1. Clouds and Climate: Forced Changes to Clouds SOEE3410 Ken Carslaw Lecture 4 of a series of 5 on clouds and climate Properties and distribution of clouds Cloud microphysics and precipitation Clouds and radiation Clouds and climate: forced changes to clouds Clouds and climate: cloud response to climate change

2. ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics1 Content of Lecture 10 Mechanisms Aerosol-cloud interaction Observational evidence for changes in clouds Climate models and estimated radiative forcings

3. ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics1 Reading Global indirect aerosol effects: a review, U. Lohmann, J. Feichter, Atmospheric Chemistry and Physics, 5, 715-737, 2005. Available online at http://www.copernicus.org/EGU/acp/acp/5/715/acp-5- 715.htmhttp://www.copernicus.org/EGU/acp/acp/5/715/acp-5- 715.htm The complex interaction of aerosols and clouds, H. Graf, Science, 303, 1309-1311, 27 February 2004.

4. ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics1 Changes to Clouds Forced by Aerosol............. unperturbed cloud Increased CDN (constant LWC) Albedo effect Twomey effect 1 st Indirect effect Drizzle suppression (increased LWC) Increased cloud height Increased cloud lifetime Heating increases cloud burn-off Cloud lifetime effect Albrecht effect 2 nd Indirect effect Semi-direct effect

5. ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics1 An Additional Forced Change Not yet considered by IPCC liquid Cumulonimbus Change in ice formation, latent heating

6. ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics1 Cloud Drop Number and Aerosol Composite of observations from many measurement sites

7. ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics1 An Example of CDN-Aerosol Relationship Aerosol Number (cm -3 ) CDN (cm -3 ) Observational data from Gultepe and Isaac (1999) Why doesn’t CDN increase linearly with aerosol number?

8. ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics1 Explanation for CDN-Aerosol Relationship Why doesn’t CDN increase linearly with aerosol number? Maximum supersaturation (Smax) in cloud is reduced by droplet growth Figures show global model calculations CDN Smax Aerosol

9. ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics1 Other Factors Affecting CDN Updraught speed –Very difficult to quantify at global model spatial resolutions –Also affects response to  aerosol Aerosol size distribution –Typically not simulated in a global model Aerosol composition –Until recently, just sulphate mass

10. ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics1 How aerosol size affects CDN Model calculations

11. ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics1 Droplet number vs. aerosol size and number Fixed updraught speed log(N) Diameter Solid contours = CDN; colours = aerosol mass (  g m -3 )

12. ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics1 Satellite Observations Polder satellite POLarization and Directionality of the Earth's Reflectances radiometer TOP: Aerosol index (measure of aerosol column number) BOTTOM: Cloud droplet radius Breon et al., (Science, 2002)

13. ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics1 Satellite Observations of 1 st Indirect Effect Polder Satellite data Cloud drop radius decreases with increasing aerosol number Bréon et al., Science 2002 Quaas et al., JGR 2004

14. ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics1 Oceanic vs. Continental Regions Ocean clouds are more susceptible to changes in aerosol than over land Oceans also have lower albedo (larger change in reflectivity) Land cloud drop radiuys Ocean cloud drop radius Ocean Aerosol Optical Depth Aerosol index Cloud drop radius (  m)

15. ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics1 Localised Effects Aerosol point sources in the Adelaide region of Australia Advanced Very High Resolution Radiometer (AVHRR) multi-wavelength satellite observations Green/yellow implies smaller/more numerous drops in polluted regions

16. ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics1 Inferred Changes in Precipitation Collision and coalescence suppressed in deep convective clouds Refer to Lecture 2 From Ramanathan et al., Science, 2001 polluted clouds clean clouds 1 2 3 4 5 Approx altitude (km)

17. ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics1 The Semi-Direct Effect Cloud Fraction Smoke Optical Depth Columbia Shuttle image MEIDEX, January 12, 2003 Koren et al. (2004): observational evidence for semi-direct effect based on MODIS satellite

18. ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics1 Treatment of CDN in Climate Models Jones (1994) (Met Office Model) Gultepe and Isaac (2004) Continental Marine Global Single fit equations describing CDN vs. model aerosol number

19. ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics1 Model Calculations of CDN 1860 emissions 2000 emissions

20. ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics1 Model Calculations of Change in Surface SW Energy Budget Due to aerosol direct effect and 1 st /2 nd indirect effects Cloud effects significant

21. ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics1 Global Mean Forcings From Intergovernmental Panel on Climate Change Scientific Assessment

22. ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics1 Uncertainties (1/2) Observations –Limited quantitative information from satellites Aerosol and cloud drop optical properties (no aerosol chemistry, no direct microphysics measurement) Cloud top only –Difficult to determine cause and effect What would clouds look like without increased aerosol? –Multiple changes No “control experiment” Increased aerosol loading is often associated with drier air 1 st indirect effect never observed without other changes –

23. ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics1 Uncertainties (2/2) Models –Aerosol schemes too simplistic Particle size/composition –Cloud physics incomplete Highly parameterized CDN-aerosol link too simplistic (improvement needs information that is unreliable in models; e.g., updraught speed) Rain formation –Sub-grid processes (multi-cell clouds)