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Radiative Properties of Clouds

Radiative Properties of Clouds. ENVI3410 : Lecture 9 Ken Carslaw. Lecture 3 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

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Radiative Properties of Clouds

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  1. Radiative Properties of Clouds ENVI3410 : Lecture 9 Ken Carslaw Lecture 3 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. Content of Lecture 9 • Global radiation balance and the role of clouds • Radiation interaction with cloud particles • Shortwave radiation (Cloud albedo) • Longwave radiation (emissivity) • Net effect of clouds

  3. Atmospheric Radiation Streams • Blackbody emission spectrum • E=sT4 Sun’s shortwave energy arriving at Earth Earth’s emitted longwave (infrared) energy at the top of the atmosphere

  4. weak IR absorption at 8-14 mm weak near-IR absorption strong IR absorption (GH effect) no visible light absorption

  5. Global Energy Budget ~75% by clouds

  6. Problems to Solve • How much solar SW radiation is reflected by a cloud and what physical properties of the cloud control the albedo? • Is any solar radiation absorbed by a cloud? • How much terrestrial (Earth) LW radiation is absorbed by clouds? • What is the net effect of clouds on Earth’s energy balance and future changes to that balance?

  7. Radiation Interaction With Clouds • Rays A-D are scattered (no loss of radiative energy) • Ray E is absorbed (converted to heat) • Scattering + absorption = extinction Scattered light gives cloud white appearance scattering absorption Intensity of direct beam progressively reduced inside cloud

  8. Microphysical Factors Affecting Scattering • Scattering cross-section defined as • Qsca = scattering efficiency (fraction of light scattered relative to shadow area) • Qsca depends on • size of particle relative to wavelength of light • Index of refraction (real component)

  9. Scattering Efficiency, Qsca Typical cloud droplets (terrestrial radiation) Typical cloud droplets (visible light) • Qsca ~ 2 for cloud drops in visible light • Qsca ~ variable for cloud drops in near IR From Mie calculation

  10. Scattering Versus Drop Size for Constant LWC • Assume single droplet size, Qsca = constant • If LWC is constant, then • Total light scattered in a thin cloud depends on • Therefore, scattering efficiency of a cloud depends on • Therefore, doubling N increases albedo by 25%, but thickness is also important

  11. Cloud Reflectivity in Solar Spectrum (SW) • Visible light absorption negligible • Some weak absorption in near-IR part of solar spectrum • Cloud drop size and number is important in global energy balance 100 100 cloud thickness (m) 80 80 1500 2 4 60 60 500 8 Reflectivity (%) Reflectivity (%) 16 Absorptivity (%) 40 40 drop radius (mm) 100 20 20 16 50 LWC = 0.3 g m-3 2 0 0 10 100 1000 10 100 1000 cloud drop concentration (cm-3) liquid water path (g m-2)

  12. Mean Liquid Water Path

  13. Solar Radiation Intensity Through a Cloud Upward and downward SW radiation streams through a cloud above a surface of albedo = 0 1000 S S 800 600 Height (m) Note rather slow decrease: Clouds need to be fairly thick to have a high albedo 400 200 0 0 200 400 600 800 1000 SW intensity (W m-2)

  14. Clouds and LW Radiation Wavelength / mm • Measured infrared spectrum of Earth from above a cloudless Sahara Desert 25 15 10 8 blackbody curves for different temperatures Clouds absorb in the atmospheric window

  15. Cloud Absorptivity of IR Radiation 1.0 • Clouds are very efficient absorbers of LW across the entire terrestrial spectrum 0.8 Some scattering remains, but cloud becomes close to a perfect emitter/absorber at low LWP 0.6 Absorptivity 0.4 0.2 0 0 10 100 1000 liquid water path (g m-2)

  16. Solar (SW) and Terrestrial (LW) Radiation Intensity Through a Cloud strong LW cooling at cloud top 1000 1000 S S L L 800 800 600 600 L and L in balance Height (m) Height (m) 400 400 high LW downward flux below cloud 200 200 0 0 0 200 400 600 800 1000 280 300 320 340 360 380 SW intensity (W m-2) LW intensity (W m-2)

  17. Consequences of Different SW and LW Behaviours • Clouds need to be relatively thick to have an albedo approaching 1.0 • Even relatively thin clouds are good absorbers of LW radiation • Thin cirrus clouds are effective LW absorbers but poor SW reflectors

  18. Consequences Cb: Large effect on albedo, large effect on OLR Ci: Small effect on albedo, large effect on OLR 210 Clear sky OLR through atmospheric window 250 280 290 Sc: Large effect on albedo, small effect on OLR

  19. Albedo and OLR From ERBE ERBE = Earth Radiation Budget Experiment satellite Deep Cb low Sc Deep Cb Ocean surface low Sc

  20. Net Radiation Balance

  21. Cloud Forcing Deep Cb small net radiative effect (SW cooling, LW heating) low Sc cool in summer and Warm in winter

  22. Net Effect of Clouds • SW cooling • LW heating • Not complete cancellation, and depends on cloud type and season • Next effect is global mean 15-20 Wm-2 cooling • About 4-5 times radiative effect of CO2 doubling • Changes in cloud type/cover/properties have potential to affect climate

  23. Reading/Further Investigation • Read a description of ERBE • Examine and understand further images • http://cimss.ssec.wisc.edu/wxwise/homerbe.html

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