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Retrieval of the Temperature and Humidity Profile of the Atmospheric Boundary Layer Using FTIR Spectroscopy Narayan Adhikari University of Nevada, Reno 23 April 2010 3/19/2018 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 3/19/2018 2

Vertical structure of atmosphere Abundance of gases in the troposphere: (fraction by volume in dry air) N 2: 78. 1%, O 2: 20. 9% thermosphere mesopause height (km) ------------------------------mesosphere stratopause ------------------------------stratosphere Ar & inert gases: 0. 936% Green house gases: H 2 O vapor: (0 -2)%, CO 2: 386 ppm, CH 4: 1. 7 ppm N 2 O: 0. 35 ppm, O 3: 10 ppb CFCs: 0. 1 ppb tropopause ------------------------------troposphere ------------------------------temperature (K) Distribution of gases: water vapor, cloud, aerosol: 0 -15 km atmospheric boundary layer: 50 m - 3 km N 2, O 2, Ar, CO 2: 0 -90 km O 3: 15 - 50 km (stratosphere) and surface Charged ions: Ionosphere (above 50 km) 3/19/2018 3

Black body emission* Planck’s function BB emission curves at terrestrial temperatures Wien’s displacement law Stefan-Boltzmann law BB emission curves of the Sun and Earth 90 80 Longwave (terrestrial radiation): 4 -100 m (thermal IR) T = 5780 K 70 radiative flux ( W m-2 m-1) Shortwave (solar radiation): 0. 1 – 4 m Sun ( scaled by a factor of 10 -6 ) 60 The earth emits radiation at longer wavelengths (i. e. lower energy) than the sun. (scaled by a factor of 10 -6). 50 40 30 Approx. 99% of the total solar output lies in shortwave region. Earth 20 T = 288 K Approx. 99% of the radiation emitted by the earth and its atmosphere lies in thermal infrared band. 10 0 0. 1 0. 2 0. 4 1 2 4 10 ) wavelength ( m 3/19/2018 20 50 100 *Adapted from Petty, W. Grant, second edition 5

Energy states of H 2 O and CO 2 H 2 O symmetric O-H bend symmetric O-H stretch asymmetric O-H stretch CO 2 (a) (b) (c) Symmetric mode (a) produces no dipole moment and no absorption of IR radiation by CO 2. Asymmetric modes (b) and (c) produce "dipole moment", and are responsible for IR radiation absorption by CO 2. 3/19/2018 7

Intermission !!! Quiz: What’s the difference ? ? ? (A) heat water (B) water heat Answer: (A): No convective mixing, stable water (B): Convective mixing, unstable water 3/19/2018 9

50 m - 3 km 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. 3/19/2018 Figure adapted from Stull, 1988 10

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. 3/19/2018 11

FTIR spectroscopy FTIR is the abbreviation of Fourier transform Infrared radiation. It consists of: (a) Michelson interferometer and (b) computer for Fourier transform. movable mirror X 2 path difference = x 1 - x 2 beam-splitter fixed mirror measured interferogram source X 1 interferogram computed spectrum detector note: = 1/ (cm-1) Fourier transform spectrum R( ) interferogram, ID 3/19/2018 12

Calibration of FTIR spectrometer Circulation water in hot BB window Circulation Water Out mirror Brass 5 cm Cone Paint Black 30 cm cold BB Thermistor probe FTIR spectrometer Assumed linear model for spectral response: V( ) = a( ) + b ( ) R( ) ▪ V( ): detector voltage ▪ R( ): target radiance ▪ R( ) = B( ) for perfect black body at temperature T ▪ a( ) and b( ) are calibration factors. b = (V 1 -V 2)/(B 1 -B 2) a = [ V 1(B 1 -B 2) - B 1(V 1 -V 2) ]/(B 1 -B 2) Finally the calibrated target radiance is given by R( ) = [ (B 1 - B 2) V + V 1 B 2 - V 2 B 1 ] / (V 1 - V 2) With the measurements of cold and hot black bodies, we obtain a and b as follows: 3/19/2018 13

Measurement of downwelling IR radiance with FTIR at UNR Cloudy sky, 01 Apr. , 2010 Clear sky, 06 Apr. , 2010 Strong IR absorption bands : < 650 cm-1 & : 1300 cm-1 2000 cm-1 H 2 O vapor : CO 2 : 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. O 3 absorption band: centered at 1042 cm-1 (9. 6 m ). This and H 2 O 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: 1 cm-1 = 0. 04 m and 1 m = 25 cm-1. 3/19/2018 14

contd… Brightness temperature (T b): For = 1, Tb physical temperature (T) For 1, Tb T. The temperatures at strong CO 2 and H 2 O 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. 3/19/2018 15

Retrieval methodology: overview Observed radiance Model radiance We minimize the difference: by adjusting the values of T(z) and RH(z) for Altitude (m) Retrieved temperature and humidity profile temperature (K) mixing ratio (g/kg)

Measurement of model radiance Radiant intensity at reaching the sensor at ground is: Thermal IR radiative transfer (non- scattering atmosphere) Ttop TOA 0 where Tm pm T 2 p 2 T 1 p 1 : Planck’s emission function (transmittance at ) K : absorption coefficient of an absorbing gas e. g. water Ts surface ps vapor ( obtain from HITRAN database) q(p): mixing ratio of water vapor Finally, we solve eqn. (1) using retrieval code with guess T(p) and q(p) to compute 3/19/2018 . 18

Retrieved temperature structure* 2000 FTIR measurement at Lamont, Oklahoma 12 Sept. 1996 1750 287 1500 289 1250 1000 291 750 293 500 295 250 297 299 0 Altitude (m) Temperature in Kelvin 2000 0 2 4 6 8 10 12 14 16 18 Weather balloon measurement at Lamont , Oklahoma 20 22 24 12 Sept. 1996 287 1750 1500 289 1250 A cold front passes through the site on that day. Some differences between the panels are caused by the 293 difference in frequencies of FTIR and weather balloon 295 297 soundings. 1000 291 750 500 250 0 Both cross sections show the rapid vertical temperature decrease of the atmosphere at around 0600 UTC from 0 to 1500 km. 0 2 4 6 8 10 12 14 16 18 20 22 23 299 Time (UTC) 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 3/19/2018 19

Conclusions • FTIR ABL profiles provide data for numerical forecast models. • Since the normal frequency of weather balloon launches is 12 h, 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. 3/19/2018 21

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. 3/19/2018 22

Appreciation Dr. W. Patrick Arnott Associate Professor Director, Undergraduate Atmospheric Sciences Program UNR Madhu Gyawali, Graduate Student, UNR Michael Weller Graduate Student, UNR 3/19/2018 23

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. 3/19/2018 24

Thank You! 3/19/2018 My Home Village and my High School 25