Computer Modeling Laboratory 3

Written report due: 6 February

Spectral absorption by atmospheric gases

REQUIRED MATERIALS:
Lecture 4

ADDITIONAL MATERIALS:
HITRAN database: http://www.hitran.com
AIRS: http://airs.jpl.nasa.gov

 

TASK

High spectral resolution remote sensing has a wide range of applications in studying Earth and planetary atmospheres and surfaces. Examples of satellite high spectral resolution IR sensors include the Atmospheric Infrared Sounder (AIRS) and the Infrared Atmospheric Sounding Interferometer (IASI). AIRS has 2378 spectral bands in the IR spectral range from 650 to 2700 cm-1. The IASI sensor covers the spectral range from 645 to 2760 cm-1 at a constant spectral sampling interval of 0.25 cm-1. High spectral resolution remote sensing data are used, for instance, to determine the atmospheric profile of temperature, water vapor and trace gases. An accurate radiative transfer modeling with high spectral resolution (often called line-by-line, LBL) is a required, crucial component for product retrievals from these remote measurements. The LBL-type of models are also an important component of developing and testing new instruments. LBL models need to include millions of absorption lines to accurately represent absorption/emission by atmospheric gases for a broad range of temperature and pressure. HITRAN dabase is one of the major sources of spectroscopic information required for line-by-line radiative transfer modeling.


To gain a better understanding of the nature of absorption by atmospheric gases, in this task you will work with several lines of water vapor located in the near IR. Table 1 gives line data from the HITRAN database.

  1. Using line parameters from Table 1, calculate and plot the spectral absorption coefficient kν as a function of wavenumber. What is a largest value of the absorption coefficient? What is the relative contribution of lines? Consider standard pressure and temperature.
  2. Select any individual line from Table 1. What spectral resolution of the instrument will be required to resolve this line in the pressure range from 1013 mb to 200 mb? Consider the Lorentz line profile and constant temperature.
  3. Testing of the instrument can be performed by taking measuarements along the path with known amount of absorber. What water vapor amount will be required so that measured monochromatic transmittance Tν at ν = 7281.72912 cm-1 will be 0.2?

Table 1. Subset of HITRAN line data for H2O (P =1013 mb; T=273 K)

Line center νo(1/cm) Line intensity
S (cm-1/(atm cm))
Line half-width
α (1/cm)
7280.31512 4.194E-03 0.0704
7280.47400 8.872E-04 0.0846
7281.08200 3.764E-02 0.0994
7281.72912 4.033E-03 0.0602