Transcript Emissivity e(l,T)

ATMOSPHERIC RADIATION
EMISSION 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:
Here DF is the radiation flux emitted in [l, l+Dl]
is the flux distribution function characteristic of the object

Total radiation flux emitted by object:
F   l d l
0
BLACKBODY RADIATION
•
•
Objects that absorb 100% of incoming radiation are called
blackbodies
For blackbodies, l is given by the Planck function:
Function of T
only! Often
denoted B(l,T)
F  sT 4
s  2p 5k 4/15c2h3 is the
Stefan-Boltzmann constant
lmax = hc/5kT
Wien’s law
lmax
KIRCHHOFF’S LAW:
Emissivity e(l,T) = Absorptivity
For any object:
Illustrative example:
Kirchhoff’s law allows
determination of the
emission spectrum of
any object solely from
knowledge of its
absorption spectrum
and temperature
…very useful!
SOLAR RADIATION SPECTRUM: blackbody at 5800 K
TERRESTRIAL RADIATION SPECTRUM FROM SPACE:
composite of blackbody radiation spectra for different T
Scene over
Niger valley,
N Africa
RADIATIVE EQUILIBRIUM FOR THE EARTH
Solar radiation flux intercepted by Earth = solar constant FS = 1370 W m-2
Radiative balance c
effective temperature of the Earth:
= 255 K
where A is the albedo (reflectivity) of the Earth
ABSORPTION OF RADIATION BY GAS MOLECULES
•
…requires quantum transition in internal energy of molecule.
•
THREE TYPES OF TRANSITION
– Electronic transition: UV radiation (<0.4 mm)
• Jump of electron from valence shell to higher-energy shell,
sometimes results in dissociation (example: O3+hn gO2+O)
– Vibrational transition: near-IR (0.7-20 mm)
• Increase in vibrational frequency of a given bond
requires change in dipole moment of molecule
– Rotational transition: far-IR (20-100 mm)
• Increase in angular momentum around rotation axis
Gases that absorb radiation near the spectral maximum of terrestrial
emission (10 mm) are called greenhouse gases; this requires
vibrational or vibrational-rotational transitions
NORMAL VIBRATIONAL MODES OF CO2
Δp  0
forbidden
Δp  0
allowed
Δp  0
IR spectrum
of CO2
asymmetric
stretch
bend
allowed
GREENHOUSE EFFECT:
absorption 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 EFFECT
IR
VISIBLE
Incoming
solar
FS / 4
FS / 4
Reflected
solar
FS A / 4
Energy balance equations:
• Earth system
FS (1  A) / 4  (1  f )s To4 + f s T14
Transmitted
surface
• Atmospheric layer
(1  f )s To4
f s To4  2 f s T14
Solution: 
f s T14
Atmospheric
emission
f s T14
Atmospheric
emission
1
4
 To=288 K
 F (1  A)  e f=0.77
To   S

f
 4(1  )s  T1 = 241 K
2


Atmospheric layer (T1)
abs. eff. 0 for solar (VIS)
f for terr. (near-IR)
Surface emission
FS A / 4
s To4
Earth surface (To)
Absorption efficiency 1-A in VISIBLE
1 in IR
RADIATIVE AND CONVECTIVE INFLUENCES
ON ATMOSPHERIC THERMAL STRUCTURE
In a purely radiative equilibrium atmosphere T decreases exponentially with z,
resulting in unstable conditions in the lower atmosphere; convection then
redistributes heat vertically following the adiabatic lapse rate
The ultimate models
for climate research
EQUILIBRIUM RADIATIVE BUDGET FOR THE EARTH
TERRESTRIAL RADIATION SPECTRUM FROM SPACE:
composite of blackbody radiation spectra emitted from different altitudes
at different temperatures
HOW DOES ADDITION OF A GREENHOUSE GAS WARM THE EARTH?
Example of a GG absorbing at 11 mm
1.
1. Initial state
2.
2. Add to atmosphere a GG
absorbing at 11 mm;
emission at 11 mm
decreases (we don’t see
the surface anymore at
that l, but the atmosphere)
3.
3. At new steady state, total
emission integrated over all l’s
must be conserved
e Emission at other l’s must
increase
e The Earth must heat!
EFFICIENCY OF GREENHOUSE GASES FOR GLOBAL WARMING
The efficient GGs are the ones that absorb in the “atmospheric window” (8-13
mm). Gases that absorb in the already-saturated regions of the spectrum are
not efficient GGs.
RADIATIVE FORCING OF CLIMATE CHANGE
Fin
Incoming
solar
radiation
Fout
Reflected
solar radiation
(surface, air,
aerosols,
clouds)
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 e
Radiative forcing DF = Fin – Fout
Increasing greenhouse gases g DF > 0 positive forcing
• The radiative forcing changes the heat content H of the Earth system:
DT
dH
 DF  o
dt
l
eventually leading to steady state
DTo  lDF
where To is the surface temperature and l is a climate sensitivity parameter
• Different climate models give l = 0.3-1.4 K m2 W-1, insensitive to nature of forcing;
differences between models reflect different treatments of feedbacks
CLIMATE CHANGE FORCINGS, FEEDBACKS, RESPONSE
Positive feedback from water vapor causes rough doubling of l
CLIMATE FEEDBACK FROM HIGH vs. LOW CLOUDS
Clouds reflect solar radiation (DA > 0) g cooling;
…but also absorb IR radiation (Df > 0) g warming
WHAT IS THE NET EFFECT?
sTcloud4 < sTo4
sTcloud4≈ sTo4
Tcloud≈ To
convection
sTo4
To
LOW CLOUD: COOLING
sTo4
HIGH CLOUD: WARMING
IPCC [2007]
ORIGIN OF THE ATMOSPHERIC AEROSOL
Aerosol: dispersed condensed matter suspended in a gas
Size range: 0.001 mm (molecular cluster) to 100 mm (small raindrop)
Soil dust
Sea salt
Environmental importance: health (respiration), visibility, radiative balance,
cloud formation, heterogeneous reactions, delivery of nutrients…
SCATTERING OF
RADIATION
BY AEROSOLS:
“DIRECT EFFECT”
Scattering efficiency is
maximum when
particle radius = l
eparticles in 0.1-1 mm
size range are efficient
scatterers of solar
radiation
By scattering
solar radiation,
aerosols
increase the
Earth’s albedo
2 (diffraction
limit)
EVIDENCE OF AEROSOL EFFECTS ON CLIMATE:
0
Temperature decrease following large volcanic eruptions
Observations
Temperature
Change (oC)
-0.6
-0.4
-0.2
+0.2
NASA/GISS general
circulation model
1991
1992
1993
Mt. Pinatubo eruption
1994
SCATTERING vs. ABSORBING AEROSOLS
Scattering sulfate and organic aerosol
over Massachusetts
Partly absorbing dust aerosol
downwind of Sahara
Absorbing aerosols (black carbon, dust) warm the climate by absorbing solar
radiation
AEROSOL “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
EVIDENCE OF INDIRECT EFFECT: SHIP TRACKS
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
 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
SATELLITE IMAGES OF SHIP TRACKS
NASA, 2002
Atlantic, France, Spain
AVHRR, 27. Sept. 1987, 22:45 GMT
US-west coast
OTHER EVIDENCE OF CLOUD FORCING:
CONTRAILS AND “AIRCRAFT CIRRUS”
Aircraft condensation trails (contrails) over France, photographed from the Space Shuttle (©NASA).