Abstract
Abstract: Two data-reduction approaches for the Infrared Atmospheric Sounder Interferometer satellite instrument are discussed and compared. The approaches are intended for the purpose of devising and implementing fast near real time retrievals of atmospheric thermodynamical parameters. One approach is based on the usual selection of sparse channels or portions of the spectrum. This approach may preserve the spectral resolution, but at the expense of the spectral coverage. The second approach considers a suitable truncation of the interferogram (the Fourier transform of the spectrum) at points below the nominal maximum optical path difference. This second approach is consistent with the Shannon-Whittaker sampling theorem, preserves the full spectral coverage, but at the expense of the spectral resolution. While the first data-reduction acts within the spectraldomain, the second can be performed within the interferogram domain and without any specific need to go back to the spectral domain for the purpose of retrieval. To assess the impact of these two different data-reduction strategies on retrieval of atmospheric parameters, we have used a statistical retrieval algorithm for skin temperature, temperature, water vapour and ozone profiles. The use of this retrieval algorithm is mostly intended for illustrative purposes and the user could choose a different inverse strategy. In fact, the interferogram-based data-reduction strategy is generic and independent of any inverse algorithm. It will be also shown that this strategy yields subset of interferometric radiances, which are less sensitive to potential interfering effects such as those possibly introduced by the day-night cycle (e.g., the solar component, and spectroscopic effect induced by sun energy) and unknown trace gases variability.
Highlights
The Infrared Atmospheric Sounding Interferometer (IASI) is providing data of unprecedented spectral quality and accuracy
As anticipated the data reduction strategies are intended for near real time dissemination and processing of IASI data for the retrieval of temperature, water vapour and ozone
The retrieval performance could depend on the inversion algorithm, the retrieval performance difference between the R-method and the I-method is independent of the retrieval scheme, since it depends on the information content of spectral radiances or interferometric radiances for temperature, water vapour and ozone
Summary
The Infrared Atmospheric Sounding Interferometer (IASI) is providing data of unprecedented spectral quality and accuracy (see e.g., [1]). The assimilation of IASI radiances has produced a significant positive impact on forecast quality (e.g., [2]) and on the exploitation of trace gases information for atmospheric chemistry. The instrument has a spectral coverage extending from 645 to 2,760 cm−1 , which with a sampling interval ∆σ = 0.25 cm−1 gives. Data samples are taken at intervals of 25 km along and across track, each sample having a minimum diameter of about 12 km. With a swath width on Earth’s surface of about 2,000 kilometres, global coverage is achieved in 12 hours, during which the instrument records about 650,000 spectra
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