MIPAS ground processor and data products
International Geoscience and Remote Sensing Symposium (IGARSS) 3 (1999) 1692-1696
Abstract:
As an ESA-provided payload instrument on board ENVISAT the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) will routinely acquire atmospheric limb emission spectra in the middle infrared region. Due to its high radiometric sensitivity and spectral resolution capabilities a large number of middle atmospheric constituents can be detected that are of primary interest in the various disciplines of atmospheric research. The ENVISAT ground segment concept foresees generation and near real time dissemination of MIPAS data products up to the level of pressure (p), temperature (T) and volume-mixing ratio (VMR) profiles for the high priority target species O3, H2O, CH4, N2O, HNO3. The basic algorithms have been defined and are currently being verified in various ESA supported studies. End-to-end tests based on simulated instrument raw data have been carried out, using prototype versions of the Level 1 B (processing from raw data up to calibrated radiance spectra) and Level 2 algorithm components (retrieval of vertical p, T and VMR profiles). Runtime checks have been performed for the critical profile retrieval modules. For a reference scenario (p, T plus five trace gas retrievals for a single, 16 heights elevation sequence; one iteration per retrieval), a total runtime of approx. 150 s has been achieved on an IBM RS/6000 model 397.Intercomparison of the KOPRA and the RFM radiative transfer codes
P SOC PHOTO-OPT INS 3867 (1999) 348-363
Abstract:
We discuss the intercomparison between the Karlsruhe Optimized and Precise Radiative transfer Algorithm (KOPRA) 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 Optimised Forward Model (OFM) and the RFM as well as between KOPRA and the RFM have already been performed(Ref.(1,2)). To be able to relate on these results, this validation is similarly organised: we perform subsequently more complex tests of ray-tracing, integrated column amounts, homogeneous and limb path calculations of unapodised, apodised and held-of-view (FOV) convolved spectra, using the same isolated CO2 line as well as the same six MIPAS microwindows. Additionally we compare modelling 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 fulfils the acceptance criteria requested for the OFM. In most cases the deviations are even clearly below 1 nW/(cm(2) 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 modelling of the H2O-, O-2- and N-2-continua and of CO2 line-mixing. Larger deviations of up to several nW/(cm(2) sr cm(-1)) occured for NLTE calculations on the basis of "default" atmospheric profiles with vertical resolution of I 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/(cm(2) 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/(cm(2) 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/(cm(2) sr cm(-1)).An optimized forward and retrieval model for MIPAS near real time data processing
Proceedings of SPIE - The International Society for Optical Engineering 3501 (1998) 170-185
An optimized forward and retrieval model for MIPAS near real time data processing
P SOC PHOTO-OPT INS 3501 (1998) 170-185
Abstract:
The infrared emission limb sounder MIPAS (Michelson Interferometer for Passive Atmospheric Sounding) will be operated as an ESA core instrument on the ENVISAT-1 satellite. Near real time retrieval of pressure, temperature (p,T) and volume mixing ratio (VMR) of five key species (O-3, H2O, N2O, CH4, and HNO3) from calibrated spectra will be performed in the Level 2 processor of the ENVISAT Payload Data Segment. In order to develop an optimized (with respect to speed and accuracy) retrieval algorithm suitable for the implementation in MIPAS Level 2 processor, an ESA supported study is being carried out. In the framework of this study, an optimized forward / retrieval code based on the global fit approach was implemented. In this approach all the spectra of a limb-scanning sequence are simultaneously fitted, so that error propagation in the altitude domain is avoided.The attained accuracy performances of the retrieval code are the following:Temperature error < 2 K at all the altitudes covered by the standard MIPAS scan (8-53 km)Tangent pressure error: < 3%Error on the retrieved VMR of the key species: < 5 % at most of the altitudes of scientific interest covered by the standard MIPAS scan.The run-time required to perform p,T and VMR retrieval of the five MIPAS target species from a limb-scanning sequence of 16 limb-views is less than 6 minutes on a SUN SPARCstation 20. The most effective code optimizations were implemented in the radiative transfer model and in the computation of the jacobian of the retrieval.Evidence of non-LTE in the CO2 15 μm weak bands from ISAMS and WINDII observations
Geophysical Research Letters 24:4 (1997) 361-364