ED43B-0939
CRT
Introductory Tools for Radiative Transfer Models
1
D.R. Feldman , V.
Natraj 2,
3
L. Kuai , and Y.L. Yung 3
1. Caltech, Department of Environmental Science and Engineering, 1200 E. California Blvd. MC 150-21 Pasadena, CA 91125; 626-395-6447
2. Caltech, Department of Chemical Engineering
3. Caltech, Division of Geological and Planetary Sciences
Technical support provided by Wing-Ning Yung, the RT team at AER, Inc, and Lex Berk of Spectral Sciences, Inc.; Research supported financially by the NASA Earth Systems Science Fellowship, Grant # NNG05GP90H
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Abstract
Satellite data are currently so voluminous that, despite their unprecedented quality and potential for scientific application, only a small fraction is analyzed due to two factors: researchers'
computational constraints and a relatively small number of researchers actively utilizing the data. Ultimately it is hoped that the terabytes of unanalyzed data being archived can receive
scientific scrutiny but this will require a popularization of the methods associated with the analysis. Since a large portion of the complexity of the analysis is associated with the proper
implementation of the radiative transfer model, it is reasonable and appropriate to make the model as accessible as possible to general audiences. Unfortunately, the algorithmic and
conceptual details that are necessary for state-of-the-art analysis also tend to frustrate the accessibility for those new to remote sensing. Several efforts have been made to have web-based
radiative transfer calculations, and these are useful for limited calculations, but analysis of more than a few spectra requires the utilization of home- or server-based computing resources.
We present a system that is designed to allow for easier access to radiative transfer models with implementation on a home computing platform in the hopes that this system can be utilized
in and expanded upon in advanced high school and introductory college settings. This learning-by-doing process is aided through the use of the Community Radiative Transfer (CRT) wiki
which may be able to facilitate greater interest in the field of remote sensing.
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MOTIVATION
The measured quantity from a remote observation and concurrently the result of a radiative
transfer calculation are a function of the specifics of:
For NASA, much effort has been devoted to the dissemination of the data through online
data gateways 1, but the technical character of the data being disseminated presents a
barrier to greater community participation. In particular, understanding the nature of
the measurements being made, and thus learning computational radiative transfer,
may prevent those who are new to or curious about the field of remote sensing from
being able to conduct science on their own personal computers.
Several different classes of RT codes have been written and are generally organized according
to the spectral resolution and spectral region of interest.
•
•
•
• A matrix specifying the profiles of temperature, pressure,
and radiatively active gases.
• The observer location and viewing angle.
• Gray or non-gray surface properties.
• The solar function.
• The instrumental response function.
• Range of wavelengths under consideration.
where I is the radiance, J is the emission/scattering term, k is the absorption coefficient,
and s is the pathlength. The multi-dimensional nature of and the computational
requirement for solutions in all but the most elementary instances are challenges for the
classroom setting. At the collegiate level, it is hoped that courses that cover radiative
transfer concepts rigorously will also be able to educate students with the actual tools
needed to perform remote sensing science.
CRT WIKI DESCRIPTION
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The aim of the Community Radiative Transfer (CRT) project is to facilitate the
appropriate execution of radiative transfer models so that novel science can be performed
using the vast amounts of unexplored remote sensing data. One easy tool that can be used
to achieve such a task is a wiki. The CRT wiki is technical and meant for an audience
focused on understanding radiative transfer and how it relates to remote sensing data
analysis.
The wiki is a realization of the DokuWiki package 2 and is organized into 4 main sections
(referred to internally as namespaces):
•
•
•
•
Administrative
Conceptual
Forum
Technical
For treatment of scattering or where any of the above inputs require more specification, an
inhomogeneous data structure may be a useful tool for allowing variable argument input length.
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A wiki on radiative transfer models will not be very useful for
general audiences unless these audiences can easily run the
radiative transfer models on standard platforms. The majority
of radiative transfer codes that were surveyed for this project 8
have been designed to be compiled with commercial Fortran
compilers and run on a Linux platform. However, as of 2006,
Macintosh and Windows operating systems together account
for 99 % of personal computers used 9, it is reasonable to orient
the CRT wiki so that it focuses on distributing:
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EXAMPLES FROM THE LIDORT SUITE
One of the most popular RT codes for scenarios where scattering is
important is DISORT, which is a general and flexible package that can be
used in a wide variety of atmospheric applications. LIDORT 6 incorporates
the useful features of DISORT; in addition, it is designed to give more
accurate results for a much larger range of solar and viewing geometries by
taking into account the curvature of the atmosphere. Furthermore, weighting
functions are generated simultaneously with the radiances by using analytic
differentiation of the RT equation. LIDORT has been used for ozone profile
retrievals from OMI on EOS-AURA and GOME-2 on the METOP series,
among others.
Figure 7: Screenshot of implementation of LBLRTM on
the Mac OS X platform run through AppleScript.
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VISUALIZATION
The visual representation of the results of RT codes is crucial
even for the most elementary analysis. Several visualization
packages exist and are commonly-utilized in the remote
sensing community including Matlab® and IDL® . These
support advanced plotting capabilities with straightforward
syntax.
Several different radiative transfer codes have been developed by AER, Inc. for modelling
infrared spectra, and broad- and narrow-band fluxes, and heating rates 3. The CRT wiki
introduces some of these codes focusing on the line-by-line code LBLRTM and the broadband
flux and heating-rate code RRTM.
LBLRTM has been the subject of at least 20 publications while RRTM has been the subject of
at least 21 publications. However, the intricate nature of the inputs for these codes may be
daunting to the beginner. As part of this project, a series of wrapper routines is introduced that
adhere to the input details discussed in Section (3).
Figure 6: Cartoon of angular
dependence of scattering.
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CONCLUSIONS
Figure 6: Derivation of air-mass
factors for sample GOME viewing
scenes over Pennsylvania for the
clear-sky and cloudy fractions of the
scenes as determined with the
LIDORT radiative transfer model.
Adapted from Fig. 2, Martin et al.7.
Figure 8: Example color contour plot that can be created
with 12 lines of Matlab code and provides easy, quick analysis
of 3-dimensional results, inspired by Clough et al. 10.
1. There is much remote sensing data that is left unscrutinized and a community-oriented, accessible approach to radiative transfer may be
able to address the data deluge.
2. The Community Radiative Transfer (CRT) wiki has been developed to facilitate knowledge regarding the implementation of RT codes.
The ultimate goal of the project is to facilitate personal-computer based science using satellite remote sensing data.
3. Preliminary efforts have been made to port commonly-used RT codes such as LBLRTM, MODTRAN, and LIDORT to the Mac platform.
4. While it is desirable for RT codes to output text files of their results, visualization is a central part of analysis using radiative transfer and
user-friendly tools are an essential part of the future course for the CRT project.
5. Input into the structure and focus of the CRT project is much appreciated, especially by those interested in undergraduate courses on the
subject matter.
http://www.gps.caltech.edu/~drf/wiki
for ease of use.
PLATFORM CHOICES
Figure 4: Similar to Figure (2) (a) but using the MODTRAN 4 code.
EXAMPLES FROM AER CODES
Many concepts are shared with Wikipedia, though access restriction is employed so
that while pages are viewable by all, only registered users can edit pages. The
registration process is very simple and can be done in 1 minute. The wiki is located at:
Figure 1: Screenshot of the CRT wiki index page. Page layout and searching is intended
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• Stand-alone executable models built for
Mac and Windows
• Code that runs by clicking in lieu of the
command-prompt
• Code that easily facilitates batch runs.
The medium through which the light is propagating.
The orientation of the viewer (viewing geometry).
The instrumental response function.
Despite this variety, a rubric can be developed so that the input specification file and
execution of the code standardized where it can be and flexible where it needs to be. For
atmospheric remote sensing problems, the standardized inputs may take the form of:
I  J  k * I  s
One terrestrial radiative transfer code that has been utilized extensively over
almost 20 years is the MODerate Resolution TRANsmittance code 5,
MODTRANTM. Since MODTRAN’s initial release, over 120 publications
have utilized this code due to its speed and comprehensive set of options.
Several wrapper packages have been released in order to manage the arcane
format for the input file that MODTRAN requires. For the CRT project, a
freely-available wrapper has been released with examples similar to those
associated with the routines associated with the AER RT suite of codes (see
Section (4)).
RADIATIVE TRANSFER MODEL DETAILS
Science derived from remote sensing measurements aboard satellite platforms is
currently data-rich. Each satellite instrument produces several megabytes per second of
calibrated measurement data that have the potential to yield useful scientific results,
especially in light of the novelty of many of these instruments. However, the ratio of data
to researchers is quite large, and with the advent of new instruments, this ratio only
stands to grow larger.
At its heart, radiative transfer is a deceptively simple endeavor which seeks to solve for
radiance as a result of its propagation through a medium as seen in the Fundamental
Equation of Radiative Transfer:
EXAMPLES FROM MODTRAN 4
Figure 2: Comparison between measured
Figure 3: Broadband infrared heating rates
and modeled spectra for the CAMEX mission 4
using the LBLRTM code. The above panel
shows an overlay of the two spectra while the
below panel details the difference.
calculated from the longwave RRTM code for a
clear-sky tropical model atmosphere. The color
of the line denotes the heating rate of a particular
spectral band as indicated in the legend on the
right.
10 REFERENCES
1. Ouzonov, D., Savtchenko, A., et al. (2004). Advances in Space Research. 33(7) 1109-1113.; 2. Splitbrain.org, (2006). http://wiki.splitbrain.org/wiki:dokuwiki;
3. Clough, S.A., Shepard, M.W., et al. (2005). Journal of Quantitative Spectroscopy and Radiative Transfer 91:233-244.; 4. CAMEX mission source,
http://deluge.ssec.wisc.edu/~his/his/validation/DELIVERY/contents.html; 5. Berk, A., Bernstein, L.S., Robertson, D.C., et al. (1989). GL-TR-89-0122. Air Force
Geophysics Laboratory, Bedford, MA.; 6. Spurr, R.D., Kurosu, T.P. et al. (2001). Journal of Quantitative Spectroscopy and Radiative Transfer. 68(6): 689-735.;
7. Martin, R.V., Chance, K., et al. (2002). Journal of Geophysical Research-Atmospheres. 107(D20).; 8. Soden, B., Tjemkes, J., et al. (2000). Bulletin of the
American Meteorological Society. 81(4): 797-808.; 9. Provided by netapplications.com to marketshare.hitslink.com
(http://marketshare.hitslink.com/report.aspx?qprid=5). 10. Clough, S.A., Iacono, M.J. et al. (1995). Journal of Geophysical Research-Atmospheres. 100(D8): 16519-16536.