Smoke, Clouds, Rainfall and Climate

- Two Direct and Three Indirect Effects

Meinrat O. Andreae

Max Planck Institute

for Chemistry, Mainz

Collaborators:

Paulo Artaxo, U. São Paulo

Maria Asuncao Silva Dias, U. São Paulo

Daniel Rosenfeld, Hebrew University

Earle Williams, MIT

Greg Roberts, MPI Chemistry & Caltech

Hans-P. Graf, MPI Meteorology, Hamburg

Olga Mayo-Bracero, MPI Chemistry

Bim Graham, MPI Chemistry

Jean Sciare, MPI Chemistry

and many others...

How do aerosols influence climate?

I) Direct Effects (i.e., not involving cloud)

a) Backscattering of sunlight into space

increased albedo cooling

Ib) Absorption of sunlight

• At surface: cooling

• In atmosphere: warming

• Effects:

reduced convection and cloudiness

reduced evaporation from ocean

reduced rainfall downwind

• The key parameters are the black carbon content of the aerosol and its mixing state

External Mixing +0.27

Black Carbon Core +0.54

Internal Mixing +0.78

Aerosol Mixing State of Black Carbon

Forcing(W m-2, Jacobson, 2000)

India:Haze over Ganges - Brahmaputra plain, observed by MODISNote: Haze is lighter than surface almost everywhere, especially over ocean, but darker over the low cloud patch in the upper Ganges plain

Smoke from Africa over Indian Ocean(Note the smoke being ingested by clouds)

II) Indirect Effects

• Each cloud droplet needs a "seed" or nucleus to be able to form: "Cloud Condensation Nucleus” (CCN)

• For a given cloud, the more CCN in the air, the more droplets

• Since the water supply in a cloud is limited: more droplets means smaller droplets

IIa) First Indirect Effect

• Adding CCN makes clouds with more, smaller droplets.

• These clouds are whiter, reflect more sunlight net cooling

Ship tracks off theWashington coast

IIb) Second Indirect Effect

“Overseeding“: To produce rain, cloud droplets need to be bigger than ~14 µm radius. When there are too many CCN, this radius is not reached and rainfall is suppressed. This occurs typically at CCN >800 cm-3.

Therefore:

Adding CCN increases cloud lifetime and cloud abundance Cooling

• This rain-suppression mechanism affects mainly "warm" clouds (those not containing ice phase)

• If there is enough latent heat available (tropics), the air will rise and rain-production mechanisms involving ice will take over.

• The results are– more wide-spread mixed phase clouds with lightning

– a shift in the release of latent heat from lower levels (warm clouds) to upper levels in the troposphere

– An increase in the total amount of heat released in cloud, because of ice formation

IIc) Third Indirect Effect:Aerosol Effect on Convection Dynamics

Note: IPCC-TAR does not include the 2nd indirect effect here, and does not even take the 3rd effect into account!

Is there evidence for the “Third Effect”?

• Our wet season data from Amazon basin indicate that CCN are very low in “natural” state

Maritime-type convection: “Green Ocean”

• Dry “smoky” season data show strong increase in CCN due to biomass smoke

Transition to “continental”-type clouds with strong electric activity and enhanced deep convection.

Large Scale Biosphere-Atmosphere Experiment over Amazônia

CLAIRE ‘98CLAIRE ‘98Manaus RegionManaus RegionMarch/April 1998

CLAIRE 2001CLAIRE 2001Manaus RegionManaus Region

July 2001

EUSTACH ‘99EUSTACH ‘99forest andforest and

pasture sites inpasture sites inRondôniaRondônia

7 April – 21 May15 Sept. – 1 Nov.

Aircraft ExperimentAircraft Experiment2 – 14 September

Summary of CCN Spectra

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.610

100

1000

******

**

**

Wet Season Pasture site Jaru Tower

(120 - 132) CLAIRE '98

Dry season Pasture site

Transition Jaru Tower

(134 - 143)

N

(cm

-3)

Sc (%)

What are CCN and where do they come from?

• Any aerosol particle with a certain minimal mass of soluble material (minimum size typically ~0.05 µm diameter)

CCN sources in the “clean” tropics:• Dust, Sea Spray: Over the humid continental tropics, too

few to be important

• Marine biogenic sulfate (from DMS): Probably important some of the time

• Terrestrial biogenic sulfate (from DMS and H2S): May play a key role in providing soluble aerosol component

• Secondary Organic Aerosol: May provide a significant part of aerosol mass, but mode of particle formation still under question

• Primary biogenic aerosol: Bacteria, spores, plant debris may account for a large fraction of aerosol mass and number!

Visibility ~ ??? km

NCN ~ 500 cm-3 NCCN(1%) ~ 250 cm-3

BC ~ 0.2 g m-3

The Rondonia forest site during the wet season

April - the wet and clean time of year: Note the shallow precipitating clouds, extensive warm rainout, glaciation at T>-10oC, and few lightning events

TRMM VIRS+PR, Amazon, 1998 04 13 16:28

VIRS T-Re

The “Green Ocean”: Maritime clouds over the Amazon

Dry SeasonSmoke haze

Visibility ~ 800 mNCN ~ 10,000 cm-3

NCCN(1%) ~ 2,000 cm-3

BC ~ 7 g m-3

Wet Season

Visibility ~ ??? km

NCN ~ 500 cm-3 NCCN(1%) ~ 250 cm-3

BC ~ 0.2 g m-3

Now, most particles are smoke...

… made up mostly of carbonaceous matter

About half of which is water-soluble, and therefore acts as CCN

Levoglucosan

VIRS+PR, Amazon, 1998

13 SEP 14:15

VIRS T-Re

TOMS Aerosol Index13 September 1998

September: The Fire Season

Note that clouds do not precipitate before reaching height of 6.5 km or –12oC isotherm, while containing ample cloud water.

The “Green Ocean” turns dry: Smoky clouds over the Amazon

VIRS+PR, Amazon, 1998 09 15 18:16

VIRS T-Re

PR H-Z

When the “smoky clouds” become Cb, they spark lightning and high Z

(from E. Williams et al., JGR, 2002, in press)

Effects on electrical activityCCN < 600

Moderate CAPE

CCN 600-1000Moderate/higher CAPE

Very high CCNModerate/high CAPE

Low CCNVery high CAPE

The situation is complex:

Both high CCN and high CAPE can lead to “continental”-type convection.

But “maritime”-type convection, now the most common rainfall regime in the wet season over the Amazon, cannot persist at elevated CCN

0

5

10

15

20

25

30

6 8 10 12 14 16 18 20 22 24 26 28 29

AmazonAfrica

Re

lati

ve

freq

uen

cy

Effective Radius [micron]

Plot of the relative frequency of effective cloud droplet radius over the African Congo (black bars) and the Amazon (crosshatched bars) based on AVHRR data (from J. R. McCollum et al., JAM, 2000).

Further evidence:

Congo vs. Amazon

Compare flash rates in January...

… and in September

Since the tropics are the heat engine of the atmosphere, this has worldwide consequences for circulation dynamics...

GCM simulation of the impact of biomass burning in the tropics on the global circulation in the extra-tropics. (Graf et al., 2000).

GCM simulation of the impact of biomass burning in the tropics on the global circulation in the extra-tropics. (Graf et al., 2000).

And the future of the Amazon?

Increasing development brings:

• More biomass smoke

• More NOx from engines

• More sulfate particles from power plants

• More NOx from soil due to canopy removal

• More secondary organic aerosol because of more NOx and O3

High CCN year-round

First Results from CLAIRE-2001 in the Manaus plume

• Detailed horizontal and vertical profiles of many trace gases and aerosols:– CO, CO2, NO, NO2, VOC, 13CO2

– CN, CCN, – scattering, absorption

• Detailed meteorological information

Dispersion model of the Manaus plume

The Manaus plume on AVHRR and TRMM ...

Conclusions:

• There is a wide range of direct and indirect effects of aerosols on climate

• The most important effects appear to be those that are the least well understood and quantified

• In its unpolluted state, the atmosphere over the Amazon Basin contains low CCN concentrations, and therefore has “marine”-type convection and rainfall production

• Pollution from biomass burning and other human activities has increased the CCN levels over Amazonia, esp. during the “dry” season

• This shifts convection and rainfall into a “continental” regime, with enhanced lightning and convective energy transfer at higher altitudes

• This perturbation has global effects on circulation dynamics

• Increasing aerosol emissions in the region will further enhance these effects