The DLR Virtual Lab: Scattering and Radiative Transfer Codes
The Virtual Lab (VL) project's initial goals were to provide Web access for electromagnetic scattering and radiative transfer simulation applications developed at the DLR's Remote Sensing Technology Institute and to make them more accessible for technology transfer and scientific exchange. To reduce perprogram development effort, it became clear early on that providing Web interfaces for a substantial selection of programs would only be feasible with a generic platform.
The VL hardware currently comprises a heterogeneous cluster containing seven Intel/Linux machines and one Sparc/ Solaris machine located at DLR's site in Neustrelitz. The VL's main implementation language is Python along with Zope, MySQL, and OpenLDAP. The project is based on first- and second-generation Web technology, nb. HTML 3.2 and HTTP 1.0.
A more detailed description of the VL
can be found in a recent article published by Computing
in Science & Engineering:
Remote sensing of the earth and its environment becomes more and more important in the context of sustainable development and global change monitoring. Common to all remote sensing techniques is the fact that they are based on information coming from scattered and transmitted electromagnetic waves in different spectral ranges. Therefore, realistic models of how such electromagnetic waves are transmitted and scattered under certain conditions are of particular importance in remote sensing applications.
During the past we have developed several programs allowing light scattering analysis on dielectric but, in general, nonspherical particles in the resonance region. In this region the scattering models must be based on a full-wave analysis of Maxwell's equations. This is a mathematically pretentious task. Concerning radiative transfer we have developed models to perform simulations in the infrared (line-by-line model) as well as the visible and the ultraviolet spectral range. All these programs have been successfully applied to retrieve trace gases in the earth's atmosphere, to determine cloud and aerosol properties, and to perform system studies for the detection of natural and anthropogenic high-temperature events like volcanoes and bio-mass burning. Our scattering and radiative transfer programs represent the state-of-the-art in many aspects, and they are also of interest in other fields than remote sensing (in technical and medical diagnostics, for instance)
In the VL, sophisticated programs are available allowing light scattering analysis up to the geometric optics region on various classes of nonspherical particles such as spheroids, hexagonal and irregular ice particles, and Chebyshev-like particles which are used in remote sensing to model aerosol components, microphysical properties of Cirrus clouds and hydrometeors, for instance. Amongst other, the following programs can be used:
The methodological background of mieschka, pmieschka, and CYL are the generalization of the separation of variables method [Rother, 1998] (GSVM) applied in spherical coordinates for the first two programs. In CYL this method is applied in cylindical coordinates in combination with Huygen's principle to find an approximation for finite cylindrical columns having noncircular cross-sections [Rother et al., 1999]. Essential numerical simplifications can be achieved if the scattering geometry exhibits a certain symmetry. This was demonstrated in Rother et al. , for instance.
There are two general approaches which have been widely used in the past to solve light scattering problems on nonspherical particles rigorously. These are the Finite-Difference methods (FD) and the Boundary Integral Equation methods (BIE) which have traditionally been treated separately. FD methods are based on the description of the scattering problem in terms of partial differential equations. They apply a discretization scheme to some (Method of Lines (MoL) [Rother, 1999a]) or to all spatial coordinates (conventional FD methods, see e.g. Taflove ) of the corresponding partial differential equation. BIE methods, such as Waterman's T-matrix approach, start from Green's theorem in conjunction with the Helmholtz equation and the related free-space Green's function [Tsang et al., 1985]. With the GSVM both approaches can be condensed into a common mathematical body [Rother, 1999b]. This is achieved by deriving the surface Green's function related to the scattering problem. This Green's function establishes the link between BIE methods and methods based on the solution of the corresponding partial differential equation.
Modelling the transfer of electromagnetic radiation in the atmosphere is important for meteorology, climatology, and atmospheric remote sensing. Radiative transfer in the atmosphere is driven by absorption, emission, and scattering of light at molecules, aerosols, and hydrometeors [Liou, 1980]. The change of radiation intensity (radiance) passing through the atmosphere is formally described by a complex equation which does not allow for a general solution. A large variety of radiative transfer codes have been developed in the past decades based on approximations etc. appropriate for a a certain application and/or wavenumber regime.
The following models are currently available in the VL for radiative transfer:
The following figures show an excerpt of a typical session using the MIESCHKA code for computing scattering properties for spherical and non-spherical particles. For other software components the sequence of providing the relevant input parameters is similar.
Figure 1 shows the complete specification for MIESCHKA to perform a single scattering calculation for a homogeneous water drop. Before the required input data is complete, the user is requested to fill out a sequence of individual HTML forms containing input masks.
As soon as the input for the numerical experiment is complete, the "Start Task" button will appear near the top of the screen.
Figure 2 indicates that the VL cluster has completed this particular task. This screen also echoes standard output and standard errors during the task.
After pressing the Show Results" button a plot pops up, Figure 3, serving as a quicklook. This plot shows the scattering phase function for a purely scattering water drop.
The complete results for each task are packed into a tar-file "resultfiles.tar" which can readily be downloaded to the user's local directory.
Using the button "Abort task and go back to InputDialog" allows the user to modify some of the parameters and re-run the task, leading to another plot, in this case for an absorbing water drop, see Figure 4.
For scientific questions related to:
1) Scattering codes, please contact Dr. Jochen Wauer
2) Radiative transfer codes, please contact Dr. Franz Schreier
This document was generated using the LaTeX2HTML translator Version 2002-1 (1.69)
Copyright © 1993, 1994, 1995, 1996, Nikos
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The command line arguments were:
The translation was initiated by Thomas Trautmann on 2004-04-08
Thomas Trautmann 2004-04-08