Retrieval of the Temperature and Humidity

Profile of the Atmospheric Boundary Layer

Using FTIR Spectroscopy

Narayan AdhikariUniversity of Nevada, Reno

23 April 2010

04/20/23 1

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.

04/20/23 5

,]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

04/20/23 7

...,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 ???

04/20/23 9

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.

04/20/23 12

( ) 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

04/20/23 23

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