Thomas von Clarmann

Optimized forward model and retrieval scheme for MIPAS near-real-time data processing.

An optimized code to perform the near-real-time retrieval of profiles of pressure, temperature, and volume mixing ratio (VMR) of five key species (O(3), H(2)O, HNO(3), CH(4), and N(2)O) from infrared limb spectra recorded by the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) experiment on board the European Space Agency (ESA) Environmental Satellite ENVISAT-1 was developed as part of a ESA-supported study. The implementation uses the global fit approach on selected narrow spectral intervals (microwindows) to retrieve each profile in sequence. The trade-off between run time and accuracy of the retrieval was optimized from both the physical and the mathematical points of view, with optimizations in the program structure, in the radiative transfer model, and in the computation of the retrieval Jacobian. The attained performances of the retrieval code are noise error on temperature <2 K at all the altitudes covered by the typical MIPAS scan (8-53 km with 3-km resolution), noise error on tangent pressure <3%, and noise error on VMR of the target species <5% at most of the altitudes covered by the standard MIPAS scan, with a total run time of less than 1 min on a modern workstation.

Sensitivity of trace gas abundances retrievals from infrared limb emission spectra to simplifying approximations in radiative transfer modelling

The accuracy of trace gas abundances retrieved from spectrally resolved infrared limb emission measurements relies, among other things, on the appropriate modelling of radiative transfer through the actual atmosphere. We quantify the mapping of several commonly applied simplifications in radiative transfer modelling on the trace gas abundances retrieval error at the example of the Michelson interferometer for passive atmospheric sounding (MIPAS)/environmental satellite (ENVISAT) space borne Fourier transform infra-red limb emission experiment. The Karlsruhe optimized and precise radiative transfer algorithm (KOPRA) which was used as a tool for this study will be introduced. KOPRA supports accurate modelling of the particular instrument requirements of MIPAS and the observation scenarios during the ENVISAT mission, in particular with respect to its viewing direction and its altitude coverage of the atmosphere. We show that disregarding of horizontal temperature inhomogeneities and non-local thermodynamic equilibrium effects, insufficient accuracy in modelling of field-of-view and apodisation effects, and disregarding individual profiles of isotopic species play the key roles in radiative transfer modelling and lead to systematic retrieval errors which can by far exceed the expected random retrieval error caused by measurement noise.

Stratospheric ClONO2 and HNO3 profiles inside the Arctic vortex from MIPAS‐B limb emission spectra obtained during EASOE

Vertical profiles of ClONO2 and HNO3 inside the Arctic vortex have been retrieved from infrared limb emission spectra recorded during balloon flights on January 13 and in the night of March 14/15, 1992 from Esrange, Sweden (68°N) as part of the European Arctic Stratospheric Ozone Experiment (EASOE). The instrumentation used was the cryogenic Michelson Interferometer for Passive Atmospheric Sounding, Balloon-borne version (MIPAS-B). Low ClONO2 abundances in mid-January indicate that a significant portion of ClONO2 had already been converted at that time. An unexpectedly high ClONO2 amount (1.8 to 3.1 ppbv between 16.1 and 21.5 km altitude) has been inferred from the March flight data. This implies that obviously most of the total available chlorine (ClOy) in the lower stratosphere was then in this reservoir molecule. The measured HNO3 profiles give no hint of any significant layered removal of gaseous HNO3 by condensation on particles or/and sedimentation.

Remote sensing of the middle atmosphere with MIPAS

On 1 March 2002 the Envisat research satellite has been launched successfully into its sun-synchronous orbit. One of its instruments for atmospheric composition sounding is the Michelson Interferometer for Passive Atmospheric Sounding, a limb-scanning mid-infrared Fourier transform spectrometer. Different scientific objectives of data users require different approaches to data analysis, which are discussed. A strategy on how to validate the involved algorithms and relevant strategies is presented.

A new non-LTE retrieval method for atmospheric parameters from mipas-envisat emission spectra

Abstract The Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) is a high-resolution limb sounder on board the European polar platform ENVISAT, scheduled to be launched in 2001. A large number of atmospheric trace gases relevant to stratospheric ozone chemistry and global change are expected to be retrieved from the IR spectra covering a wide spectral range. While operational data analysis under responsibility of the European Space Agency is limited to conditions of local thermodynamic equilibrium (LTE), the analysis of limb radiances affected by non-LTE is left to scientific institutions. In this paper we present an innovative non-LTE retrieval method as part of the MIPAS semi-operational data processor developed at the Institut fur Meteorologie und Klimaforschung (IMK). The new approach enables the treatment of vibrational, rotational, and spin non-LTE as well as a dependence of the non-LTE state distribution on the retrieval target quantities. In a case study, the method has been tested for its application to the non-LTE analysis of 5.3 μm MIPAS radiances. The fundamental ro-vibrational band of nitric oxide emitting at 5.3 μm shows strong non-LTE effects arising from vibrational excitation of stratospheric NO and superposed thermospheric non-LTE emissions. A conventional non-LTE retrieval approach using ab initio vibrational temperatures of NO cannot be applied due to rotational and spin non-LTE in the thermosphere, and the dependence of stratospheric vibrational temperatures on the NO abundance itself. The ability of the developed non-LTE inversion tool to retrieve stratospheric NO abundances is demonstrated by retrieval simulations. The further application of this method to the simultaneous retrieval of NO and kinetic temperature in the thermosphere and the retrieval of important non-LTE process parameters has also been tested.

Simultaneous measurements of HDO, H2O, and CH4 with MIPAS‐B: Hydrogen budget and indication of dehydration inside the polar vortex

For the first time, vertical profiles of HDO inside the Arctic vortex along with CH4 and H2O were retrieved from nighttime infrared limb emission spectra measured by the Michelson Interferometer for Passive Atmospheric Sounding, Balloonborne instrument (MIPAS-B) from Kiruna (Sweden, 68°N) on February 11, 1995 and March 24, 1997. The deuterium to hydrogen ratio (D/H) of water vapor shows a strong depletion in comparison to that of standard mean ocean water (SMOW), particularly in the lower stratosphere for the February 1995 flight. This extraordinarily strong depletion indicates an additional isotopic effect due to dehydration by polar stratospheric cloud particles. The maximum dehydration occurs at a lower altitude than that of the denitrification measured on the same flight. A dehydration of up to 0.7(±0.4) ppmv is seen in the compact correlation between CH4 and H2O. For the March 1997 results the exceptionally low D/H ratios and a deviation from the linear H2O-CH4 correlation could not be found. The H2O results of the February 1995 flight show a peak mixing ratio of 7.1 ppmv at 17.1 hPa and a minimum of 3.6 ppmv at 137.5 hPa. The analysis of the March flight shows a similar profile, but the vertical gradient is less pronounced. The total hydrogen budget of the stratosphere was examined by evaluating the quantity [H] = [H2O] + 2[CH4], revealing values of around 7.25 ppmv on average for both flights. All profiles reflect the subsidence of polar vortex air.

Optimized spectral microwindows for data analysis of the Michelson Interferometer for Passive Atmospheric Sounding on the Environmental Satellite

For data analysis of the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) atmospheric limb emission spectroscopic experiment on Environmental Satellite microwindows, i.e., small spectral regions for data analysis, have been defined and optimized. A novel optimization scheme has been developed for this purpose that adjusts microwindow boundaries such that the total retrieval error with respect to measurement noise, parameter uncertainties, and systematic errors is minimized. Dedicated databases that contain optimized microwindows for retrieval of vertical profiles of pressure and temperature, H2O, O3, HNO3, CH4, N2O, and NO2 have been generated. Furthermore, a tool for optimal selection of subsets of predefined microwindows for specific retrieval situations has been provided. This tool can be used further for estimating total retrieval errors for a selected microwindow subset. It has been shown by use of this tool that an altitude-dependent definition of microwindows is superior to an altitude-independent definition. For computational efficiency a dedicated microwindow-related list of spectral lines has been defined that contains only those spectral lines that are of relevance for MIPAS limb sounding observations.

stratospheric trace gas concentrations in the Arctic polar night derived by FTIR‐spectroscopy with the Moon as IR light source

In winter 1992/93 we have performed ground-based FTIR measurements in the Arctic (79°N, 12°E) to derive column densities of stratospheric trace gases within the polar vortex. Due to the polar night the moon had to serve as infrared light source instead of the sun. It was possible to perform FTIR-measurements for about a week around full moon. Column densities of N2O, CH4, HF, HCl, O3, NO2, HNO3 and ClONO2 have been obtained. The spectra were recorded at 0.01 cm−1 or 0.02 cm−1 resolution using MCT or InSb detectors. Measurements with an aerosol lidar performed at the same site show that polar stratospheric clouds (PSCs) appeared several times during the polar night. It seems that the concentrations of the stratospheric trace gases are strongly influenced by the occurrence of PSCs.

Tomographic retrieval of atmospheric parameters from infrared limb emission observations

Typical inversion of limb-sounding measurements assumes local horizontal homogeneity of the atmosphere. This simplification corresponds to spectral radiance errors that can exceed the noise level of a typical infrared instrument by a factor of 10 and causes errors in retrieved state parameters. To avoid these errors and to take the horizontal structure of the atmosphere into account, a two-dimensional (2D) tomographic sequential estimation approach is described. Application to temperature retrievals from simulated measurements yields typical retrieval errors of the order of 1 K, and a one-dimensional retrieval with the same synthetic measurements shows differences to the true values up to 10 K in regions with strong horizontal inhomogeneities. The horizontal resolution of the 2D retrieval is even better (up to 40 km) than the horizontal tangent point spacing.

Karlsruhe optimized and precise radiative transfer algorithm. Part I: requirements, justification, and model error estimation

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 ESAs 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.