radiative properties of clouds

Radiative Properties of CloudsSOEE3410

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 Dynamics 1

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


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

SW LW


strong IRabsorption(GH effect)

weak IRAbsorption in the 8-14 mm “window”

no visible lightabsorption

weak near-IRabsorption


Global Energy Budget

~75% by clouds


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?


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 progressivelyreduced inside cloud

scattering

absorption


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

scaQr 2s


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

31

3

343

4

NLWCrrNLWC

33

4

32

2 ;3

4 rLWCN

NLWCr

scaQrN 2

131

or rN

N=drops/volume


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

10 100 10000

20

40

60

80

100

24

816

liquid water path (g m-2)

Ref

lect

ivity

(%)

10 100 10000

20

40

60

80

100

cloud drop concentration (cm-3)R

efle

ctiv

ity (%

)

50

100

500

1500

drop radius (mm)cloud thickness (m)

LWC = 0.3 g m-316

2

Absorptivity (%

)

Typical clouds

Typical clouds


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


Solar Radiation Intensity Through a Cloud

Upward and downward SW radiation streams through a cloud above a surface of albedo = 0

0 200 400 600 800 10000

200

400

600

800

1000

SW intensity (W m-2)

Hei

ght (

m)

SS

Note rather slow decrease:Clouds need to be fairly thick to have a high albedo


Clouds and Longwave (LW) Radiation

• Measured infrared spectrum of Earth from above a cloudless Sahara Desert

25 15 10 8 Wavelength / mm

blackbody curves for different temperatures

Clouds absorb in the atmospheric window


Cloud Absorptivity of LW Radiation

0 10 100 1000 0

0.2

0.4

0.6

0.8

1.0

liquid water path (g m-2)

Abs

orpt

ivity

Some scattering remains,but cloud becomesclose to a perfectemitter/absorber above quite low LWP

• Clouds are very efficient absorbers of LW across the entire terrestrial spectrum


Solar (SW) and Terrestrial (LW) Radiation Intensity Through a Cloud

0 200 400 600 800 10000

200

400

600

800

1000

SW intensity (W m-2)

Hei

ght (

m)

SS

280 300 320 340 360 3800

200

400

600

800

1000

LW intensity (W m-2)

Hei

ght (

m)

L L

L and L inbalance

high LW downward fluxbelow cloud

strong LW cooling at cloud top


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


Consequences

290

280

250

210Clear sky

OLR through “atmospheric

window”

4TEemit s

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

Tem

p/K

Alti

tude

/km

0

1

5

15


Albedo and Outgoing Longwave Radiation (OLR) From ERBE

ERBE = Earth Radiation Budget Experiment satellite

low Sc Deep Cb

Ocean surface low Sc

Deep Cb

cirrus


Cloud Forcing (net radiative effect of clouds)

low Sc cool in summer andWarm in winter

Deep Cb small net radiative effect(SW cooling, LW heating)

Cirrus warming


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


Reading/Further Investigation

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

http://cimss.ssec.wisc.edu/wxwise/homerbe.html


Questions for this lecture

1. What are the approximate wavelengths of solar shortwave and terrestrial longwave radiation streams?

2. Which radiation stream (SW or LW) is absorbed more in the atmosphere?

3. Which radiation stream (SW or LW) is absorbed more in a cloud?4. What is the main process that attenuates SW radiation as it passes

through a cloud?5. 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?

6. Why is the “atmospheric window” important for climate change?7. Explain the reason why thin cirrus clouds can warm the climate.

radiative properties of clouds

Documents

climate changeenvi3410

climate system

cloud response

solar radiation

solar sw radiation

refractionfor large

earths energy balance

net effect of clouds