B. M. Dinelli

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.

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.

Ab initio ro-vibrational levels of H3+ beyond the Born-Oppenheimer approximation

Abstract The value of the adiabatic correction to the Born-Oppenheimer electronic energy is calculated as a function of geometry for H3+ using SCF wavefunctions. A mass-dependent adiabatic function is combined with the near-Born-Oppenheimer electronic structure calculations of Rohs, Kutzelnigg, Jaquet and Klopper and the rotation-vibration energy levels of H3+ and D3+ are calculated. The levels for H3+ are significantly better than any previous ab initio estimates but are less accurate than those obtained by recent spectroscopically determined effective H3+ potentials. The adiabatic correction is less important for the heavier D3+. For both ions rotational levels are obtained to near experimental accuracy. Small, systematic shifts in the vibrational bands may be attributable to residual errors in the Born-Oppenheimer potential.

SPECTROSCOPICALLY DETERMINED BORN-OPPENHEIMER AND ADIABATIC SURFACES FOR H3+, H2D+, D2H+, AND D3+

High resolution spectroscopic data for H3+, H2D+, D2H+, and D3+ is used to determine effective, mass‐dependent potential energy surfaces for each isotopomers. These surfaces are expressed as a sum of the mass‐independent Born–Oppenheimer (BO) potential and a mass dependent adiabatic correction. For H3+ and D3+ the adiabatic correction is a single surface of the same symmetry and functional form as the BO surface. For H2D+ and D2H+ a second, lower symmetry surface is required. Fits to all three surfaces were started from recent, high quality ab initio calculations. The standard deviations for fits using all the available data with J≤9 are 0.015 cm−1 for H3+, 0.010 cm−1 for H2D+ and D2H+ combined, and 0.015 cm−1 for D3+. These values are close to the intrinsic experimental error of much of the data and improve on the corresponding ab initio surfaces by at least an order of magnitude. The fits are very compact: nearly 1600 data are fitted by adjusting 36 constants and freezing 51 at their ab initio values.

Asymmetric adiabatic correction to the rotation–vibration levels of H2D+ and D2H+

Calculations on H2D+ and D2H+ have shown that the energy levels of these asymmetric isotopomers of H3+ cannot be reproduced using effective potential energy surfaces with D3h symmetry. It is shown that for these ions the adiabatic correction to the Born–Oppenheimer approximation has an asymmetric component which can be expressed as a mass‐independent surface multiplied by a mass factor. An expression for this function is obtained from ab initio calculations. Use of this adiabatic correction is found to resolve the discrepancy with the levels of H2D+ and D2H+. The ab initio calculations reported reproduce the observed H2D+ transitions with an average error (obs−calc) of −8 MHz for the rotational transitions, −0.06 cm−1 for the ν1 band, −0.13 cm−1 for ν2, and −0.19 cm−1 for ν3. These errors are nearly constant for all transitions within a vibrational band. This gives a very accurate ab initio framework for predicting unobserved transition frequencies.

Bands of H3+ up to 4ν2 : rovibrational transitions from first principles calculations

Abstract Band origins and Einstein A coefficients are presented for transitions linking all the vibrational levels of H3+ up to 4ν2(l = 4). The importance of these data for astrophysical modeling is discussed. For H3+, rovibrational wavefunctions and energy levels are obtained for J up to 16. These are used to synthesize sample rovibrational spectra with particular emphasis on the ν1 + 2ν2(l = 2) ← 0 and 4ν2(l = 2) ← 0 bands, which would appear to be good candidates for laboratory observation.

Observation and analysis of the ν3 band of NH+3

The ν3 degenerate vibration–rotation band of the ammonia cation NH+3 was observed and analyzed. The spectrum was detected in direct absorption using a tunable difference frequency spectrometer combined with velocity modulation. The ion was produced in a 6 kHz ac discharge with a gas mixture of He:H2:NH3 (∼250:8:1) and with a total pressure of ∼6 Torr. Spin–rotation splittings for most Q‐branch transitions were well resolved and spin–rotation interaction constants were determined. A symmetric rotor Hamiltonian with A1–A2 splittings and l resonance was used to analyze the spectrum. The spectral pattern indicates that NH+3 is a planar molecular with D3h symmetry, consistent with a 2A‘2 ground electronic state.

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.

Measurement of stratospheric distributions of H2 16O, H2 18O, H2 17O, and HD16O from far infrared spectra

The stratospheric concentration profiles of H216O and of the isotopic species H218O, H217O, and HD16O, are retrieved from far-infrared emission spectra recorded by a balloon-borne Fourier transform spectrometer with 0.0035 cm−1 resolution. The retrievals were carried out using the global-fit procedure on a statistically significant number of transitions for each isotope in the 40–85 cm−1 spectral region. The concentration and the relative mixing ratio distributions of each isotope are given along with the resulting isotopic ratios. A comparison of these results with the natural isotopic abundances shows that there is a depletion of up to 56±8% of the D-containing isotope of water, while no significant deviation is obtained for the other isotopes within the error bars. A comparison of our results with previously reported measurements is also given.