CHAPTER 1: Introduction

 
• 1.1 General
• 1.2 Features of SCIATRAN

1.1 General

This user's guide describes how SCIATRAN can be installed and used on UNIX workstations.

It is strongly recommended to carefully read this user's guide before using SCIATRAN.

SCIATRAN is a radiative transfer program developed at the Institute of Remote Sensing (ife) / Institute of Environmental Physics ( iup), University of Bremen, Bremen, Germany.

SCIATRAN Version 1.2 is an extension of the GOMETRAN radiative transfer model [37, 4, 38, 28, 43, ?, 14, 27].

SCIATRAN has been developed in FORTRAN 77 on a SUN Ultra Sparc workstation under Solaris 2.x. SCIATRAN has also been tested on the University of Bremen FB1 CRAY J916 supercomputer running UNICOS, and on a DEC Alpha station running OpenVMS AXP (TM) V6.1. Nevertheless, ife/iup cannot make any warranties that the program is free of errors.

SCIATRAN has been designed to allow fast and accurate simulation of radiance spectra as measured or expected to be measured from space with the passive remote sensing UV-Vis-NIR spectrometers GOME (Global Ozone Monitoring Experiment) [12] (spectral range: 240-790 nm; viewing mode: nadir) and SCIAMACH [5] (240-2400 nm; nadir, limb, solar and lunar occultation). GOME has been successfully launched onboard ESA's satellite ERS-2 in April 1995. SCIAMACHY is expected to be launched onboard ESA's satellite ENVISAT-1 in mid 2001. Further details concerning GOME and SCIAMACHY can be found on our web-site. SCIATRAN is designed to be the forward model for retrieval of atmospheric constituents from GOME/SCIAMACHY satellite data.

During the last years several papers have been published and diploma and PhD thesis were generated demonstrating that GOMETRAN/SCIATRAN is a valuable tool to be used for many applications [37, 4, 38, 28, 43, 6, 32, 7, 8, 12, 20, 21, 19, 22, 14, 24, 28 ], such as the retrieval of atmospheric constituents from remote radiance measurements, for UV flux calculations at the Earth's surface or for photolysis rate calculations. Many of these papers also contain comparison with other models or measurements. Therefore, we consider this model to be sufficiently validated. Nevertheless, validation is a never ending and still ongoing task, especially important when applying the program to "new" areas or after program modification.

1.2 Features of

This section gives an overview about the general features of SCIATRAN. Details can be found in the sample user interface ASCII files (*.inp files) which have been included in this user's guide. Modifying these files (using any ASCII editor) enables the user to set many parameters related to viewing geometry, atmospheric composition, algorithm flow, etc. The sample input files contain detailed explanations necessary to understand SCIATRAN's capabilities and to successfully use the program.

Basically 240 nm - 2400 nm, several spectral windows can be selected. The sub-range fully supported (e.g., by correlated-k parameters, see below) is related to the GOME/SCIAMACHY spectral channels, i.e., 240-1750 nm (channels 1-6), 1940-2040 nm (channel 7), and 2260-2385 nm (channel 8).

1. Plane-parallel mode:
   Plane-parallel atmosphere (neglecting all effects due to the sphericity of the Earth), valid for solar zenith angles less than about 75 deg.
2. Pseudo-spherical mode (default):
   Plane-parallel atmosphere but (solar) source term calculated in spherical geometry (incl. refraction). This gives accurate results for solar zenith angles less than about 92 deg in conjunction with a (satellite) ``near-nadir'' viewing geometry, i.e. about +/- 35 deg (top-of-atmosphere) line-of-sight zenith angle (see, e.g., [36]). This means that an accurate simulation of limb radiance spectra is not possible with this version but will be possible in the near future (see [23, 36]) A description of the pseudo-spherical approach can be found in [14] (in German).

Note: In principle it is, of course, also possible to extract the complete radiation field and not only the top-of-atmosphere radiance. This might be necessary for actinic flux calculations (full sphere ($4\pi$) integrated radiance) performed, e.g., in order to calculate photolysis frequencies [4], or to simulate the radiance as measured from the ground (e.g., for airmass factor calculations for the interpretation of ground based zenith sky scattered light DOAS (= Differential Optical Absorption Spektroscopy) measurements) (see also [12, 5]).

This user's guide mainly focuses on how SCIATRAN can be used for the simulation of quantities directly related to GOME and SCIAMACHY, i.e., the top-of-atmosphere radiance and related quantities, e.g., airmass factors and/or weighting functions. SCIATRAN Version 1.2 also supports the ground based viewing mode but with some limitations (e.g., no weighting function calculations).

O$$₃, NO$$₂, ClO, OCLO, BrO, HCHO, SO$$₂, NO$$₃, O$$₄, O$$₂, and H$$₂O, CO, CH$$₄, and N$$₂O are implemented at present.

O$$₂, H$$₂O, CO, CH$$₄, and N$$₂O are in the following called ``line-absorber''. They are treated somewhat differently than the so called ``continuum-absorber'' (O$$₃, NO$$₂, ClO, OCLO, BrO, HCHO, SO$$₂, NO$$₃, O$$₄).

The absorption cross-section of line-absorbers depends strongly on wavelength, pressure and temperature and can be calculated from spectroscopic line parameters like line position, line intensity, air-broadened half-width etc. (obtained from, e.g., the HITRAN spectroscopic data base [35]). Two program modes are implemented in order to accurately consider line-absorptions: (i) an accurate line-by-line and (ii) a significantly faster correlated-k (c-k) mode (see [6, 7, 8] for details).

For continuum absorbers, cross-sections as measured in the laboratory are used, which are in the present version assumed to be independent of pressure but might depend on temperature. At present only the temperature dependence of the absorption cross sections of O$$₃ and NO$$₂ is taken into account. If temperature dependent cross sections of other continuum absorber will become available, the user can easily make use of them by modifying the appropriate user input file (see sample file xsections.inp).

Two aerosol parameterizations are implemented: The widely used LOWTRAN 7 aerosol scheme including Henyey-Greenstein phase functions (``LOWTRAN aerosol scheme'') [39, 26] or, alternatively, an aerosol parameterisation developed for GOMETRAN by R. Hoogen, ife (``GOMETRAN/SCIATRAN'' aerosol scheme) [18] which has been extended by J. Kauss, ife, in order to cover the spectral range of SCIAMACHY ([24], in German).

Lambertian reflector with (wavelength dependent) albedo. The height of the surface w.r.t. the sea level can be specified.

The user can make a selection between eight different water cloud types classified according to Stephens [42]. Clouds can be treated as (i) scattering and absorbing layers of finite vertical extent (``Clouds As Layers'' (CAL) scheme), i.e., similar to the aerosol parameterisation, and/or (ii) as reflecting lower boundary (``Cloud As Boundary'' (CAB) scheme) [27, 28]. The CAL scheme is accurate but rather slow. The CAB scheme might be used for the simulation of satellite observations in case of ground pixels covered by optically thick clouds. It does not provide any information on the radiance field below the cloud top. The CAB scheme takes into account the angular dependence of the reflected light (i.e., the non-Lambertian reflectivity of clouds) and transmission losses through the cloud (using ``escape functions''). The CAB scheme does not take into account absorption inside the cloud.

It is also possible to select a simple Lambertian surface as lower boundary and to specify its height as well as its albedo. This also might be used to simulate a ``cloud'' (or an elevated surface).

Note: Due to data base errors (wrong escape function values) it is presently not possible to use the CAB scheme for clouds with finite optical depth. No problems are expected when using the CAB scheme for clouds with infinite optical depth.

Full multiple scattering treatment (intensity and weighting functions) and several single scattering options.

Fast (``quasi-analytical'') calculation of weighting functions (i.e., the derivative of the radiance w.r.t. atmospheric or surface parameters) of trace gas concentrations, aerosol scattering and absorption, Rayleigh scattering, pressure, temperature and albedo for retrieval purposes [38].

The user has access to several flags and parameters, for example to optimise speed and accuracy or to easily manipulate the composition of the atmosphere (for example, several components, like trace gases or aerosols, can easily be switched on or off).

For each of the above mentioned trace gases airmass factors can be calculated (see [12, 5]).

Actinic flux ($4\pi$, i.e. spherically integrated radiance) (see [4]) as well as the ``normal'' up- and down-welling flux (i.e., the flux w.r.t. a unit area perpendicular to the Earth's surface) can be calculated.

Major new features in this version (e.g., w.r.t. GOMETRAN++ Version 2.9) are the line-by-line and c-k or ESFT (= Exponential Sum Fitting of Transmittance functions) schemes which enable the modelling of line absorptions (see above). Detail can be found in [6, 7, 8]. Further information is given in the ESFT include file Execute/esft.inc and in the control.inp sample file. Both files are part of this guide.

The aerosol as well as the cloud data base has recently been extended and implemented in order to cover the spectral range of SCIAMACHY (240-2400 nm) [24].

Rotational Raman scattering (``Ring effect'') has also been implemented [43] as well as thermal emission. Both effects can independently be switched on or off.

• Not implemented are non-thermal (non-LTE) emissions such as the NO-$\gamma$ band emissions around 260 nm or the O$$₂(a$1\Delta$_g) emission around 1.27 nm.
• The temperature dependence of all absorption cross-section - except ozone in the Hartley-Huggins bands - is presently neglected in the temperature weighting function.
• The pressure dependence of all absorption cross-section is presently neglected in the pressure weighting function.

• Extension to full spherical geometry (limb single and multiple scattering options).
• Solar and lunar occultation transmittance modes.

All these extensions are necessary to cover all scanning modes of SCIAMACHY. The present model is only appropriate for the nadir observations.

Figure 1.1 shows a sun-normalized radiance spectrum (``intensity'') covering the entire spectral range of SCIAMACHY calculated using SCIATRAN's correlated-k mode.

 
Figure 1.1: Simulated SCIAMACHY sun-normalized radiance measurement in nadir mode as calculated with SCIATRAN (correlated-k (c-k) mode; after convolution with 0.3 nm FWHM Gaussian slit function). Scenario: U.S. Standard Atmosphere, multiple scattering, solar zenith angle 60$o$, albedo 0.1, tropospheric maritime and stratospheric background aerosol, no clouds. Top: The strong decrease of the intensity below 300 nm is due to ozone absorption. The remaining features are due to oxygen and water vapor absorption. Middle: strong CO$$₂ absorption band with overlapping H$$₂O absorption. Bottom: the clearly visible structures are due to H$$₂O and CH$$₄ absorption with overlapping weak N$$₂O and CO absorption in the first and second part of channel 8, respectively.