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The possibility to derive microphysical properties of polar stratospheric clouds from future MIPAS (Michelson Interferometer for Passive Atmospheric Sounding) -- ENVISAT measurements was investigated. Available refractive index data for PSC candidates were intercompared in order to estimate their reliability. Especially for NAT the laboratory measurements differ significantly and for ternary H2SO4/HNO3/H2O solutions only one source of data exists. For simulating limb-spectra, a Mie model was implemented in the forward code KOPRA (Karlsruhe Optimized and Precise Radiative transfer Algorithm) in such a way that in parallel to the radiance spectra the derivatives with respect to a variety of microphysical aerosol parameters can be generated. Broadband forward calculations for small and large aerosols were made for various refractive indices. For large particles the PSC signal in the spectrum was up to forty times larger than the noise level. The signal for small particles was around the spectral noise. By minimizing the total retrieval error an automatic microwindow selection was performed for different PSC scenarios. Under the assumption of known radius and width of the aerosol size distribution resulting errors for number density retrieval were less than 3% (9 X 10-4 cm-3) for large and around 50% (7 cm-3) for small particles. For large particles it is possible to perform a two-parameter fit of number density and mode radius with errors less than 10%. Due to the Rayleigh- limit a distinction between radius and number density is not possible for small particles. However, the volume density can be derived with 12% (0.4 micrometer3/cm-3) uncertainty for large and 20% (2.2 X 10-2 micrometer3/cm-3) for small aerosols. These conclusions are valid as long as the aerosol layer is not optically thick which in our examples was the case for water ice (type II) PSCs of number density 0.2 cm-3 and radius 2.6 micrometer.© (2001) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Hyperspectral remote sensing is a challenge to data analysis due to the large data rate. Since it will often not by possible to analyze the entity of measured spectral data, so-called 'micro windows' are selected which contain the bulk of information on the target state parameters to be retrieved, while any interfering signal is minimized. We discuss the benefits of a quantitative method for automatic selection of optimized sets of such micro windows for the analysis of Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) data. While MIPAS, which will be operated onboard the polar orbiting environmental satellite (ENVISAT), is not a hyperspectral instrument, it serves as an example to study the power of the micro window approach. The MIPAS instrument will measure the IR emission of various atmospheric trace gases by limb scans covering the altitude region from 6 to 68 km altitude.© (2001) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

We discuss the intercomparison between the Karlsruhe Optimized and Precise Radiative transfer Algorithm (KO-PRA) and the Reference Forward Model (RFM) codes, which have been designed for analysis of MIPAS-ENVISAT data. The purpose of this intercomparison is to validate the KOPRA algorithm, i.e. to identify and to remove possible errors in the KOPRA (or RFM) code and to quantify the reason of remaining differences. Similar comparisons between the MIPAS Optimized Forward Model (OFM) and the RFM as well as between KOPRA and the RFM have already been performed. To be able to relate on these results, this validation is similarly organized: we perform subsequently more complex tests of ray-tracing, integrated column amounts, homogeneous and limb path calculations of unapodised, apodised and field-of-view (FOV) convolved spectra, using the same isolated CO2 line as well as the same six MIPAS microwindows. Additionally we compare modeling of CO2 line-mixing, non-local thermodynamic equilibrium (NLTE), trace gas continua and cross-section spectra. The KOPRA-RFM residuals are below a quarter of the noise- equivalent spectral radiance (NESR) for the isolated CO2 line as well as for the MIPAS microwindows, i.e. KOPRA fulfills the acceptance criteria requested for the OFM. In most cases the deviations are even clearly below 1 nW/(cm2 sr cm-1), that is more than one order of magnitude below the acceptance threshold. This is valid for unconvolved as well as for ALS (apodised line shape) and FOV convolved spectra. There is also good agreement in modeling of the H2O-, O2- and N2-continua and of CO2 line-mixing. Larger deviations of up to several nW/(cm2 sr cm-1) occurred for NLTE calculations on the basis of 'default' atmospheric profiles with vertical resolution of 1 or 2.5 km. These differences were found to be due to different layer-averaging of the vibrational temperatures and could be considerably reduced by calculations with a higher vertical resolution of 250 m. Cross-section spectra agree well, if the tabulated data are given independent of pressure, e.g. for ClONO2 and N2O5, and cover the atmospheric temperatures. Due to different temperature extrapolation the deviations increase up to 10 nW/(cm2 sr cm-1) for atmospheric temperatures outside the measuring range. The RFM is not yet adjusted to cross-sections tabulated for non- equidistant temperatures and for atmospheric pressures, like CFC-data in the HITRAN96 database. If these data are used, larger differences arise, e.g. up to 30 nW/(cm2 sr cm-1) between CFC-12 spectra. Avoidance of interpolation by performing homogeneous path calculations for p,T of one of the tabulated cross-section datasets reduces the deviations to below 0.5 nW/(cm2 sr cm-1).© (1999) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

We present the Karlsruhe Optimized and Precise Radiative transfer Algorithm (KOPRA) which has been specifically developed for data analysis of the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) going to be launched on ESA's polar-orbiting Environmental Satellite 1 (ENVISAT-1) in 1999. KOPRA has been designed to account for the particular instrument requirements of MIPAS and the observation scenarii during the ENVISAT mission, in particular with respect to the viewing direction and the altitude coverage of the atmosphere. The conceptual details of KOPRA, which reflect the requirements set up by the instrument design details, the observation scenarii, and the link to a retrieval concept with high flexibility, are presented. The forward model error due to discarding individual physical processes and properties of the atmosphere as well as an over-all error budget with respect to these parameters is assessed in order to demonstrate the improvements of retrieval accuracy expected by usage of KOPRA.

Two sets of subroutines for calculating absorption cross section spectra and transmission or radiance fields are presented. These libraries will be used in the operational data processing of the MIPAS/ENVISAT level-2 off-line processor and are part of the Karlruhe Optimized and Precise Radiative transfer Algorithm (KOPRA). The objective in developing these libraries was to accommodate flexibility and simplicity in use without substantial loss of accuracy and efficiency. The ADDLIN library uses an efficient method for calculating absorption cross section spectra line-by-line to arbitrary high numerical accuracy. Computational efficiency is achieved by calculating each spectral line on its own optimum set of sampling points. Absorption cross section spectra are stored on flexible, non-equidistant frequency grids. The TRANSF package provides integration routines that can be quickly configured for a variety of specific applications and measurement scenarios. The routines operate on the non-equidistant frequency grids produced by ADDIN and allow the researcher to implement the radiative transfer in a simple and almost natural way. Computational efficiency results from the reduced number of sampling points on the non-equidistant frequency grids produced by ADDIN and allow the researcher to implement the radiative transfer in a simple and almost natural way. Computational efficiency results from the reduced number of sampling points on the non-equidistant frequency grids compared to equidistant frequency grids of uniform intervals.

The new Karlsruhe Optimized and Precise Radiative transfer algorithm (KOPRA) is a line-by-line model for use in retrieval processors for atmospheric observations. It simulates IR spectra by taking into account physical properties of the atmospheric observations. It simulates IR spectra by taking into account physical properties of the atmosphere and of the instrument. Besides spectrum calculation, KOPRA has the capability to determine the derivatives of the spectrum with respect to many retrieval parameters. Comparisons between analytical and numerical derivatives, which are generally within a few percent, demonstrate that approximates due to run time optimized implementations are small. Furthermore, a flexible scheme is presented for handling various parameterizations of atmospheric profiles as implemented in the code in order to support different retrieval approaches.

High resolution limb emission spectra were measured by the balloon-borne FTIR spectrometer MIPAS-B in the Arctic vortex in March 1992. These spectra are significantly affected by the Mt. Pinatubo stratospheric aerosol. A method has been developed for separating the radiance signal of emission lines of trace gases from the aerosol continuum. By applying this method to the MIPAS-B spectra aerosol extinction coefficients have been retrieved at approximately 60 spectral positions in the 750-980 cm-1 and 1180-1380 cm-1 spectral ranges. The spectral shape of the mid- IR aerosol extinction has been derived for the tangent altitudes 11.3 km, 14.5 km and 16.1 km. On the basis of Mie- calculations it has been demonstrated that the spectral aerosol extinction coefficient is sensitive to the aerosol composition as well as to the particle size distribution. An algorithm has been developed to retrieve microphysical parameters by least-squares fitting of the derived spectral extinction coefficients to Mie-generated extinction coefficients. The retrieved aerosol parameters indicate the significance of scattering even in the mid-IR. Related corrections on the basis of multiple scattering calculations were performed. Retrieved compositions and effective volume densities of the Pinatubo aerosol in the Arctic vortex are presented.© (1996) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

The accurate knowledge of pressure and temperature profiles is a precondition for the retrieval of race gas profiles from limb emission measurements. A method was investigated which allows the pressure and temperature retrieval from limb emission spectra as expected to be measured by the MIPAS-ENVISAT instrument. Regardless which retrieval scheme will be used, the simultaneous retrievability of pressure and temperature depends largely on the proper selection of microwindows. Microwindows which contain a large amount of information on these target quantities while being insensitive to systematic errors are considered to be the most appropriate ones. A microwindow selection which minimizes the pressure temperature retrieval error has been carried out for the instrument specifications of MIPAS- ENVISAT. Errors under consideration were random noise, calibration uncertainties, and neglection of possible breakdown of thermodynamic equilibrium.© (1996) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.