Anu Dudhia

Abstract:

The recent development of satellite instruments that obtain spectrally resolved measurements of the atmosphere has highlighted the problem of how to determine the best subsets, or microwindows, of such spectra for retrievals of temperature and composition. A technique is described that maximizes the information content (or some other figure of merit) based on the modeling of the propagation of systematic as well as random error terms through the retrieval process. Apart from selecting microwindows, this technique can also prioritize existing microwindows for different circumstances and provides a full error analysis of the retrieval. A practical application is demonstrated for the Michelson Interferometer for Passive Atmospheric Sounding limb-viewing interferometer, but the technique is equally applicable to nadir-viewing instruments.

Abstract:

Limb infrared spectra calculated by the Optimized and Precise Radiative Transfer Algorithm (KOPRA), GENLN2, Reference Forward Model (RFM), and Simulation Program for Infrared Radiative Transfer (SPIRT) radiative transfer algorithms were intercompared for both local thermodynamic equilibrium (LTE) and non-local thermodynamic equilibrium (non-LTE) conditions, in the latter case for given profiles of vibrational temperatures. The overall agreement is usually better than approximately 0.5%, except for isolated phenomena that have been assessed in more detail. Most spectral radiance discrepancies occur also under LTE conditions and are attributed to different treatment of far wings of lines, integration schemes, and mass-weighted averaging of atmospheric temperature and pressure along slant path segments as well as different spectral sampling during the calculation of monochromatic radiances. Copyright 2002 by the American Geophysical Union.

Abstract:

Limb infrared spectra calculated by the KOPRA, GENLN2, RFM, and SPIRT radiative transfer algorithms were intercompared with special emphasis on non-local thermodynamic equilibrium emission. The overall agreement is usually better than approximately 0.5 percent except for single phenomena which have been assessed in more detail. Most spectral radiance discrepancies occur also under local thermodynamic equilibrium conditions and are attributed to different treatment of far wings of lines, integration schemes and mass-weighted averaging of atmospheric state temperature and pressure along slant path segments, as well as different spectral sampling during the calculation of monochromatic radiances. Additional differences in the non-local thermodynamic equilibrium case are presumably due to the different treatment of inhomogeneities of vibrational temperatures with height during numeric integration of radiative transfer.

Abstract:

Atmospheric measurements from new high spectral resolution remote sensing instruments will contain a huge amount of information. To use these data efficiently it is necessary to select a subset of the measurements for use in the retrieval of atmospheric profiles. The use of sections of spectrum, 'microwindows', increases the efficiency of forward model calculations. An objective scheme has been developed to select microwindows, which are optimal in spectral and altitude range, using the reduction in retrieval error. The method has been applied to MIPAS-B measurements, a balloon-borne Fourier transform infrared spectrometer. Results show reasonable retrieval errors. An assessment of the best achievable accuracy has been made, and the most important error sources are thus demonstrated.

Abstract:

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.