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

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 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/m 2 from that calculated using ECMWF. The error with which the far infrared flux is determined by REFIR-PAD is about 0.4 W/m 2 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.

Analysis of Water Vapor Absorption in the Far‐Infrared and Submillimeter Regions Using Surface Radiometric Measurements From Extremely Dry Locations

“…Six instruments making spectrally resolved radiation measurements were deployed to the Cerro Toco site for RHUBC‐II. Two instruments provided measurements throughout the far‐IR, the Radiation Explorer in the Far‐Infrared‐Prototype for Applications and Development (REFIR‐PAD; Palchetti et al, , ) and the Far‐Infrared Spectroscopy of the Troposphere (FIRST; Mlynczak et al, ). Closure analysis related to the REFIR‐PAD measurements is presented in this paper (section ), and this analysis is validated using FIRST observations (section ).…”

Remote measurement of high preeruptive water vapor emissions at Sabancaya volcano by passive differential optical absorption spectroscopy

“…Likewise, the water vapor VCD above a given location will typically vary with season as the air temperature changes [Gaffen et al, 1992]. The water vapor concentration decreases rapidly with altitude in the troposphere [Committee for the Extension of the U.S. Standard Atmosphere, 1976;Cahen et al, 1982;Palchetti et al, 2007], as temperature also decreases. Strikingly, a temporary decrease in global average H 2 O VCDs was even observed ensuing the 1991 eruption of Mount Pinatubo, as global-mean lower tropospheric temperature cooled by about a half degree Celsius for several years [Soden et al, 2002].…”

Water vapor sounding with the far infrared REFIR-PAD spectroradiometer from a high-altitude ground-based station during the ECOWAR campaign

“…The REFIR‐PAD instrument is a Fourier transform spectroradiometer designed to operate both from balloon platform, in the nadir and limb view observation geometries [ Palchetti et al , 2006, 2007], and from the ground in the zenith looking geometry [ Bianchini et al , 2007]. The instrument is characterized by a compact, uncooled design, with a misalignment‐compensated optical scheme that can provide wideband (100–1100 cm −1 ) atmospheric emission spectra with a 0.5 cm −1 spectral resolution (unapodized).…”