Clouds and Climate Forced Changes to Clouds SOEE

Clouds and Climate: Forced Changes to Clouds SOEE 3410 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

Content of Lecture 8 • Forcing mechanisms through aerosol-cloud interactions • Observational evidence for changes in clouds • Climate models and estimated radiative forcings ENVI 3410 : Coupled Ocean & Atmosphere Climate Dynamics 1

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/5/715/acp-5715. htm • The complex interaction of aerosols and clouds, H. Graf, Science, 303, 1309 -1311, 27 February 2004. ENVI 3410 : Coupled Ocean & Atmosphere Climate Dynamics 1

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 Cloud lifetime effect Albrecht effect 2 nd Indirect effect ENVI 3410 : Coupled Ocean & Atmosphere Climate Dynamics . Heating increases cloud burnoff Semi-direct effect 1

An Additional Forced Change • Not yet considered by IPCC Cumulonimbus Change in ice formation, latent heating See the Graf paper liquid ENVI 3410 : Coupled Ocean & Atmosphere Climate Dynamics 1

Cloud Drop Number and Aerosol • Composite of observations from many measurement sites ENVI 3410 : Coupled Ocean & Atmosphere Climate Dynamics 1

An Example of CDN-Aerosol Relationship CDN (cm-3) Observational data from Gultepe and Isaac (1999) • Why doesn’t CDN increase linearly with aerosol number? Aerosol Number (cm-3) ENVI 3410 : Coupled Ocean & Atmosphere Climate Dynamics 1

Aerosol • Calculate cloud drop number based on a fixed updraught speed CDN 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 Smax ENVI 3410 : Coupled Ocean & Atmosphere Climate Dynamics 1

Other Factors Affecting CDN • Updraught speed – Very difficult to quantify at global model spatial resolutions • Aerosol size distribution – Typically not simulated in a global model • Aerosol composition – Mass of main components (sulphate, organics, salt etc) ENVI 3410 : Coupled Ocean & Atmosphere Climate Dynamics 1

How aerosol size affects CDN • Model calculations ENVI 3410 : Coupled Ocean & Atmosphere Climate Dynamics 1

Droplet number vs. aerosol size and number • Fixed updraught speed log(N) Solid contours = CDN; colours = aerosol mass (mg m-3) ENVI 3410 : Coupled Ocean & Atmosphere Climate Dynamics Diameter 1

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) ENVI 3410 : Coupled Ocean & Atmosphere Climate Dynamics 1

Satellite Observations of 1 st Indirect Effect • Polder Satellite data • Cloud drop radius decreases with increasing aerosol number A rough measure of aerosol number concentration Bréon et al. , Science 2002 Quaas et al. , JGR 2004 ENVI 3410 : Coupled Ocean & Atmosphere Climate Dynamics 1

Cloud drop radius (mm) Oceanic vs. Continental Regions Ocean Aerosol Optical Depth Ocean cloud drop radius Land cloud drop radiuys Aerosol index • Ocean clouds are more susceptible to changes in aerosol than over land • Oceans also have lower albedo (larger change in reflectivity) ENVI 3410 : Coupled Ocean & Atmosphere Climate Dynamics 1

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 ENVI 3410 : Coupled Ocean & Atmosphere Climate Dynamics 1

Inferred Changes in Precipitation 5 3 2 1 Approx altitude (km) 4 • Collision and coalescence suppressed in polluted deep convective clouds • Refer to Lecture 2 polluted clouds From Ramanathan et al. , Science, 2001 clean clouds ENVI 3410 : Coupled Ocean & Atmosphere Climate Dynamics 1

The Semi-Direct Effect Cloud Fraction Koren et al. (2004): observational evidence for semi-direct effect based on MODIS satellite Smoke Optical Depth Columbia Shuttle image MEIDEX, January 12, 2003 ENVI 3410 : Coupled Ocean & Atmosphere Climate Dynamics 1

Treatment of CDN in Climate Models • Single fit equations describing CDN vs. model aerosol number Jones (1994) (Met Office Model) Global Gultepe and Isaac (2004) Continental Marine ENVI 3410 : Coupled Ocean & Atmosphere Climate Dynamics 1

Model Calculations of CDN 1860 emissions 2000 emissions ENVI 3410 : Coupled Ocean & Atmosphere Climate Dynamics 1

Model Calculations of Change in Surface SW Energy Budget • Due to aerosol direct effect and 1 st/2 nd indirect effects • Cloud effects significant ENVI 3410 : Coupled Ocean & Atmosphere Climate Dynamics 1

Global Mean Forcings From Intergovernmental Panel on Climate Change Scientific Assessment ENVI 3410 : Coupled Ocean & Atmosphere Climate Dynamics 1

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 – ENVI 3410 : Coupled Ocean & Atmosphere Climate Dynamics 1

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) – Aerosol is short-lived and transported/removed according to complex weather patterns ENVI 3410 : Coupled Ocean & Atmosphere Climate Dynamics 1

Questions for this lecture • Explain why cloud drop number concentration doesn’t increase linearly with aerosol number concentration • On slide 15, explain why the radiative properties of the clouds change in the yellow regions. Construct your argument based on constant LWC. • Why might LWC not be the same in the yellow and pink regions (clue in slide 16) • Why are marine clouds more “susceptible” to changes in cloud drops than continental clouds? • On slide 19, why is the map of changes in cloud drop number so patchy? Hence explain why the aerosol indirect effect is hard to predict in models. ENVI 3410 : Coupled Ocean & Atmosphere Climate Dynamics 1