Luca Palchetti

Breadboard of a Fourier-transform spectrometer for the Radiation Explorer in the Far Infrared atmospheric mission

In preparation for a possible space mission, a breadboard version named REFIR-BB of the Radiation Explorer in the Far Infrared (REFIR) instrument has been built. The REFIR is a Fourier-transform spectrometer with a new optical layout operating in the spectral range 100-1100 cm(-1) with a resolution of 0.5 cm(-1), a 7-s acquisition time, and a signal-to-noise ratio of better than 100. Its mission is the spectral measurement in the far infrared of the Earths outgoing emission, with particular attention to the long-wavelength spectral region, which is not covered by either current or planned space missions. This measurement is of great importance for deriving an accurate estimate of the radiation budget in both clear and cloudy conditions. The REFIR-BB permits the trade-off among all instrument parameters to be studied, the optical layout to be tested, and the data-acquisition strategy to be optimized. The breadboard could be used for high-altitude ground-based campaigns or could be flown for test flights on aircraft or balloon stratospheric platforms. The breadboards design and the experimental results are described, with particular attention to the acquisition strategy and characterization of the interferometer. Tests were performed both in laboratory conditions and in vacuum. Notwithstanding a loss of efficiency above 700 cm(-1) caused by the poor performance of the photolithographic polarizers used as beam splitters, the results demonstrate the feasibility of using the spectrometer for space applications.

Retrieval of foreign-broadened water vapor continuum coefficients from emitted spectral radiance in the H2O rotational band from 240 to 590 cm −1

The paper presents a novel methodology to retrieve the foreign-broadened water vapor continuum absorption coefficients in the spectral range 240 to 590 cm(-1) and is the first estimation of the continuum coefficient at wave numbers smaller than 400 cm(-1) under atmospheric conditions. The derivation has been accomplished by processing a suitable set of atmospheric emitted spectral radiance observations obtained during the March 2007 Alps campaign of the ECOWAR project (Earth Cooling by WAter vapor Radiation). It is shown that, in the range 450 to 600 cm(-1), our findings are in good agreement with the widely used Mlawer, Tobin-Clough, Kneizys-Davies (MT CKD) continuum. Below 450 cm(-1) however the MT CKD model overestimates the magnitude of the continuum coefficient.

Spectral noise due to sampling errors in Fourier-transform spectroscopy

An assessment is made of the spectral noise in Fourier-transform spectroscopy caused by sampling errors in the interferogram acquisition. Numerical evaluations are performed in the case of the REFIR (radiation explorer in the far infrared) instrument developed for the measurement of the long-wavelength Earth emissions from satellite platforms. In this case the slow response of a room-temperature pyroelectric detector, the relatively short acquisition time, the broadband operation, and the wish for a relaxed requirement of the mirror drive accuracy make sampling error an important issue. Different sampling methods can be considered for reduction of the spectral noise induced by sampling errors. The effects of different sampling methods are quantified and discussed for the selection of the most-suitable option for this instrument. We find that only sampling methods that introduce some compensation (either analog or digital) of the frequency dependence of amplitude and phase components of the acquisition-system responsivity provide satisfactory results. In particular, the equal time sampling followed by a digital filter and numerical resampling has been examined minutely with a simulation model used to perform sensitivity tests of the main parameters that characterize the procedure.

Measurement of the water vapour vertical profile and of the Earth's outgoing far infrared flux

Our understanding of global warming depends on the accuracy with which the atmospheric components that modulate the Earth’s radiation budget are known. Many uncertainties still exist as regards the radiative effect of water in the different spectral regions, among which is the far infrared, where very few observations have been made. An assessment is shown of the atmospheric outgoing flux obtained from a balloon-borne platform with wideband spectrallyresolved nadir measurements at the top of the atmosphere over the full spectral range, from 100 to 1400 cm −1, made by a Fourier transform spectrometer with uncooled detectors. From these measurements, we retrieved 15 pieces of information regarding water vapour and temperature profiles and surface temperature, with a major improvement in our knowledge of water vapour in the upper troposphere. The retrieved atmospheric state made it possible to calculate the emitted radiance also at frequencies and zenith angles that have not been observed and to determine the outgoing spectral radiation flux. This proves that spectrally resolved observations can be used to derive accurate information on the integrated flux. While the retrieved temperature was in agreement with ECMWF analysis, the retrieved water vapour profile differed significantly; depending on the time and the location, the derived flux in the far infrared (20–600 cm −1) differed by 2–3.5 W/m2 from that calculated using ECMWF. The error with which the far infrared flux is determined by REFIR-PAD is about 0.4 W/m2 and is caused mainly by calibration uncertainties, while detector noise has a negligible effect. This proves that uncooled detectors are adequate for top-of-the-atmosphere radiometry. Correspondence to: L. Palchetti (l.palchetti@ifac.cnr.it)

A wide-band nadir-sounding spectroradiometer for the characterization of the Earth's outgoing long-wave radiation

A balloon-borne wide-band Fourier transform spectrometer named REFIR-PAD (Radiation Explorer in the Far InfraRed, Prototype for Applications and Development) has been developed at CNR-IFAC to perform the characterisation of the Earths outgoing long-wave radiation in the far-infrared region. The spectroscopic characterisation of this region is expected to increase greatly our level of knowledge of the radiative effects of water content in the upper troposphere. The REFIR-PAD instrument provides spectrally-resolved nadir-sounding radiance measurements in the 100-1400 cm-1 range, with a 0.5 cm-1 resolution, covering the most part of the Earths long-wave emission and including both the far-infrared and the better known middle-infrared region. REFIR-PAD was flown as a piggy-back payload on the CNES IASI-LPMAA stratospheric balloon gondola in June 2005 from Teresina, Brazil. The data collected in this mission, will provide valuable information for the development of a future space mission aimed to the operational monitoring of the upper troposphere water vapour and clouds in order to identify their climate signatures.

Design and mathematical modelling of the space-borne far-infrared Fourier transform spectrometer for REFIR experiment

A far infrared Fourier spectrometer named REFIR Radiation explorer in the far infrared has been designed for space-borne observations of the atmospheric radiance in the broad spectral range 100-1000 cm y1 . The requirements are discussed and the adopted instrumental configuration is presented. The variability of the source and the narrow frequency bandwidth of the detectors as well as some novel features of the selected configuration require some dedicated analysis for the establishment of the instrument performances which are herewith described. q 1999 Elsevier Science B.V. All rights reserved. PACS: 07.87.q v; 93.85.q q

Effect of beam-splitter emission in Fourier-transform emission spectroscopy

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.

Far-Infrared Radiative Properties of Water Vapor and Clouds in Antarctica

AbstractWater vapor and clouds are among the most important greenhouse components whose radiative features cover all the broad spectral range of the thermal emission of the atmosphere. Typically more than 40% of the total thermal emission of Earth occurs in the far-infrared (FIR) spectral region from 100 to 667 cm−1 (wavelengths from 100 to 15 µm). Nevertheless, this spectral region has not ever been fully covered down to 100 cm−1 by space missions, and only a few ground-based experiments exist because of the difficulty of performing measurements from high altitude and very dry locations where the atmosphere is sufficiently transparent to observe the FIR emission features. To cover this lack of observations, the Italian experiment “Radiative Properties of Water Vapor and Clouds in Antarctica” has collected a 2-yr dataset of spectral measurements of the radiance emitted by the atmosphere and by clouds, such as cirrus and polar stratospheric clouds, from 100 to 1,400 cm−1 (100–7 µm of wavelength), including...

Validation of H2O continuum absorption models in the wave number range 180–600 cm−1 with atmospheric emitted spectral radiance measured at the Antarctica Dome-C site

This work presents the results concerning the analysis of a set of atmospheric emitted (down welling) spectral radiance observations in the spectral range 180 to 1100 cm(-1) acquired at the Dome-C site in Antarctica during an extensive field campaign in 2011-2012. The work has been mainly focused on retrieving and validating the coefficients of the foreign contribution to the water vapour continuum absorption, within a spectral range overlapping the water vapour rotational band. Retrievals have been performed by using a simultaneous physical retrieval procedure for atmospheric and spectroscopic parameters. Both day (summer) and night (winter) spectra have been used in our analysis. This new set of observations in the far infrared range has allowed us to extend validation and verification of state-of-art water vapour continuum absorption models down to 180 cm(-1). Results show that discrepancies between measurements and models are less than 10% in the interval 350-590 cm(-1), while they are slightly larger at wave numbers below 350 cm(-1). On overall, our study shows a good consistency between observations and state-of-art models and provides evidence toward needing to adjust absorptive line strengths. Finally, it has been found that there is a good agreement between the coefficients retrieved using either summer or winter spectra, which are acquired in far different meteorological conditions.

SAFIRE-A (spectroscopy of the atmosphere by far-infrared emission-airborne): optimized instrument configuration and new assessment of improved performance.

An upgraded configuration of the SAFIRE-A Fourier transform far-infrared spectrometer was recently set up, and significant improvements in instrument performance were attained during several testing and scientific flights onboard the high-altitude research aircraft M55-Geophysica. New features were implemented in specific instrument subsystems, such as the pointing system, the reference laser interferometer, and the onboard calibration unit, to increase the overall instrument functionality and to obtain reliable operation from both the high-frequency (approximately 120 cm(-1)) and the low-frequency (approximately 23 cm(-1)) detection channels. Other changes, such as those made in the onboard recording system or in the postflight data-transfer procedure, were aimed at expanding the capability of unattended operation and at providing a user-friendly interface for data downloading and ground servicing. A detailed description of these modifications is given, along with a quantitative assessment of the SAFIRE-A instrument performance.