Retrieval of the Temperature and Humidity Profile of the Atmospheric Boundary Layer Using FTIR Spectroscopy Narayan Adhikari University of Nevada, Reno 23 April /8/20151

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 8/8/20152

Vertical structure of atmosphere height (km) temperature (K) tropopause stratopause mesopause troposphere stratosphere mesosphere thermosphere Distribution of gases:  water vapor, cloud, aerosol: 0-15 km  N 2, O 2, Ar, CO 2 : 0-90 km  O 3 : km (stratosphere) and surface  Charged ions: Ionosphere (above 50 km) atmospheric boundary layer: 50 m - 3 km 8/8/20153 Abundance of gases in the troposphere: (fraction by volume in dry air)  N 2 : 78.1%, O 2 : 20.9%  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: 0.35ppm, O 3 : 10 ppb  CFCs: 0.1 ppb

Global energy exchange between the Earth-atmosphere system and space ( 1- D flux model ) a sw a lw F0F0 F1F1 F2F2 F3F3 F4F4 F5F5 F7F7 atmosphere surface  A longwaveshortwave sun TsTs TaTa  For radiative equilibrium,  Locally, at night F 0 to F 3 are all zero, so net flux on the ground is: What would happen if there were no atmosphere? Ans: The Earth would be uninhabitable !!! space F6F6 Fig. adapted from Petty W.Grant second edition

Black body emission*  Shortwave (solar radiation): 0.1 – 4  m  Longwave (terrestrial radiation):  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. 8/8/ 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 ) radiative flux ( W m -2  m -1 ) ( scaled by a factor of ) BB emission curves of the Sun and Earth Sun T = 5780 K Earth T = 288 K (scaled by a factor of ).

Solar radiation spectrum from the TOA to the sea level* (cloud-free atmosphere) 8/8/20156  UV visibleinfrared sun light at the top of the atmosphere 5250  C blackbody spectrum radiation at the sea level Spectral irradiance (W m -2  m -1 ) Wavelength,  m Rayleigh scattering  The black solid line represents the perfect black body emission curve at 5250  C.  The yellow portion represents the solar radiation spectra at the top of the atmosphere, prior to atmospheric effect.  The red portion shows the radiation reaching the ground.  Solar irradiance is removed by Rayleigh scattering and absorption by gases as indicated.  Clouds strongly attenuate solar radiation. *Adapted from Liou, K.N., second edition.

Energy states of H 2 O and CO 2 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 CO 2.  Asymmetric modes (b) and (c) produce "dipole moment", and are responsible for IR radiation absorption by CO 2. H2OH2O CO 2 symmetric O-H bend 8/8/2015 7

Thermal IR radiation transfer in the atmosphere  s, 0  1, z 1  2, z 2  m-1, z m-1 T m-1, p m-1 T top, 0  m, z m where  e (  a +  s ) is the extinction coefficient, non negative  Optical thickness,  between levels z 1 and z 2 is:  The total optical path between the surface and the m th level of the atmosphere is given by  Transmittance of radiation from level z 1 to z 2 or vice versa is given by  The total transmittance between the surface and the m th level of the atmosphere is given by  = optical depth  s = total optical depth z = height T = temperature p = pressure 8/8/  Beer-Bougher-Lambert law of radiation extinction: where  = cos ,  is the zenith angle, 0   1 T s, p s 0, z top T 1, p 1 T 2, p 2 T m, p m TOA I0I0 I surface

Intermission !!! Quiz: What’s the difference ??? 8/8/20159 heat 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. 8/8/ Figure adapted from Stull, m - 3km

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. 8/8/

FTIR spectroscopy interferogram, I D Fourier transform spectrum R( ) FTIR is the abbreviation of Fourier transform Infrared radiation. It consists of: (a) Michelson interferometer and (b) computer for Fourier transform. 8/8/ path difference  = x 1 - x 2 measured interferogram computed spectrum source detector movable mirror beam-splitter fixed mirror X2X2 X1X1 interferogram note: = 1/ (cm -1 )

Calibration of FTIR spectrometer Brass Cone Black Paint Circulation water in Circulation Water Out 5 cm 30cm 8/8/  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.  With the measurements of cold and hot black bodies, we obtain a and b as follows: 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 ) FTIR spectrometer hot BB cold BB window mirror Thermistor probe

Measurement of downwelling IR radiance with FTIR at UNR 8/8/ Cloudy sky, 01 Apr., 2010 Clear sky, 06 Apr., 2010  Strong IR absorption bands :  H 2 O vapor : < 650 cm -1 & :1300 cm -1   2000 cm -1  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: 1cm -1 = 0.04  m and 1  m = 25 cm -1.

contd… 8/8/  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.  Brightness temperature (T b ):  For  = 1, T b  physical temperature (T)  For   1, T b  T.

Prerequisites for remote sensing techniques At what wavelengths is the cloud-free atmosphere appreciably transparent? At which wavelenth is the cloud-free atmosphere strongly absorbing, and which components are responsible for absorption and emission? How do the extinction (absorption and scattering) properties of clouds vary with wavelengths? 8/8/ Significant absorption bands of some gases:  H 2 O : 6.3  m, 2.7  m  CO 2 : 15  m, 4.3  m  O 3 : 0.28  m, 9.6  m,  m  CH 4 : 3.3  m, 7.6  m  N 2 O : 4.5  m, 7.8  m Atmosphere CO 2, H 2 O vapor, CH 4, N 2 O, O 3 etc.. emission FTIR IR radiation Ground remote sensor

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

Measurement of model radiance 8/8/  Radiant intensity at reaching the sensor at ground is: where : Planck’s emission function (transmittance at )  K : absorption coefficient of an absorbing gas e.g. water vapor ( obtain from HITRAN database)  q(p): mixing ratio of water vapor p2p2 p m surface 0 TsTs T1T1 T2T2 TmTm T top TOA p1p1 psps  Finally, we solve eqn. (1) using retrieval code with guess T(p) and q(p) to compute. 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 Altitude (m) Time (UTC) FTIR measurement at Lamont, Oklahoma 12 Sept Weather balloon measurement at Lamont, Oklahoma 12 Sept  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 8/8/201519

Retrieved water vapor mixing ratio* FTIR measurement at Lamont, Oklahoma 12 Sept Weather balloon measurement at Lamont, Oklahoma 12 Sept  Both panels show a rapid increase in absolute water vapor at 0600 UTC.  The upper panel clearly shows an elevated layer of moisture between 1600 and 2100 UTC at 1 km.  Weather balloons miss the air mass transition as they are not launched during the frontal passage. *Adapted from Smith L. William, 1999, JAOT Altitude (m) Time (UTC) mixing ratio in g/kg 8/8/201520

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. 8/8/201521

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. 8/8/201522

Appreciation 8/8/ Dr. W. Patrick Arnott Associate Professor Director, Undergraduate Atmospheric Sciences Program UNR Madhu Gyawali, Graduate Student, UNR Michael Weller Graduate 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. 8/8/201524

Thank You! My Home Village and my High School 8/8/201525