ABSTRACT
The processes of discretization, interpolation, and resampling are frequently used in data analysis. Here the formalism of functional spaces is used as a framework for the description and characterization of both the measurement operation and these subsequent processes. The tools provided by this formalism are applied to the problem of resampling of atmospheric volume mixing ratio vertical profiles obtained with limb-sounding measurements. In particular, a resampling method that uses the conservation of the vertical column as a constraint is presented and compared with other methods. The effects of the resampling process in terms of error propagation and loss of vertical resolution are also evaluated.
ABSTRACT
We propose a new approach to the analysis of limb-scanning measurements of the atmosphere that are continually recorded from an orbiting platform. The retrieval is based on the simultaneous analysis of observations taken along the whole orbit. This approach accounts for the horizontal variability of the atmosphere, hence avoiding the errors caused by the assumption of horizontal homogeneity along the line of sight of the observations. A computer program that implements the proposed approach has been designed; its performance is shown with a simulated retrieval analysis based on a satellite experiment planned to fly during 2001. This program has also been used for determining the size and the character of the errors that are associated with the assumption of horizontal homogeneity. A computational strategy that reduces the large number of computer resources apparently demanded by the proposed inversion algorithm is described.
ABSTRACT
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.
ABSTRACT
A simple model of an emission Fourier-transform spectrometer is provided for a beam splitter with infinitesimal thickness but with general optical properties otherwise. Using the principle of conservation of energy, we derived the rigorous expressions that relate the interferograms from three different signals (source, reference, and beam-splitter emission), which are simultaneously observed by the detector. The relationship among phase and amplitude of the three interferograms is evaluated. The results indicate that the behavior of a Fourier-transform emission spectrometer may be more complicated than what is assumed in some models. Calibration procedures should reflect the implications of this rigorous approach.
ABSTRACT
A recovery procedure has been developed to correct instrument line-shape distortions observed in Fourier transform spectroscopy. The procedure can be described as a phase-error correction performed in the spectral domain to correct for path-difference-dependent phase errors observed in sharp spectral features. The technique has been applied successfully to high-resolution atmospheric emission spectra. The inherent broadening of the real features has been separated accurately from instrumental distortions. Using models for the path-difference-dependent error sources and data from two narrow window regions at 50 and 118 cm(-1), we show that the distortion has a simple dependence on the spectral frequency.