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

ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics1 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

ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics1 Atmospheric Radiation Streams Blackbody emission spectrum E=  T 4 Sun’s shortwave energy arriving at Earth Earth’s emitted longwave (infrared) energy at the top of the atmosphere SW LW

ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics1 strong IR absorption (GH effect) weak IR Absorption in the 8-14  m “window” no visible light absorption weak near-IR absorption

ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics1 Global Energy Budget ~75% by clouds

ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics1 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?

ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics1 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 Intensity of direct beam progressively reduced inside cloud scattering absorption

ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics1 Microphysical Factors Affecting Scattering Scattering cross-section defined as Q sca = scattering efficiency (fraction of light scattered relative to shadow area) Q sca depends on –size of particle relative to wavelength of light –Index of refraction

ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics1 Scattering Efficiency, Q sca Q sca ~ 2 for cloud drops in visible light Therefore Q sca ~ variable for cloud drops in near IR Typical cloud droplets (visible light) Typical cloud droplets (terrestrial radiation) From Mie calculation

ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics1 Scattering Versus Drop Size for Constant Water Content Assume single droplet size, Q sca = constant 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

ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics1 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 liquid water path (g m -2 ) Reflectivity (%) cloud drop concentration (cm -3 ) Reflectivity (%) drop radius (  m) cloud thickness (m) LWC = 0.3 g m Absorptivity (%) Typical clouds

ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics1 Mean Liquid Water Path (LWP) (measured in g m -2 )

ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics1 Solar Radiation Intensity Through a Cloud Upward and downward SW radiation streams through a cloud above a surface of albedo = SW intensity (W m -2 ) Height (m) SS SS Note rather slow decrease: Clouds need to be fairly thick to have a high albedo

ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics1 Clouds and Longwave (LW) Radiation Measured infrared spectrum of Earth from above a cloudless Sahara Desert Wavelength /  m blackbody curves for different temperatures Clouds absorb in the atmospheric window

ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics1 Cloud Absorptivity of LW Radiation liquid water path (g m -2 ) Absorptivity Some scattering remains, but cloud becomes close to a perfect emitter/absorber above quite low LWP Clouds are very efficient absorbers of LW across the entire terrestrial spectrum

ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics1 Solar (SW) and Terrestrial (LW) Radiation Intensity Through a Cloud SW intensity (W m -2 ) Height (m) SS SS LW intensity (W m -2 ) Height (m) LL LL L  and L  in balance high LW downward flux below cloud strong LW cooling at cloud top

ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics1 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

ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics1 Consequences Clear sky OLR through “atmospheric window” Ci: Small effect on albedo, large effect on OLR Sc: Large effect on albedo, small effect on OLR Cb: Large effect on albedo, large effect on OLR Temp/K Altitude/km

ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics1 Albedo and Outgoing Longwave Radiation (OLR) From ERBE ERBE = Earth Radiation Budget Experiment satellite low ScDeep Cb Ocean surfacelow Sc Deep Cb cirrus

ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics1 Net Radiation Balance at the top of the atmosphere

ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics1 Cloud Forcing (net radiative effect of clouds) low Sc cool in summer and Warm in winter Deep Cb small net radiative effect (SW cooling, LW heating) Cirrus warming

ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics1 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 CO 2 doubling Changes in cloud type/cover/properties have potential to affect climate

ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics1 Reading/Further Investigation Read a description of ERBE Examine and understand further images