Retrieval of the Temperature and Humidity
Profile of the Atmospheric Boundary Layer
Using FTIR Spectroscopy
Narayan AdhikariUniversity of Nevada, Reno
23 April 2010
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Overview
• Basics of radiation transfer in the atmosphere• Atmospheric boundary layer and its evolution• FTIR spectroscopy• Measured IR emission spectra • Retrieval of atmospheric boundary layer profile• Conclusions• Future work
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Vertical structure of atmospherehe
ight
(k
m)
temperature (K)
-------------------------------------------------------------
------------------------------------------------------------- -------------------------------------------------------------
tropopause
stratopause
mesopause -------------------------------------------------------------
troposphere
stratosphere
mesosphere
thermosphere
Distribution of gases:
water vapor, cloud, aerosol: 0-15 km
N2, O2, Ar, CO2: 0-90 km
O3: 15- 50 km (stratosphere) and surface
Charged ions: Ionosphere (above 50 km)
atmospheric boundary layer: 50 m - 3 km
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Abundance of gases in the troposphere: (fraction by volume in dry air)
N2: 78.1%, O2: 20.9%
Ar & inert gases: 0.936%
Green house gases:
H2O vapor: (0-2)%,
CO2: 386 ppm, CH4: 1.7 ppm
N2O: 0.35ppm, O3: 10 ppb
CFCs: 0.1 ppb
Black body emission*
Shortwave (solar radiation): 0.1 – 4 m
Longwave (terrestrial radiation): 4 -100 m (thermal IR)
The earth emits radiation at longer wavelengths (i.e. lower energy) than the sun.
Approx. 99% of the total solar output lies in shortwave region.
Approx. 99% of the radiation emitted by the earth and its atmosphere lies in thermal infrared band.
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,]1)(exp[
2)(
5
2
−=
Tkch
chTB
Bλλ
λ
,.max constT =λ,4TF σ=
Planck’s function
Wien’s displacement law
Stefan-Boltzmann law
*Adapted from Petty, W. Grant, second edition
BB emission curves at terrestrial temperatures
wavelength (m)
rad
iativ
e
flux
( W
m-2
m-1
)
( scaled by a factor of 10-6 )
BB emission curves of the Sun and Earth
0.1 0.2 0.4 1 2 4 10 20 50 100
30
10
20
40
50
0
60
80
70
90Sun
T = 5780 K
Earth
T = 288 K
(scaled by a factor of 10-6).
Energy states of H2O and CO2
symmetric O-H stretch asymmetric O-H stretch
(a) (b) (c)
Symmetric mode (a) produces no dipole moment and no absorption of IR radiation by CO2 .
Asymmetric modes (b) and (c) produce "dipole moment", and are responsible for IR radiation absorption by CO2.
H2O
CO2
symmetric O-H bend
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...,2,1,0,8
)1(
2
12
22 =
+== l
Ihll
IErot πω
...,2,1,0,2
1=⎟
⎠
⎞⎜⎝
⎛ += νν fhEvib
Intermission !!!
Quiz: What’s the difference ???
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heat
heat
water water
(A) (B)
Answer:
(A): No convective mixing, stable water
(B): Convective mixing, unstable water
Atmospheric boundary layer and its evolution
During daytime, solar heating of the earth surface persistent turbulence and convective mixing of the air well mixed layer in the atmosphere up to few kilometers altitude of the troposphere.
The mixing height or the thickness of ABL depends on the nature of the surface, amount of heat energy and humidity of a place.
At night, the ground cools off thermals and turbulence cease mixed layer changes into residual layer a stable boundary layer of cool air is formed near the ground.
Surface layer the lowest part of ABL and actual region of mixing.
04/20/23 10 Figure adapted from Stull,1988
50 m
- 3
km
Why do we care about the profile of ABL?
• ABL is the area of the atmosphere in which we live, and all of our activities take place there.
• It is the region where heat, momentum, water vapor, and other trace substances are exchanged with the Earth’s surface.
• It is where nearly all of our weather is produced.
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FTIR spectroscopy
interferogram, ID
Fouriertransform
spectrum R(ν)
FTIR is the abbreviation of Fourier transform Infrared radiation. It consists of: (a) Michelson interferometer and (b) computer for Fourier transform.
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( ) dvvRID ∫∞
Δ=Δ0
)2(cos)( πν
path difference = x1 - x2
( ) ∫∞
=0
)2(cos dvIR D πν
measured interferogram
computed spectrum
source
detector
movable mirror
beam-splitter fixed mirrorX2
X1
interferogram
note: ν = 1/λ (cm-1)
Calibration of FTIR spectrometer
Brass Cone
Black Paint
Circulation water in Circulation Water Out
5 cm
30cm
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Assumed linear model for spectral response:
V(ν) = a(ν) + b (ν) R(ν)
▪ V(ν): detector voltage ▪ R(ν): target radiance ▪ R(ν) = B(ν) for perfect black body at temperature T ▪ a(ν) and b(ν) are calibration factors.
With the measurements of cold and hot black bodies, we obtain a and b as follows:
b = (V1-V2)/(B1-B2) a = [ V1(B1-B2) - B1(V1-V2) ]/(B1-B2)
Finally the calibrated target radiance is given by
R(ν) = [ (B1 - B2) V + V1B2 - V2B1 ] / (V1 - V2)
FTIR spectrometer
hot BB
cold BB
window
mirror
Thermistor probe
Measurement of downwelling IR radiance with FTIR at UNR
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Cloudy sky, 01 Apr., 2010 Clear sky, 06 Apr., 2010
Strong IR absorption bands :
H2O vapor : ν < 650 cm-1 &
:1300 cm-1 ν 2000 cm-1
CO2 : near 667 cm-1 ( or 15 m)
The atmosphere seems to be opaque at these spectral
regions.
Atmospheric “dirty’ window region for IR radiation 800 – 1300 cm-1
The atmosphere is more transparent at this region and
FTIR records emission from the higher atmosphere.
O3 absorption band: centered at 1042 cm-1 (9.6 m ). This and H2O vapor absorption lines make the window region dirty.
April 06 shows less radiance than April 01. Significant
difference is observed at the window region. Note: 1cm-1 = 0.04 m and 1m = 25 cm-1.
contd…
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The temperatures at strong CO2 and H2O absorption spectral regions refer to that of lowest levels of the atmosphere (285 K ).
April 01 is slightly warmer than April 06.
The funny ‘cold’ spike at the center of the ozone absorption band corresponds to an unique region of relative transparency.
⎟⎟⎠
⎞⎜⎜⎝
⎛+
=
ν
ννν
Rch
K
chTb 322
1ln
)(
Brightness temperature (Tb):
For = 1, Tb physical temperature (T)
For 1, Tb T.
Retrieval methodology: overview
Observed radiance mRννR
We minimize the difference: by adjusting the values of T(z) and RH(z) for
mRR νν −
temperature (K) mixing ratio (g/kg)
Alti
tud
e (
m)
Model radiance
mRν
Retrieved temperature and humidity profile
Measurement of model radiance
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Radiant intensity at ν reaching the sensor at ground is:
where
: Planck’s emission function
(transmittance at ν)
Kν: absorption coefficient of an absorbing gas e.g. water vapor ( obtain from HITRAN database)
q(p): mixing ratio of water vapor
p2
pm
surface
0
mRνTs
T1
T2
Tm
TtopTOA
p1
ps
Finally, we solve eqn. (1) using retrieval code with guess T(p) and q(p) to compute . mRν
( )[ ] ( )1),(0
dpdp
ppdtpTBR
sp
sv
m ∫−= νν
( )[ ]⎟⎟⎠
⎞⎜⎜⎝
⎛−
=1
2 32
KT
hc
e
hcpTB
νν
ν
])()(/1exp[)],([exp),( ∫=−=p
pss
s
dppqpkgppppt νν τ
Thermal IR radiative transfer (non- scattering atmosphere)
Retrieved temperature structure*
Comparison of an FTIR boundary layer temperature retrievals to an interpolated weather balloon temperature-time cross section (weather balloon launches are indicated by the long dashed lines).
*Adapted from Smith L. William, 1999, JAOT
Alti
tude
(m
)
1750
1500
1250
1000
750
2000
1750
1500
1250
1000
750
500
250
0
500
250
0
287
289
291
293
295
297299
287
291
289
293
295297
299
Time (UTC)
2 4 6 8 10 12 14 16 18 20 22 24
FTIR measurement at Lamont, Oklahoma 12 Sept. 1996
0
0 4 6 8 10 12 14 16 18 20 22 232
2000
Weather balloon measurement at Lamont , Oklahoma 12 Sept. 1996
Both cross sections show the rapid vertical temperature decrease of the atmosphere at around 0600 UTC from 0 to 1500 km.
A cold front passes through the site on that day.
Some differences between the panels are caused by the difference in frequencies of FTIR and weather balloon soundings.
Temperature in Kelvin
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Conclusions
• FTIR ABL profiles provide data for numerical forecast models.
• Since the normal frequency of weather balloon launches is 12h, the FTIR provides much better temporal resolution of the ABL features than the weather balloon does.
• FTIR measurements allow for retrieval of the temperature and water vapor vertical profiles during rapid air mass transitions.
• FTIR sounding radiances reinforcing with satellite sounding radiances can yield entire tropospheric vertical profiles of temperature and water vapor.
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Future work
• Use of FTIR measurements in our own retrieval code to obtain the temperature and humidity structure of the atmospheric boundary layer (ABL).
• With FTIR measurement, we can frequently update the primary meteorological parameters of Reno which will be helpful to:
- monitor the air quality by estimating potential air pollution dilution in Reno.
- predict daily weather of Reno.
- study the diurnal and seasonal variation of air quality in Reno.
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Appreciation
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Dr. W. Patrick ArnottAssociate Professor
Director, Undergraduate Atmospheric Sciences Program UNR
Madhu Gyawali,Graduate Student, UNR
Michael WellerGraduate Student, UNR
References
• Smith, W.L., W.F. Feltz, R.O. Knuteson, H.E. Revercomb, H.B. Howell, and H.M. Woolf, 1998: The retrieval of planetary boundary layer structure using ground-based infrared spectral radiance measurements. J.Atmos. Oceanic Technol., 16
• W.F. Feltz, W.L. Smith, R.O. Knuteson, H.E. Revercomb, H.M. Woolf, and H.B. Howell, 1995: Meteorological applications of the Atmospheric Emitted Radiance Interferometer(AERI). J. APP., Meteor., 37
• Smith, W.L., 1970: Iterative solution of the radiative transfer equation for the temperature and absorbing gas profile of an atmosphere. App. Opt., 9, 9.
• W. F. Feltz, W. l. Smith, R.O. Knuteson, and B. Howell, 1996: AERI temperature and water vapor retrievals: Improvements using an integrated profile retrieval approach. Session Papers.
• Liou K.N., 2002: An Introduction to atmospheric Radiation Second Edition. Academic press.
• Wallace J.M., Hobbs P.V.,: Atmospheric Science An Introductory survey second edition. Academic Press.
• Han Y., J. A. Shaw, J. H. Churnside, P.D. Brown and S.A. Clough,1997: Infrared spectral radiance measurements in the tropical Pacific atmosphere.
• Petty W. Grant: A first course in Atmospheric Radiation Second Edition. Sundog Publishing.
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Thank You!
My Home Village and my High School04/20/23 25