Regional analysis of Regional analysis of multi-year aerosol multi-year aerosol

indirect effectsindirect effects

Dr. Thomas JonesDr. Thomas JonesUniversity of Alabama in HuntsvilleUniversity of Alabama in Huntsville

January 13, 2009January 13, 20098989thth Annual AMS Conference, Phoenix, AZ Annual AMS Conference, Phoenix, AZ

Special Symposium on Aerosol–Cloud–Climate Interactions: 4.4

Aerosol Indirect EffectsAerosol Indirect Effects Aerosol interact with atmospheric and cloud Aerosol interact with atmospheric and cloud

properties in many waysproperties in many ways First (Twomey 1977)First (Twomey 1977)

more aerosols increase # of CCN thereby reducing droplet more aerosols increase # of CCN thereby reducing droplet sizesize

Second (Albrecht 1989)Second (Albrecht 1989) Reduced droplet size leads to delay in onset of C&C reducing Reduced droplet size leads to delay in onset of C&C reducing

precipitation efficiency and increasing cloud lifetimeprecipitation efficiency and increasing cloud lifetime Semi-direct (Ackerman et al. 2000)Semi-direct (Ackerman et al. 2000)

Absorbing aerosols (dust or black carbon) can warm key Absorbing aerosols (dust or black carbon) can warm key atmospheric layers affecting stability and likelihood of cloud atmospheric layers affecting stability and likelihood of cloud formationformation

Convective (Andreae et al. 2004)Convective (Andreae et al. 2004) Reduction in stratiform precipitation efficiency can lead to Reduction in stratiform precipitation efficiency can lead to

an increase in convective precipitation.an increase in convective precipitation.

Importance of aerosol Importance of aerosol type & surrounding type & surrounding

conditionsconditions Certain types of aerosols affect cloud Certain types of aerosols affect cloud

properties differentlyproperties differently Sulfates and sea salt make excellent CCNSulfates and sea salt make excellent CCN Dust aerosols are less effective CCN and also Dust aerosols are less effective CCN and also

have the offsetting effect of absorbing rather have the offsetting effect of absorbing rather than reflecting solar radiationthan reflecting solar radiation

Atmospheric conditionsAtmospheric conditions If sufficient moisture and synoptic forcing are If sufficient moisture and synoptic forcing are

not sufficient for cloud formation, then aerosol not sufficient for cloud formation, then aerosol indirect effects would not be observedindirect effects would not be observed

Thus, increasing AOT may not necessarily lead Thus, increasing AOT may not necessarily lead to larger values of AIE…to larger values of AIE…

Regional analysis of AIERegional analysis of AIE Goals:Goals:

Calculate AIE for several regions around the Calculate AIE for several regions around the globe, each containing unique aerosol and globe, each containing unique aerosol and atmospheric conditionsatmospheric conditions

Separate total AOT into (primarily) anthropogenic Separate total AOT into (primarily) anthropogenic and dust components to analyze the relative and dust components to analyze the relative indirect effects for both aerosol typesindirect effects for both aerosol types

Apply to a multi-year data set to study the effects Apply to a multi-year data set to study the effects of changing aerosol and atmospheric conditions of changing aerosol and atmospheric conditions on AIE on AIE

Using case study examples, determine if aerosol Using case study examples, determine if aerosol and cloud layers are located at the appropriate and cloud layers are located at the appropriate levels to explain observed effectslevels to explain observed effects

For example, for the first AIE to be occurring, then For example, for the first AIE to be occurring, then aerosols and clouds should be in the same atmospheric aerosols and clouds should be in the same atmospheric layerlayer

DataData Aerosol and cloud properties are obtained from Aerosol and cloud properties are obtained from

the CERES-SSF product over several regionsthe CERES-SSF product over several regions Contains collection 4 AOT Contains collection 4 AOT Cloud parameters derived using the Minnis et al. (2003) Cloud parameters derived using the Minnis et al. (2003)

algorithmalgorithm We only use liquid-water cloud informationWe only use liquid-water cloud information

Atmospheric conditions are obtained from the Atmospheric conditions are obtained from the NCEP Re-analysis productNCEP Re-analysis product Temperature, moisture, and vertical velocity at a Temperature, moisture, and vertical velocity at a

2.5°x2.5° resolution2.5°x2.5° resolution Both products are combined into a monthly Both products are combined into a monthly

averaged dataset between Mar. 2000 and Dec. averaged dataset between Mar. 2000 and Dec. 20052005 Only Terra results are shown, but data from Aqua Only Terra results are shown, but data from Aqua

produces similar resultsproduces similar results

RegionsRegions

Arabian Sea: (Mix of dust and anthropogenic aerosols)Arabian Sea: (Mix of dust and anthropogenic aerosols) Bay of Bengal: (Primarily anthropogenic)Bay of Bengal: (Primarily anthropogenic) Eastern Atlantic: (Dust aerosols)Eastern Atlantic: (Dust aerosols)

DustMix

AN

MethodologyMethodology Compare time-series and correlation statistics Compare time-series and correlation statistics

between AOT and cloud properties to between AOT and cloud properties to determine if expected relationships exist.determine if expected relationships exist. Anthropogenic and dust AOT separated using a Anthropogenic and dust AOT separated using a

modified version of the Kaufman method. modified version of the Kaufman method. (Large (Large

uncertainties, but does give qualitative feel for differences)uncertainties, but does give qualitative feel for differences)

Calculate aerosol direct and indirect effects at Calculate aerosol direct and indirect effects at top of atmosphere using a modified form of top of atmosphere using a modified form of method outlined by Quaas et al. (2008)method outlined by Quaas et al. (2008) We use a different diurnal averaging scheme and We use a different diurnal averaging scheme and

have modified the anthropogenic equations to have modified the anthropogenic equations to derive dust effects as wellderive dust effects as well

Only AIE values from first indirect effect are Only AIE values from first indirect effect are calculated calculated

Arabian SeaArabian Sea

AOT - Rc Correlation < 0.0

High COT Summer AIE

Arabian SeaArabian Sea Aerosol concentrations and types change Aerosol concentrations and types change

dramatically as a function of seasondramatically as a function of season High AOT (0.6) associated with dust in the summer, High AOT (0.6) associated with dust in the summer,

lower AOT (0.2) associated with anthropogenic lower AOT (0.2) associated with anthropogenic aerosols in the winter.aerosols in the winter.

Correlation between AOT and RCorrelation between AOT and Rcc is most is most negative during summer (-0.3)negative during summer (-0.3) Indicative of first AIEIndicative of first AIE Increase in COT at this time consistent with thisIncrease in COT at this time consistent with this Both dust and anthropogenic AIE have similar Both dust and anthropogenic AIE have similar

magnitudes (peak values of -1.5 Wmmagnitudes (peak values of -1.5 Wm-2-2)) Direct effect from anthropogenic aerosols Direct effect from anthropogenic aerosols

greatest in winter, when AIE is negligible.greatest in winter, when AIE is negligible.

Arabian Sea cont…Arabian Sea cont… However, atmospheric conditions also play an However, atmospheric conditions also play an

important roleimportant role Humidity and upward vertical motion maximized in Humidity and upward vertical motion maximized in

summer, creating conditions more favorable for summer, creating conditions more favorable for cloud formation.cloud formation.

Strong upward vertical motion appears to overcome Strong upward vertical motion appears to overcome any semi-direct effects due to dust absorption.any semi-direct effects due to dust absorption.

Indirect effects are Indirect effects are notnot observed in winter, observed in winter, when conditions are less favorable for cloud when conditions are less favorable for cloud formationformation Despite high anthropogenic AOT content and Despite high anthropogenic AOT content and

anthropogenic DREanthropogenic DRE

Bay of BengalBay of BengalAOT - Rc Correlation > 0.0

AIE ~ 0 Wm-2

Bay of BengalBay of Bengal Seasonal variations in AOT less than observed in Seasonal variations in AOT less than observed in

Arabian Sea, anthropogenic aerosol dominate aerosol Arabian Sea, anthropogenic aerosol dominate aerosol typetype Some influx of dust aerosols in summerSome influx of dust aerosols in summer

Cloud fraction, COT, and cloud height are all Cloud fraction, COT, and cloud height are all maximized in summer, but the correlation between Rmaximized in summer, but the correlation between Rcc and AOT is and AOT is notnot consistently negative during this time. consistently negative during this time. Of interest is that CTP in this region are ~30 hPa higher Of interest is that CTP in this region are ~30 hPa higher

than in the Arabian Seathan in the Arabian Sea Despite favorable conditions for cloud formation in Despite favorable conditions for cloud formation in

summer, only weak AIE was observed.summer, only weak AIE was observed. Based on some case study examples, it appears the Based on some case study examples, it appears the

discontinuity between aerosol and cloud layers may play a discontinuity between aerosol and cloud layers may play a part in this resultpart in this result

ConclusionsConclusions Aerosol indirect effects are more likely to be Aerosol indirect effects are more likely to be

observed when atmospheric conditions are observed when atmospheric conditions are more favorable for cloud development.more favorable for cloud development. No clouds, no AIE!No clouds, no AIE!

Relative vertical distributions of aerosol and Relative vertical distributions of aerosol and cloud layers is also very importantcloud layers is also very important If aerosols are not in the same layer as clouds, then If aerosols are not in the same layer as clouds, then

first AIE is not likely to occur.first AIE is not likely to occur. Important application for CALIPSO & CloudSat dataImportant application for CALIPSO & CloudSat data

Semi direct effects of absorbing aerosols can Semi direct effects of absorbing aerosols can offset those of the aerosols acting as CCNoffset those of the aerosols acting as CCN When dust AOT is very high such as in North When dust AOT is very high such as in North

AtlanticAtlantic

QuestionsQuestions

I know, at lot to digest in 12 I know, at lot to digest in 12 minutes…I’m writing a paper with minutes…I’m writing a paper with all the detailsall the details

Dr. Sundar ChristopherDr. Sundar Christopher Dr. Johannes Quaas Dr. Johannes Quaas LARC (CERES-SSF data)LARC (CERES-SSF data)

Acknowledgments:Acknowledgments: