1 / 22

Radiative Transfer Band Models Overview

Explore the concepts of equivalent width, weak/strong line approximations, and band models in radiative transfer. Learn about Curtis-Godson approximation, MODTRAN, and statistical band models for simplifying spectral transmittance computations.

stanleya
Download Presentation

Radiative Transfer Band Models Overview

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. PHY2505 - Lecture 8 Radiative Transfer Band Models

  2. Outline Equivalent width Weak line/strong line approximations Band models Curtis-Godson approximation MODTRAN

  3. Equivalent width Consider a homogenous atmospheric layer. Here the spectral absorption coefficient does not depend on path length. The spectral transmittance T for a band of width Dv is And spectral absorptance, A Equivalent width, W [cm-1]: a measure of absorptance, A, over the spectral intervalDv

  4. Equivalent width of a Lorentz line

  5. Equivalent width of a Lorentz line

  6. “Weak line” limit

  7. “Strong line” limit

  8. Can find experimentally from “curves of growth” Strong/weak line: limits of validity

  9. Band models • A band is a spectral interval of a width Dv small enough to use a mean value of the Planck function Bv(T) but large enough to contain several absorption lines • Band models are introduced to simplify computation of spectral transmittance • Now we have found out how to calculate the equivalent width of a single line, need to consider how we deal with a band of many lines • Two main cases: • Lines with regular positions • Lines with random positions

  10. Regular Elasser band model This gives See Liou p139-141 for derivation

  11. Principle of statistical band models

  12. Principle of statistical band models where d is the mean spacing For multiple lines, transmission is exponential in W

  13. Goody statistical model

  14. Goody statistical model: weak and strong line limits

  15. Here, the spectral lines are rearranged over a given spectral interval and a histogram produced Absoprtion coefficient for representative lines is multiplied by a weighting function representing frequency of occurance of this type of line Typically useful to use 4 divisions per decade on log scale … See Liousection 4.3for a discussion of the limits of validity for this approximation Correlated k distribution Liou, FIG 4.5

  16. Curtis – Godson approximation

  17. Curtis – Godson approximation

  18. Curtis – Godson approximation

  19. Curtis – Godson approximation

  20. MODTRAN4 A. Berk *, G.P. Anderson #, P.K. Acharya *, J.H. Chetwynd #, M.L. Hoke #,L.S. Bernstein *, E.P. Shettle ^, M.W. Matthew *, and S.M. Adler-Golden # US Air Force Research Laboratory *Naval research Laboratory 2cm-1 resolution • Atmosphere: gas profiles, temperature, pressure profiles, aerosol/cloud type & vertical distribution • Surface type & measurement geometry • Select calculation methods: eg. correlated K method, scattering (DISTORT)

  21. Who still uses band models (MODTRAN)? UV/VIS atmospheric instruments where scattering important Eg. SCISAT-1 MAESTRO McElroy, C.T. , A spectroradiometer for the measurement of direct and scattered solar spectral irradiance from on-board the NASA ER-2 high-altitude research aircraft, Geophys. Res. Lett., 22, 1361-1364 (1995). Multispectral imagers cloud, ozone, water vapour retrieval Eg. MODIS Justice, C.et al, The Moderate Resolution Imaging Spectroradiometer (MODIS): Land remote sensing for global change research, IEEE Trans. Geosci. Remote Sens., 36, 1228-1249 (1998). Hyperspectral imagers for atmospheric corrections Eg. AVIRIS Berk, A, et al, MODTRAN Cloud and Multiple Scattering Upgrades with Application to AVIRIS, Remote Sens. Environ., 65, 367-375 (1998).

  22. MODTRAN dialogue windows

More Related