ATMOSPHERIC RADIATIONATMOSPHERIC RADIATION

Here is the radiation flux emitted in [

is the flux distribution function characteristic of the object

Total radiation flux emitted by object:

EMISSION OF RADIATIONEMISSION OF RADIATION

• Radiation is energy transmitted by electromagnetic waves; all objects emit radiation

• One can measure the radiation flux spectrum emitted by a unit surface area of object:

0

d

BLACKBODY RADIATIONBLACKBODY RADIATION

• Objects that absorb 100% of incoming radiation are called blackbodies

• For blackbodies, is given by the Planck function:

k 4/15c2h3 is theStefan-Boltzmann constant

max = hc/5kT Wien’s law

Function of Tonly! Often denoted B(T)

max

KIRCHHOFF’S LAW: KIRCHHOFF’S LAW: Emissivity Emissivity TT) = Absorptivity) = Absorptivity

For any object: …very useful!

Illustrative example:

Kirchhoff’s law allowsdetermination of the emission spectrum of any object solely from knowledge of its absorption spectrum and temperature

SOLAR RADIATION SPECTRUM: blackbody at 5800 KSOLAR RADIATION SPECTRUM: blackbody at 5800 K

TERRESTRIAL RADIATION SPECTRUM FROM SPACE:TERRESTRIAL RADIATION SPECTRUM FROM SPACE:composite of blackbody radiation spectra for different composite of blackbody radiation spectra for different TT

Scene overNiger valley,N Africa

RADIATIVE EQUILIBRIUM FOR THE EARTHRADIATIVE EQUILIBRIUM FOR THE EARTH

Solar radiation flux intercepted by Earth = solar constant FS = 1370 W m-2

Radiative balance effective temperature of the Earth:

= 255 K

where A is the albedo (reflectivity) of the Earth

ABSORPTION OF RADIATION BY GAS MOLECULESABSORPTION OF RADIATION BY GAS MOLECULES

• …requires quantum transition in internal energy of molecule.

• THREE TYPES OF TRANSITION

– Electronic transition: UV radiation (<0.4 m)

• Jump of electron from valence shell to higher-energy shell, sometimes results in dissociation (example: O3+hO2+O)

– Vibrational transition: near-IR (0.7-20 m)

• Increase in vibrational frequency of a given bond

requires change in dipole moment of molecule

– Rotational transition: far-IR (20-100 m)

• Increase in angular momentum around rotation axis

Gases that absorb radiation near the spectral maximum of terrestrial emission (10 m) are called greenhouse gases; this requires vibrational or vibrational-rotational transitions

NORMAL VIBRATIONAL MODES OF CONORMAL VIBRATIONAL MODES OF CO22

forbidden

allowed

allowed

Δp 0

Δp 0

Δp 0

IR spectrumof CO2

bend

asymmetricstretch

GREENHOUSE EFFECT:GREENHOUSE EFFECT:absorption of terrestrial radiation by the atmosphereabsorption of terrestrial radiation by the atmosphere

• Major greenhouse gases: H2O, CO2, CH4, O3, N2O, CFCs,…

• Not greenhouse gases: N2, O2, Ar, …

SIMPLE MODEL OF GREENHOUSE EFFECTSIMPLE MODEL OF GREENHOUSE EFFECT

Earth surface (To) Absorption efficiency 1-A in VISIBLE 1 in IR

Atmospheric layer (T1)abs. eff. 0 for solar (VIS) f for terr. (near-IR)

/ 4SF

Incoming solar

/ 4SF

Reflectedsolar

/ 4SF A

/ 4SF A4oT

Surface emission

4(1 ) of T

Transmittedsurface

41f T41f T

Atmosphericemission

Atmosphericemission

Energy balance equations:• Earth system

4 41(1 ) / 4 (1 )S oF A f T f T

• Atmospheric layer4 4

12of T f T

Solution:1

4

(1 )

4(1 )2

So

F AT

f

To=288 K f=0.77T1 = 241 K

VISIBLE IR

RADIATIVE AND CONVECTIVE INFLUENCESRADIATIVE AND CONVECTIVE INFLUENCESON ATMOSPHERIC THERMAL STRUCTUREON ATMOSPHERIC THERMAL STRUCTURE

In a purely radiative equilibrium atmosphere T decreases exponentially with z, resulting in unstable conditions in the lower atmosphere; convection thenredistributes heat vertically following the adiabatic lapse rate

The ultimate models for climate research

EQUILIBRIUM RADIATIVE BUDGET FOR THE EARTHEQUILIBRIUM RADIATIVE BUDGET FOR THE EARTH

TERRESTRIAL RADIATION SPECTRUM FROM SPACE:TERRESTRIAL RADIATION SPECTRUM FROM SPACE:composite of blackbody radiation spectra emitted from different altitudes composite of blackbody radiation spectra emitted from different altitudes

at different temperaturesat different temperatures

HOW DOES ADDITION OF A GREENHOUSE GAS WARM THE EARTH?HOW DOES ADDITION OF A GREENHOUSE GAS WARM THE EARTH?

1.1. Initial state

2. 2. Add to atmosphere a GG absorbing at 11 m; emission at 11 m decreases (we don’t see the surface anymore at that but the atmosphere)

3. At new steady state, total emission integrated over all ’s must be conserved Emission at other ’s must increase The Earth must heat!

3.

Example of a GG absorbing at 11 m

EFFICIENCY OF GREENHOUSE GASES FOR GLOBAL WARMINGEFFICIENCY OF GREENHOUSE GASES FOR GLOBAL WARMING

The efficient GGs are the ones that absorb in the “atmospheric window” (8-13 m). Gases that absorb in the already-saturated regions of the spectrum are not efficient GGs.

RADIATIVE FORCING OF CLIMATE CHANGERADIATIVE FORCING OF CLIMATE CHANGE

Incomingsolar

radiation

Reflected solar radiation (surface, air, aerosols, clouds)

Fout

Fin

IR terrestrial radiation ~ T4; absorbed/reemitted by greenhouse gases, clouds, absorbing aerosols

EARTH SURFACE

• Stable climate is defined by radiative equilibrium: Fin = Fout

• Instantaneous perturbation Radiative forcing F = Fin – Fout

• Different climate models give = 0.3-1.4 K m2 W-1, insensitive to nature of forcing; differences between models reflect different treatments of feedbacks

Increasing greenhouse gases F > 0 positive forcing

• The radiative forcing changes the heat content H of the Earth system:

oTdHF

dt

where To is the surface temperature and is a climate sensitivity parameter

eventually leading to steady state oT F

CLIMATE CHANGE FORCINGS, FEEDBACKS, RESPONSECLIMATE CHANGE FORCINGS, FEEDBACKS, RESPONSE

Positive feedback from water vapor causes rough doubling of

CLIMATE FEEDBACK FROM HIGH vs. LOW CLOUDSCLIMATE FEEDBACK FROM HIGH vs. LOW CLOUDS

convection

To

Tcloud≈ To

Clouds reflect solar radiation (A > 0) cooling;…but also absorb IR radiation (f > 0) warming

WHAT IS THE NET EFFECT?

To4

Tcloud4≈ To

4

LOW CLOUD: COOLING

Tcloud4 < To

4

To4

HIGH CLOUD: WARMING

IPCC [2007]

ORIGIN OF THE ATMOSPHERIC AEROSOLORIGIN OF THE ATMOSPHERIC AEROSOL

Soil dustSea salt

Aerosol: dispersed condensed matter suspended in a gasSize range: 0.001 m (molecular cluster) to 100 m (small raindrop)

Environmental importance: health (respiration), visibility, radiative balance,cloud formation, heterogeneous reactions, delivery of nutrients…

SCATTERING OF SCATTERING OF RADIATION RADIATION BY AEROSOLS:BY AEROSOLS:“DIRECT EFFECT”“DIRECT EFFECT”

By scattering solar radiation, aerosols increase the Earth’s albedo

Scattering efficiency is maximum when particle radius = particles in 0.1-1 msize range are efficient scatterers of solar radiation

2 (diffraction limit)

Mt. Pinatubo eruption

1991 1992 1993 1994

-0.6

-0

.4

-0.

2

0

+0

.2Te

mp

era

ture

C

ha

nge

(oC

)

Observations

NASA/GISS general

circulation model

Temperature decrease following large volcanic eruptions

EVIDENCE OF AEROSOL EFFECTS ON CLIMATE:EVIDENCE OF AEROSOL EFFECTS ON CLIMATE:

SCATTERING vs. ABSORBING AEROSOLSSCATTERING vs. ABSORBING AEROSOLS

Scattering sulfate and organic aerosolover Massachusetts

Partly absorbing dust aerosoldownwind of Sahara

Absorbing aerosols (black carbon, dust) warm the climate by absorbing solarradiation

AEROSOL “INDIRECT EFFECT” FROM CLOUD CHANGESAEROSOL “INDIRECT EFFECT” FROM CLOUD CHANGES

Clouds form by condensation on preexisting aerosol particles (“cloud condensation nuclei”)when RH>100%

clean cloud (few particles):large cloud droplets• low albedo• efficient precipitation

polluted cloud (many particles):small cloud droplets• high albedo• suppressed precipitation

Particles emitted by ships increase concentration of cloud condensation nuclei (CCN) Increased CCN increase concentration of cloud droplets and reduce their avg. size

Increased concentration and smaller particles reduce production of drizzle Liquid water content increases because loss of drizzle particles is suppressed

Clouds are optically thicker and brighter along ship track

N ~ 100 cm-3

W ~ 0.75 g m-3

re ~ 10.5 µm

N ~ 40 cm-3

W ~ 0.30 g m-3

re ~ 11.2 µm

from D. Rosenfeld

EVIDENCE OF INDIRECT EFFECT: SHIP TRACKSEVIDENCE OF INDIRECT EFFECT: SHIP TRACKS

AVHRR, 27. Sept. 1987, 22:45 GMTUS-west coast

NASA, 2002Atlantic, France, Spain

SATELLITE IMAGES OF SHIP TRACKSSATELLITE IMAGES OF SHIP TRACKS

Aircraft condensation trails (contrails) over France, photographed from the Space Shuttle (©NASA).

OTHER EVIDENCE OF CLOUD FORCING:OTHER EVIDENCE OF CLOUD FORCING:CONTRAILS AND “AIRCRAFT CIRRUS”CONTRAILS AND “AIRCRAFT CIRRUS”