The spectrally-resolved characterization of the atmospheric emission has a fundamental role in the study of the
Earth radiation balance, and only a measurement performed in a wide spectral range enables us to separate
the contributions to the radiative balance due to the different altitudes, constituents and physical phases. The
REFIR-PAD (Radiation Explorer in the Far InfraRed-Prototype for Application and Development) Fourier
transform spectroradiometer can perform a characterization of the broadband radiative signature of clouds and
aerosols, with the only limitation of the need of low levels of water vapour like those that are obtainable in high
altitude stations during winter. The capabilities of this kind of measurement have been assessed in a series of
test campaign performed in Tuscany during the winter of 2006/2007 when atmospheric emission spectra have
been acquired in various transparency conditions and the evidence of transparency in the far-infrared region
below 600 cm-1 has been demonstrated. REFIR-PAD operates in the spectral range extending from 100 to 1400
cm-1 with a resolution of 0.5 cm-1, using room-temperature detectors and optics and a compact,
misalignment-compensated
design. The instrument, developed at IFAC-CNR, Florence, has been successfully deployed in
several campaigns, both in the ground based zenith-looking geometry and in the nadir-looking balloon borne
configuration. The operating spectral range of the REFIR-PAD spectroradiometer encompasses great part of the
atmospheric emission spectrum, from the relatively unexplored far-infrared region below 400 cm-1, dominated
by water vapour rotational band, to the atmospheric transparency window, where a number of atmospheric
instruments are already operating and can provide intercomparison data.
On June 30th, 2005 the REFIR-PAD (Radiation Explorer in the Far InfraRed-Prototype for Application and
Development) Fourier transform spectroradiometer performed the first wide-band spectral characterization of the
top-of-atmosphere emitted radiation in the far-infrared with an uncooled instrument. The nadir emitted radiance
has been measured down to 100 cm-1, thus covering a spectral interval that, until now, was nearly unexplored,
and up to 1400 cm-1, including the well characterized atmospheric window region, in which it is possible to
perform comparison and intercalibration with operative instruments. The measurements were performed at an
altitude of 34 km, from a stratospheric balloon launched in tropical region, near Teresina (Brazil). The acquired
spectra have a spectral resolution of 0.5 cm-1. It should be noted that despite the operating spectral range
extending to the far-infrared region, REFIR-PAD does not require any cooled components, thanks to the use of
pyroelectric detectors and an optical scheme that compensates for the instrument self-emission. This work shows
the results of the analysis of the spectra, focusing on the far infrared portion of the atmospheric emitted radiance.
The retrieval of the vertical profiles of water vapour and temperature during the flight is presented. The vertical
resolution of the retrieval is 2 km in the upper troposphere-lower stratosphere (UTLS) region, and lower at
higher altitudes. The comparison with ECMWF for validation is also shown. Besides the characterization of
temperature and water vapour, from the analysis of the emitted radiance useful information can be gathered
about cloud and aerosol contribution to radiation budget.
The REFIR-PAD (Radiation Explorer in the Far InfraRed-Prototype for Applications and Development) Fourier transform spectroradiometer has successfully performed, at the end of June, 2005, a stratospheric balloon flight from Teresina, Brazil. The instrument has provided 8 hours worth of nadir-looking spectra acquired with a resolution of 0.5 cm-1 in the 100 to 1400 cm-1 spectral range, thus covering both the far-infrared range, containing the radiative signature of the upper tropospheric water vapour, and the better-known mid-infrared range, which provides validation with existing instruments. From the analysis of the calibrated spectra we obtain valuable information on the contribution to the Earth's outgoing long-wave radiation of water, both in the vapour and cloud form, in a region of the atmosphere, the upper-troposphere/lower-stratosphere, in which this contribution has a critical role.
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 Earth's 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 Earth's 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.
KEYWORDS: Interferometers, Sensors, Fourier transforms, Signal to noise ratio, Spectroscopy, Calibration, Signal detection, Pyroelectric detectors, Far infrared, Space operations
In the framework of the Radiation Explorer in the Far InfraRed space mission for the characterization in the far infrared of the Earth outgoing emission, a breadboard version of the Fourier transform spectrometer, which is the core instrument of the payload package, has been developed. The Fourier transform spectrometer operates in the spectral range of 100-1100 cm-1 with a resolution of 0.5 cm-1, 6.5 s acquisition time, and signal-to-noise ratio better than 100. It is a compact prototype designed both for laboratory applications and for field campaigns, in particular for operations in high-altitude ground-based sites and on-board of stratospheric balloon platforms.
This paper describes the instrument characterization performed in laboratory conditions and under vacuum. The study has allowed to study the trade-off among all the instrument parameters and to test the new optical design of the interferometer, with particular attention to the photolithographic beam splitters and the room-temperature pyroelectric detectors.
The instrument was operated for the first time in a field campaign in June 2004 from 1247~m altitude ground-based site located in the South of Italy. The results of this test and the comparison with the measurements taken with a BOMEM spectrometer, that partially overlaps the REFIR bandwidth in the higher spectral region, are reported. Info can be found at http://radiation.ifac.cnr.it.
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