Radiative Properties of Clouds

1. Radiative Properties of Clouds SOEE3410 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 This Lecture • Global radiation balance and the role of clouds • Radiation interaction with cloud particles • Shortwave radiation (Cloud albedo) • Longwave radiation (emissivity) • Net radiative effect of clouds You should understand the role of clouds in the climate system, the different behaviour of long and shortwave radiation, and the different radiative effects of different cloud types

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 LW SW

4. weak IR Absorption in the 8-14 mm “window” 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 • For large cloud drops Qsca 2

9. Scattering Versus Drop Size for Constant Water Content • If Liquid Water Content (LWC [kg m-3]) is constant, then • Total light scattered in a thin cloud depends on • Therefore, scattering efficiency of a cloud depends on • Therefore, doubling N (r decreases) increases albedo by 25% • doubling r (N decreases) decreases albedo by 50% • But thickness is also important N=drops/volume

10. Cloud Reflectivity in Solar Spectrum (Shortwave, SW) • Visible light absorption negligible • Some weak absorption in near-IR part of solar spectrum • Cloud drop size and number are important in global energy balance 100 100 drop radius (mm) cloud thickness (m) 80 80 1500 2 4 Typical clouds 60 60 500 Typical clouds 8 Reflectivity (%) Reflectivity (%) 16 Absorptivity (%) 40 40 100 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)

11. Mean Liquid Water Path (LWP)(measured in g m-2)

12. 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)

13. Clouds and Longwave (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

14. Cloud Absorptivity of LW 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 above quite low LWP 0.6 Absorptivity 0.4 0.2 0 0 10 100 1000 liquid water path (g m-2)

15. 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)

16. 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

17. Consequences Cb: Large effect on albedo, large effect on OLR Altitude/km Ci: Small effect on albedo, large effect on OLR Temp/K 210 Clear sky OLR through “atmospheric window” 15 5 250 280 1 290 0 Sc: Large effect on albedo, small effect on OLR

18. Albedo and Outgoing Longwave Radiation (OLR) From ERBE ERBE = Earth Radiation Budget Experiment satellite Deep Cb cirrus low Sc Deep Cb Ocean surface low Sc

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

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

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

22. Questions for this lecture • What are the approximate wavelengths of solar shortwave and terrestrial longwave radiation streams? • Which radiation stream (SW or LW) is absorbed more in the atmosphere? • Which radiation stream (SW or LW) is absorbed more in a cloud? • What is the main process that attenuates SW radiation as it passes through a cloud? • As you leave this lecture, what clouds can you see, what is their approximate droplet concentration, and what effect are they likely to be having on climate? • Why is the “atmospheric window” important for climate change? • Explain the reason why thin cirrus clouds can warm the climate.