Regional analysis of multi-year aerosol indirect effects

1. Special Symposium on Aerosol–Cloud–Climate Interactions: 4.4 Regional analysis of multi-year aerosol indirect effects Dr. Thomas Jones University of Alabama in Huntsville January 13, 2009 89th Annual AMS Conference, Phoenix, AZ

2. Aerosol Indirect Effects • Aerosol interact with atmospheric and cloud properties in many ways • First (Twomey 1977) • more aerosols increase # of CCN thereby reducing droplet size • Second (Albrecht 1989) • Reduced droplet size leads to delay in onset of C&C reducing precipitation efficiency and increasing cloud lifetime • Semi-direct (Ackerman et al. 2000) • Absorbing aerosols (dust or black carbon) can warm key atmospheric layers affecting stability and likelihood of cloud formation • Convective (Andreae et al. 2004) • Reduction in stratiform precipitation efficiency can lead to an increase in convective precipitation.

3. Importance of aerosol type & surrounding conditions • Certain types of aerosols affect cloud properties differently • Sulfates and sea salt make excellent CCN • Dust aerosols are less effective CCN and also have the offsetting effect of absorbing rather than reflecting solar radiation • Atmospheric conditions • If sufficient moisture and synoptic forcing are not sufficient for cloud formation, then aerosol indirect effects would not be observed • Thus, increasing AOT may not necessarily lead to larger values of AIE…

4. Regional analysis of AIE • Goals: • Calculate AIE for several regions around the globe, each containing unique aerosol and atmospheric conditions • Separate total AOT into (primarily) anthropogenic and dust components to analyze the relative indirect effects for both aerosol types • Apply to a multi-year data set to study the effects of changing aerosol and atmospheric conditions on AIE • Using case study examples, determine if aerosol and cloud layers are located at the appropriate levels to explain observed effects • For example, for the first AIE to be occurring, then aerosols and clouds should be in the same atmospheric layer

5. Data • Aerosol and cloud properties are obtained from the CERES-SSF product over several regions • Contains collection 4 AOT • Cloud parameters derived using the Minnis et al. (2003) algorithm • We only use liquid-water cloud information • Atmospheric conditions are obtained from the NCEP Re-analysis product • Temperature, moisture, and vertical velocity at a 2.5°x2.5° resolution • Both products are combined into a monthly averaged dataset between Mar. 2000 and Dec. 2005 • Only Terra results are shown, but data from Aqua produces similar results

6. Regions • Arabian Sea: (Mix of dust and anthropogenic aerosols) • Bay of Bengal: (Primarily anthropogenic) • Eastern Atlantic: (Dust aerosols) Mix Dust AN

7. Methodology • Compare time-series and correlation statistics between AOT and cloud properties to determine if expected relationships exist. • Anthropogenic and dust AOT separated using a modified version of the Kaufman method. (Large uncertainties, but does give qualitative feel for differences) • Calculate aerosol direct and indirect effects at top of atmosphere using a modified form of method outlined by Quaas et al. (2008) • We use a different diurnal averaging scheme and have modified the anthropogenic equations to derive dust effects as well • Only AIE values from first indirect effect are calculated

8. Arabian Sea AOT - Rc Correlation < 0.0 Summer AIE High COT

9. Arabian Sea • Aerosol concentrations and types change dramatically as a function of season • High AOT (0.6) associated with dust in the summer, lower AOT (0.2) associated with anthropogenic aerosols in the winter. • Correlation between AOT and Rc is most negative during summer (-0.3) • Indicative of first AIE • Increase in COT at this time consistent with this • Both dust and anthropogenic AIE have similar magnitudes (peak values of -1.5 Wm-2) • Direct effect from anthropogenic aerosols greatest in winter, when AIE is negligible.

10. Arabian Sea cont… • However, atmospheric conditions also play an important role • Humidity and upward vertical motion maximized in summer, creating conditions more favorable for cloud formation. • Strong upward vertical motion appears to overcome any semi-direct effects due to dust absorption. • Indirect effects are not observed in winter, when conditions are less favorable for cloud formation • Despite high anthropogenic AOT content and anthropogenic DRE

11. Bay of Bengal AOT - Rc Correlation > 0.0 AIE ~ 0 Wm-2

12. Bay of Bengal • Seasonal variations in AOT less than observed in Arabian Sea, anthropogenic aerosol dominate aerosol type • Some influx of dust aerosols in summer • Cloud fraction, COT, and cloud height are all maximized in summer, but the correlation between Rc and AOT is not consistently negative during this time. • Of interest is that CTP in this region are ~30 hPa higher than in the Arabian Sea • Despite favorable conditions for cloud formation in summer, only weak AIE was observed. • Based on some case study examples, it appears the discontinuity between aerosol and cloud layers may play a part in this result

13. Conclusions • Aerosol indirect effects are more likely to be observed when atmospheric conditions are more favorable for cloud development. • No clouds, no AIE! • Relative vertical distributions of aerosol and cloud layers is also very important • If aerosols are not in the same layer as clouds, then first AIE is not likely to occur. • Important application for CALIPSO & CloudSat data • Semi direct effects of absorbing aerosols can offset those of the aerosols acting as CCN • When dust AOT is very high such as in North Atlantic

14. Questions • I know, at lot to digest in 12 minutes…I’m writing a paper with all the details Acknowledgments: • Dr. Sundar Christopher • Dr. Johannes Quaas • LARC (CERES-SSF data)