Abstract
We present the first analysis of water vapour profiles derived from nadir measurements by the infrared imaging Fourier transform spectrometer GLORIA (Gimballed Limb Observer for Radiance Imaging of the Atmosphere). The measurements were performed on 27 September 2017, during the WISE (Wave driven ISentropic Exchange) campaign aboard the HALO aircraft over the North Atlantic in an area between 37°–50°N and 20°–28°W. From each nadir recording of the 2-D imaging spectrometer, the spectral radiances of all non-cloudy pixels have been averaged after application of a newly developed cloud filter. From these mid-infrared nadir spectra, vertical profiles of H2O have been retrieved with a vertical resolution corresponding to five degrees of freedom below the aircraft. Uncertainties in radiometric calibration, temperature and spectroscopy have been identified as dominating error sources. Comparing retrievals resulting from two different a priori assumptions (constant exponential vs. ERA 5 reanalysis data) revealed parts of the flight where the observations clearly show inconsistencies with the ERA 5 water vapour fields. Further, a water vapour inversion at around 6 km altitude could be identified in the nadir retrievals and confirmed by a nearby radiosonde ascent. An intercomparison of multiple water vapour profiles from GLORIA in nadir and limb observational modes, IASI (Infrared Atmospheric Sounding Interferometer) satellite data from two different retrieval processors, and radiosonde measurements shows a broad consistency between the profiles. The comparison shows how fine vertical structures are represented by nadir sounders as well as the influence of a priori information on the retrievals.
Highlights
Satellite instruments observing the upwelling thermal emission of the Earth’s surface and atmosphere with high spectral resolution are established as important tools for weather forecast, air quality, and atmospheric research
The maximum optical path difference (OPD) of the interferometer can be set between 8 cm and 0.8 cm which corresponds to a spectral sampling (defined here as 1/(2 OPD)) of 0.0625 cm−1 and 0.625 cm−1, respectively
We have examined for the first time the capability of the GLORIA
Summary
Satellite instruments observing the upwelling thermal emission of the Earth’s surface and atmosphere with high spectral resolution are established as important tools for weather forecast, air quality, and atmospheric research. 2021, 13, 3675 mid-infrared spectral region was the High-resolution Interferometer Sounder (HIS) [11] Another airborne instrument is NAST-I (NPOESS Airborne Sounder Testbed Interferometer), which showed its capability to measure atmospheric profiles of water vapour, ozone, carbon monoxide, and the surface temperature [12]. Measurements of ARIES [13] (Airborne Research Interferometer Evaluation System); helped to characterize the properties of clouds [14] and to validate IASI with the observations of water vapour and temperature profiles [15,16]. To sound the atmosphere below, GLORIA is capable to switch its line of sight to nadir mode on the HALO aircraft, making it the first airborne instrument with limb as well as nadir observational capabilities
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