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 MODIS Note: 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 the Washington 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 2 nd indirect effect here, and does not even take the 3 rd 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 ‘98 Manaus Region March/April 1998 CLAIRE 2001 Manaus Region July 2001 EUSTACH ‘99 forest and pasture sites in Rondônia 7 April – 21 May 15 Sept. – 1 Nov. Aircraft Experiment 2 – 14 September

Summary of CCN Spectra

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 H 2 S): 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 N CN ~ 500 cm -3 N CCN(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>-10 o C, and few lightning events TRMM VIRS+PR, Amazon, :28 VIRS T-Re The “Green Ocean”: Maritime clouds over the Amazon

Dry Season Smoke haze Visibility ~ 800 m N CN ~ 10,000 cm -3 N CCN(1%) ~ 2,000 cm -3 BC ~ 7  g m -3 Wet Season Visibility ~ ??? km N CN ~ 500 cm -3 N CCN(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, SEP 14:15 VIRS T-Re TOMS Aerosol Index 13 September 1998 September: The Fire Season Note that clouds do not precipitate before reaching height of 6.5 km or –12 o C isotherm, while containing ample cloud water. The “Green Ocean” turns dry: Smoky clouds over the Amazon

VIRS+PR, Amazon, :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 activity CCN < 600 Moderate CAPE CCN Moderate/higher CAPE Very high CCN Moderate/high CAPE Low CCN Very 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

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 NO x from engines More sulfate particles from power plants More NO x from soil due to canopy removal More secondary organic aerosol because of more NO x and O 3  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, CO 2, NO, NO 2, VOC, 13 CO 2 –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