Abstract
Abstract. 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 spectrally-resolved 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.
Highlights
Water is the principal molecule that governs the climate on Earth
The results of the first flight performed by REFIR-PAD in June 2005 in tropical region have been shown
The instrument measured spectra of the outgoing longwave radiation from 100 to 1400 cm−1. This spectrally resolved measurement made possible the retrieval of the atmospheric state with sufficient precision to improve the accuracy with which the integrated outgoing radiation flux can be calculated, proving that spectral information can be used to infer the angular distribution of radiance
Summary
Water is the principal molecule that governs the climate on Earth. This is because, through the hydrological cycle, it is involved in all phenomena from energy transport to radiative effects governing the climate system (Pierrehumbert, 2002). Palchetti et al.: Spectral observation of the outgoing longwave radiation that are difficult to observe in laboratory conditions and require a field validation Within this context, Sinha and Harries (1995) pointed out the lack of validation of far infrared (FIR) model line parameters of water vapour in atmospheric conditions, and stressed that FIR parameterisation in climate models should be validated by means of observational programmes. The available operative sensors give no direct information on cloud microphysics and cirrus clouds represent a major observational gap In this context, in June 2005, the first wideband spectrallyresolved measurements of the atmospheric thermal emission, including the FIR portion, were performed from a stratospheric balloon platform. REFIR-PAD measurements, described, were performed using an FTS with a Mach-Zehnder configuration that covered the 100–1400 cm−1 spectral range with a resolution of 0.475 cm−1 As described, this spectral measurement enabled us to perform the retrieval of temperature and water vapour vertical profiles up to the upper troposphere level.
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