickness, which reduces the solar radiation at the surface and causes a cooling that favors precipitation formation via the ice phase (Lohmann, 2004).

19. 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)

20. 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

21. 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?
Cirrus cloud normally form in a region where there are almost no IN. They freeze homogenously. Increasing the freezing nuclei would increase the cirrus cloudness.
However, a much stronger indirect aerosol effect on cirrus clouds is possible if several ice nuclei types with different freezing thresholds compete during the freezing process.

22. AIE on vertical structure
Temp. change
Aerosol cooling extends up to the tropopause
Max cooling
Mid & High lat. – BL
Tropics -- UT
More cooling in NH then in SH
Changes of the vertical temperature profile due to aerosol effects in K between pre-industrial and present-day conditions
40oS-40oN; 40oN–90oN; 40oS–90oS

23. AIE on vertical structure
Temp. change
+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
Changes of the vertical temperature profile due to aerosol effects in K between pre-industrial and present-day conditions
40oS-40oN; 40oN–90oN; 40oS–90oS

24. 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
Figure 3. Monthly mean surface solar radiation in United States. Blue line represents 1961 to 1970, green 1971 to 1980, and red 1981 to 1990 period for (a) clear sky and (b) overcast sky conditions.
Liepert 2002

25. 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…
Fsw = Flw + Fl + Fs + Fcond
Fcond approx = 0
Hence Flw + Fl + Fs must decrease. Since the feedback of aerosol on cloud may lead to increase in Flw, the only way to have the balance is if Fl decrease

26. 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

27. 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

28. 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.

29. 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”

30. 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

31. 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

32. AIE – forcing or response?
One question: is forcing really ADDICTIVE???
Answer: NO!!!

33. 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!!!

34. Feedback and Response of AIE cannot be separated!!!

35. 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

36. Summary

38. Constraints on the indirect aerosol effect
The indirect effect on water clouds only by sulphate and carbonaceous aerosol is between -1 and -4.4W/m^2
The inverse model(from historic records of ocean and atmospheric warming) and thermodynamic consideration(which believe that if averages is made over anthropocene so that the internal variability is taken out, then the aerosol effects will solely be due to GHG forcing on surf temp and ocean heat cont.) gives 0to-2 and -0.7to-1.7 respectively
This implies that, infact, the indirect effect estimated by this models clearly involves other forms of the aerosol effect that is currently not included in the GCM

39. Semi-direct aerosol effect -- Climatic effect of black carbon is strongly positive. – since it absorbs solar radiation within the cloud and lead to cooling of earth surface
Dispersion effect -- Liu and Daum (2002) estimated that the magnitude of the Twomey effect can be reduced by 10–80% by including the influence that an increasing number of cloud droplets has on the shape of the cloud droplet spectrum (dispersion effect).