M. Carlotti

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.

Global-fit approach to the analysis of limb-scanning atmospheric measurements.

A method for the retrieval of concentration profiles of atmospheric constituents from spectra, recorded by balloon-borne spectrometers with the limb-scanning technique, is presented. The method uses a nonlinear least-squares fit procedure to fit simultaneously the whole concentration profile on a limb-scanning sequence of spectra. A use in interferometric measurements of the stratospheric emission is shown and a comparison is discussed with the results obtained from the analysis of the same data set, by using the onion-peeling method in which the error propagation, over concentrations, is taken into account. With the global-fit, error bars smaller than with the onion-peeling analysis are obtained. Computational details are also discussed.

Geo-fit approach to the analysis of limb-scanning satellite measurements

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.

First results of MIPAS/ENVISAT with operational Level 2 code

Abstract Michelson interferometer for passive atmospheric sounding (MIPAS) is operating on board of the ENVISAT satellite and is acquiring for the first time high spectral resolution middle infrared emission limb sounding spectra of the Earth atmosphere from space. An optimized code was developed for the Level 2 near real time analysis of MIPAS data. The code is designed to provide, in an automated and continuous way, atmospheric vertical profiles of temperature, pressure and concentrations of O3, H2O, CH4, HNO3, N2O and NO2, in the altitude range from 12 to 68 km. The performances of the code are herewith derived from the analysis of the first measurements acquired with this instrument. The assumptions made for the development of the optimized code are verified with the real data. The diagnostics of the instrument performances provide indications that there is good agreements with the results obtained by the Level 1 analysis. Consistent geophysical data are retrieved which is a first step towards a more complete assessment of retrieval accuracy. The tests have identified the possibility of measurement improvements by way of some secondary operations such as a correction of the frequency scale and the use of cloud filtering. However, no change in the algorithm baseline appears to be necessary.

The far infrared spectrum of 14N16O2

High resolution Fourier transform spectra in the 8–200 cm-1 spectral region have been used to analyse the pure rotation spectrum of nitrogen dioxide. In this way, the spin rotation levels of the (000) state were accurately measured for Ka up to 14 and N up to 54. Using a hamiltonian which takes the spin-rotation and the hyperfine operators explicitly into account, it has been possible to derive a complete set of molecular parameters (rotational, spin-rotation and hyperfine constants) for the (000) state of 14N16O2 from these experimental data and from the available microwave measurements. Numerous perturbations due to the hyperfine Fermi contact operator were analysed as well as a local resonance [42 0 42, J = 41·5] ↔ [41 2 40, J = 41·5] due to the electron spin-rotation interaction. Finally, a synthetic spectrum of the (000) ← (000) band of 14N16O2 including all hyperfine transitions has been computed, covering the 0–235 cm-1 spectral region.

The spectrum of CH3OH between 100 and 200 cm−1: Torsional and “forbidden” transitions

Abstract In the framework of a systematic investigation of the infrared and far infrared spectrum of CH3OH by high-resolution Fourier transform spectroscopy, we present a catalog of 6000 absorption lines between 100 and 200 cm−1 (which means practically all the measured lines in this region which are above a reasonable peakfinder threshold), and a list of 955 recently assigned lines below 100 cm−1, which had not been included in a previous paper. The Taylor expansion coefficients for evaluating the energies of the levels involved in the transitions are also given. Most of the lines occurring in the 100–200 cm−1 region involve torsionally excited states. Another interesting feature of this region is the presence of a forbidden Q branch (Δn = 1 and ΔK = 0 simultaneously). Two further forbidden Q branches, occurring at higher wavenumbers, are also presented.

Far infrared spectrum and spectroscopic constants of AsH3 in the ground state

Abstract The far ir spectrum of arsine, AsH 3 , was recorded in the range 25–100 cm −1 with a resolution of approximately 0.004 cm −1 . Δ J = +1, Δ K = 0 rotational transitions were measured and assigned up to J ″ = 12. These transitions, together with the presently available microwave and submillimeter-wave data and ground state combination differences, were analyzed on the basis of a rotational Hamiltonian which includes Δ k = ±3 and Δ k = ±6 interaction terms. The derived ground state molecular parameters reproduced the transition frequencies of both allowed and “perturbation allowed” transitions within the accuracy of the measurements. The equilibrium structure was determined for the AsH 3 molecule.

Detection of atomic oxygen and further line assignments in the far-infrared stratospheric spectrum

The analysis of the high resolution far infrared emission spectrum of the stratosphere has made possible the identification of two features due to atomic oxygen. Several other new features have been assigned on the basis of updated line compilations. The possible detection of HBr, HO2 and H2O2 in the far infrared is discussed.

The Fourier spectrum of CH3OH: The region between 8 and 40 cm−1

Abstract The experimental Fourier spectrum of CH 3 OH has been investigated between 8 and 40 cm −1 . Good agreement was found between the experimental measurements and the results of the computational routines available up to now when low J values ( J ≲ 10) are involved. At higher J , the line assignments are possible by means of Taylor expansions of the energy levels. A catalog of almost 1500 lines, two-thirds of which have been assigned, is presented.

Multi-target retrieval (MTR): the simultaneous retrieval of pressure, temperature and volume mixing ratio profiles from limb-scanning atmospheric measurements

In this paper we describe a retrieval approach for the simultaneous determination of the altitude distributions of p, T and VMR of atmospheric constituents from limb-scanning measurements of the atmosphere. This analysis method, named multi-target retrieval (MTR), has been designed and implemented in a computer code aimed at the analysis of MIPAS-ENVISAT observations; however, the concepts implemented in MTR have a general validity and can be extended to the analysis of all type of limb-scanning observations. In order to assess performance and advantages of the proposed approach, MTR has been compared with the sequential analysis system implemented by ESA as the level-2 processor for MIPAS measurements. The comparison has been performed on a common set of target species and spectral intervals. The performed tests have shown that MTR produces results of better quality than a sequential retrieval. However, the simultaneous retrieval of p, T and water VMR has not lead to satisfactory results below the tropopause, because of the high correlation occurring between p and water VMR in the troposphere. We have shown that this problem can be fixed extending the MTR analysis to at least one further target whose spectral features decouple the retrieval of pressure and water VMR. Ozone was found to be a suitable target for this purpose. The advantages of the MTR analysis system in terms of systematic errors have also been discussed.