key concepts : infrared transmission and emission by the atmosphere gases
DESCRIPTION
CH 8: ATMOSPHERIC EMISSION: PRACTICAL CONSEQUENCES OF THE SCHWARZSCHILD EQUATION FOR RADIATION TRANSFER WHEN SCATTERING IS NEGLIGIBLE. Key Concepts : Infrared transmission and emission by the atmosphere gases. Learn how to read meteorology in infrared spectra. - PowerPoint PPT PresentationTRANSCRIPT
Pat Arnott, ATMS 749, UNR, 2008
CH 8: ATMOSPHERIC EMISSION: PRACTICAL CONSEQUENCES OF THE SCHWARZSCHILD
EQUATION FOR RADIATION TRANSFER WHEN SCATTERING IS NEGLIGIBLE
Key Concepts:
• Infrared transmission and emission by the atmosphere gases.
• Learn how to read meteorology in infrared spectra.
• Learn about the basic concepts involved with retrieval of atmospheric temperature and humidity - weighting functions.
Pat Arnott, ATMS 749, UNR, 2008
Some Energy States of Water Molecules
http://www.lsbu.ac.uk/water/vibrat.html
http://en.wikipedia.org/wiki/Libration
Pat Arnott, ATMS 749, UNR, 2008
Atmospheric Transmission: Beer’s Law: I(x)=I0e(-abs x)
What are the main sources for each gas?
Which gases are infrared active and contribute to greenhouse warming?
Which gases significantly absorb solar radiation?
Nitrous oxide is emitted by bacteria in soils and oceans, and thus has been a part of Earth's atmosphere for eons. Agriculture is the main source of human-produced nitrous oxide: cultivating soil, the use of nitrogen fertilizers, and animal waste handling can all stimulate naturally occurring bacteria to produce more nitrous oxide. The livestock sector (primarily cows, chickens, and pigs) produces 65% of human-related nitrous oxide. [1] Industrial sources make up only about 20% of all anthropogenic sources, and include the production of nylon and nitric acid, and the burning of fossil fuel in internal combustion engines. Human activity is thought to account for somewhat less than 2 teragrams of nitrogen oxides per year, nature for over 15 teragrams.
Gas concentrations from ‘typical’ midlatitude summer atmosphere.
Pat Arnott, ATMS 749, UNR, 2008
Clouds at Visible and IR (e.g. 10 um) Wavelengths
Pat Arnott, ATMS 749, UNR, 2008
Optics of N identical (particles / volume)
Light beam area = A
dz
z
z+dz
Power removed in dz: = I(z) N A dz ext
Bouger-Beer“law”(direct beam only!)
Pat Arnott, ATMS 749, UNR, 2008
CH 8: ATMOSPHERIC EMISSION: PRACTICAL CONSEQUENCES OF THE SCHWARZSCHILD
EQUATION FOR RADIATION TRANSFER WHEN SCATTERING IS NEGLIGIBLE
What process subtracts radiation?What process adds radiation?
What equation is used to calculate optical depth for a gaseous atmosphere?
Pat Arnott, ATMS 749, UNR, 2008
FTIR Radiance: Atmospheric IR Window
13 microns 8 microns
Pat Arnott, ATMS 749, UNR, 2008
DEFINITION OF THE BRIGHTNESS TEMPERATURE
TB
Measured Radiance at wavenumber v =Theoretical Radiance of a Black Body at temperature TB
Pat Arnott, ATMS 749, UNR, 2008
FTIR Brightness Temperatures
Pat Arnott, ATMS 749, UNR, 2008
Atmosphere Emission
Measurements, Downwelling
Radiance
Notes:
1. Wavelength range for CO2, H20, O3, CH4.
2. Envelope blackbody curves.
3. Monster inversion in Barrow.
4. Water vapor makes the tropical window dirty.
Pat Arnott, ATMS 749, UNR, 2008
Ideal Weighting Function Wi: Where in the atmosphere the main contribution to the radiation at wavenumber i
comes from.
Pat Arnott, ATMS 749, UNR, 2008
Downwelling Intensity Emitted by the Atmosphere to the Detector (Radiance)
z dz
ftir
emissivity=absdz/cos
=cos
B[T(z)]
blackbody radiance,T = temperature.
emission transmission
weightingfunction
Pat Arnott, ATMS 749, UNR, 2008
Weighting Functions for Satellite Remote Sensing using the
strong CO2 absorption near 15.4 um. (from Wallace and Hobbs,
2nd edition)
Ii =B(Ts)exp−τ absAll Atmos
( ) (surface)
+ B[T(z)]exp(−τabs(z))0
∞
∫ abs(z)dz (atmos)
or
I i =B(Ts)exp−τ absAll Atmos
( ) (surface)
+ B[T(z)]0
∞
∫ Wi(z)dz (atmos)
TeB(Te)
Satellite with FTIR Looking Down
Pat Arnott, ATMS 749, UNR, 2008
Chapter 8 Homework:
1. Calculate and plot weighting functions as in the previous slide, but for the FTIR spectrometer at the ground looking up.(500 to 850 cm-1 region).
2. Explain in detail, using these weighting function, how we can diagnose the temperature inversion in the Barrow Alaska graph.
3. Bring questions to class related to how this is done.
4. Extra credit: Calculate and plot weighting functions for the stratospheric ozone emission spectral region in the atmospheric window region (spectral region between 1000 and 1100 cm-1.)
Pat Arnott, ATMS 749, UNR, 2008
Simple Theory for W(z) at the Ground
Where is theHUGEapproximation?Why?
Pat Arnott, ATMS 749, UNR, 2008
Simple Theory for W(z) at the Ground
Where is theHUGEapproximation?Why?
Pat Arnott, ATMS 749, UNR, 2008
http://www.spectralcalc.com/calc/spectralcalc.php
volume mixing ratio = 0.01 (CO2) = 0.1 (others)
Pat Arnott, ATMS 749, UNR, 2008
http://www.spectralcalc.com/calc/spectralcalc.php
volume mixing ratio = 0.01 (CO2)
Can save text file!
Pat Arnott, ATMS 749, UNR, 2008
Calculate the absorption cross section per molecule from the transmittance calculations and this theory.
Pat Arnott, ATMS 749, UNR, 2008
CO2 Spectrum: Line Strength and Broadening Effects
Pat Arnott, ATMS 749, UNR, 2008
CO2 Spectrum: Line Strength and Broadening Effects
Pat Arnott, ATMS 749, UNR, 2008
abs0, P=1013.25 mb, T=296 K. ONLY CO2!!! =0.5 cm-1.
380 ppm CO2
Pat Arnott, ATMS 749, UNR, 2008
http://www.spectralcalc.com/atmosphere_browser/atmosphere.php
Calculate N(z), thenNH20(z), Nco2(z), etc.
Calculate abs(z) depth.
Calculate W(z)
Pat Arnott, ATMS 749, UNR, 2008
RENO FTIR SPECTRA
Pat Arnott, ATMS 749, UNR, 2008
Weighting Functions for the FTIR at the Ground Looking Up
H=6 km
Pat Arnott, ATMS 749, UNR, 2008
Weighting Functions for the FTIR at the Satellite Looking Down
H=6 km
Pat Arnott, ATMS 749, UNR, 2008
Theoretical Absorption Cross Sections for the indicated gases, averaged to 1 cm -1 resolution for clarity.
Pat Arnott, ATMS 749, UNR, 2008
Theoretical Absorption Cross Sections for the indicated gases, averaged to 1 cm -1 resolution for clarity.
Pat Arnott, ATMS 749, UNR, 2008
RENO FTIR SPECTRA
Which day is more moist?
Which day is warmer near the surface?
Pat Arnott, ATMS 749, UNR, 2008
Coincident FTIR Measurements, Down
and Up.
Pat Arnott, ATMS 749, UNR, 2008
More Examples of FTIR Data from a Satellite
Pat Arnott, ATMS 749, UNR, 2008
Comments on Figure 8.3.
The very strong CO2 line at 15 microns typically gives the gas temperature closest to the FTIR spectrometer.
Pat Arnott, ATMS 749, UNR, 2008
Self Study Questions
Pat Arnott, ATMS 749, UNR, 2008
FTIR Data from the NASA ER2 with Responsible Gases labeled.
IR Window 8-13 microns. IR radiation from
the Earth’s surface escapes
to space (cooling the
Earth). Absorption by
O3 near 9 microns ‘dirties’
the window.
(From Liou, pg 120).
Pat Arnott, ATMS 749, UNR, 2008
Atmospheric Temperature Profile: US “Standard” Atmosphere.
From Liou
Cirrus cloud level.High cold clouds, visible optical depth range0.001 to 10, emits IR to surface in the IR window.
Pat Arnott, ATMS 749, UNR, 2008
Cirrus Clouds: Small Crystals at Top, -40 C to -60 C
nucleation
Growth and fall
Evaporation
Pat Arnott, ATMS 749, UNR, 2008
FTIR Data from the NASA ER2, Clear and Cloudy Sky. (From Liou’s book). The ice cloud with small ice crystals has emissivity << 1, so the
ground below is partially seen. Clouds reduce the IR making it to space in the atmospheric
window region.
IR Atmospheric window region
Pat Arnott, ATMS 749, UNR, 2008
Ice Refractive
Index
Red shows the atmospheric window region. The resonance in the window region is useful for remote sensing. The real part goes close to 1, making anomalous diffraction theory a fairly reasonable approach for cross sections.
Pat Arnott, ATMS 749, UNR, 2008
Skin Depth and Absorption Efficiency
Pat Arnott, ATMS 749, UNR, 2008
Cloud Emissivity in General and Zero Scattering Approximation.
CLOU
CLOUD
I0 Incident Irradiance
I0 tTransmittedIrradiance
L=
DirectBeam
Diffuse+
1 D ideas....
I0 rReflectedIrradiance
GENERAL CLOUD MODELTRANSMITTED≡I0 tTRANSMITTED=Direct+DiffuseDirectBeam=I0exp(−extL)
Diffuse=I0 t−exp(−extL)⎡
⎣
⎢⎢
⎤
⎦
⎥⎥
REFLECTED≡I0 r
Absorptivity(a)=Emissivity(e)e=1−t−r=a
ZeroScattering(gas)CloudModelt=exp(−absL)e=1−t
Pat Arnott, ATMS 749, UNR, 2008
Cirrus with Small Crystals IR Transmission Model
Message: Curve has basic shape of the IR spectrum for small cirrus, primarily a transmission problem of ground radiance through the cloud, with a small emission correction. ASSUMES ZERO SCATTERING.
Pat Arnott, ATMS 749, UNR, 2008
Cirrus with Small Crystals IR Emission Model
Message: Curve has basic shape of the IR spectrum for small cirrus, primarily a transmission problem of ground radiance through the cloud, with a small emission correction.
Te
B(Te)
Cirrus Cloud
Satellite with FTIR Looking Down
B(Te) tc B(Tc) (1-tc)
Pat Arnott, ATMS 749, UNR, 2008
Cirrus with Small Crystals IR Emission Model
Te=300 KTcirrus=213 KCrystal D= 10 umCrystal Conc=10,000 / LiCloud Thickness = 1 km
Te
B(Te)
Cirrus Cloud
Satellite with FTIR Looking Down
B(Te) tc B(Tc) (1-tc)
Pat Arnott, ATMS 749, UNR, 2008
IR Cooling Rates(from Liou)
Message:
Clouds are good absorbers and emitters of IR radiation. MLS is a moist midlatitude profile, SAW is a dry subarctic winter profile.
Cooling rate is from the vertical divergence of the net irradiance absorbed and emitted.
ρcp ∂T∂t=−dFnetdz