Spectrally Resolved Fluxes from IASI Data: Retrieval Algorithm for Clear-Sky Measurements

Abstract

3Space-based measurements of the Outgoing Longwave Radiation (OLR) are essential for the study of the Earth's climate system. While the CERES instrument provides accurate measurements of this quantity, its measurements are not spectrally resolved. Here we present a high-resolution OLR product (sampled at 0.25 cm −1 ), derived from measurements of the IASI satellite sounder. The applied methodology relies on pre-calculated Angular Dependent Models (ADMs). These are usually calculated for ten to hundreds of different scene types (characterized by surface and atmosphere parameters).To guarantee accurate results in the range 645-2300 cm −1 covered by IASI, we constructed ADMs for over 140,000 scenes. These were selected from one year of CAMS reanalysis data. A dissimilarity-based selection algorithm was applied to choose these scenes as different from each other as possible, thereby maximizing the performance on real data, whilst keeping the number of scenes manageable. A comparison of the IASI OLR integrated over the 645-2300 cm −1 range was performed with the longwave broadband OLR products from CERES and the AIRS instrument. The latter are systematically higher due to the contribution of the far infrared to the total IR spectral range, but as expected exhibit generally high spatial correlations with the IASI OLR, except for some areas in the tropical region. We also compared the IASI OLR against the spectrally resolved OLR derived from AIRS. A good agreement was found above 1200 cm −1 while AIRS OLR appeared to be systematically higher in the atmospheric window region, likely related to differences in overpass time or to the use of a different cloud detection algorithm.The top-of-the-atmosphere (TOA) thermal flux, also referred to as the Earth's Outgoing Long- 45 wave Radiation (OLR) [W m −2 ], represents the total radiation emitted by the Earth-atmosphere 46 system into space. As part of the Earth's radiation budget, it reflects how the Earth-atmosphere sys-47 65 by using empirical angular distribution models (ADMs) which allow characterizing the angular 66 dependence of the Earth's radiation field for different scene types (denoted s) with distinct surface 67 and atmospheric anisotropic characteristics. Each of these ADMs consists of a set of anisotropic 68 factors (R s,ν (θ ,φ )) defined as the ratio of the equivalent Lambertian flux to the actual flux (Smith 69 et al. 1986; Suttles et al. 1988) that directly link the directional radiance observations to the OLR 70(for simplicity, the dependencies on x, y and t for L ν (θ , φ ) and F ν are omitted hereafter):The ADMs can be derived directly from the radiance measurements relying on the so-called 72 sorting-into-angular bins method (Suttles et al. 1989) or calculated from synthetic spectra con-73 structed for different angles of view using radiative transfer models (e.g. Huang et al. 2008). 74 The very first OLR measurements were collected by the Explorer VII satellite launched in 75 1959 (Suomi 1960), which operated during a period of 7 months. It was not un...

Artificial Neural Networks to Retrieve Land and Sea Skin Temperature from IASI

“…The interest in exploiting highly spectrally resolved IASI data to study climate variability has been previously highlighted [21][22][23][24]; preliminary T skin trends from Metop-A IASI measurements were presented at the SPIE conference [25]. Although the spectral signature of climate variability and T skin anomalies have been studied for similar instruments [26,27], relatively little has been done to generate systematic climate-data records for surface and atmospheric parameters with IASI measurements.…”

Spectrally resolved outgoing longwave radiation and its applications for the study of climate

“…In the framework of the IASI-FT project, I have developed an algorithm to derive the SR-OLR with a very high spectral sampling (0.25 cm -1 ) from the measurements of the Infrared Atmospheric Sounding Interferometer (IASI) instruments (Clerbaux et al, 2019;Hilton et al, 2012) onboard the Eumetsat Metop series of polar-orbiting satellites (Whitburn et al, 2020). This is the first retrieval algorithm that allows the SR-OLR to be derived with such a high spectral sampling, regardless of the satellite instrument.…”

“…As previously mentioned, the fundamental quantity measured by satellite infrared sounders is the directional radiance. (Whitburn et al, 2020).…”

“…In contrast, the effect of increasing atmospheric CO 2 concentrations is clearly visible on the trend maps of two different CO 2 sensitive channels (top right and middle left panels): as the rise of CO 2 leads to an increase of the fraction of SR-OLR absorbed, the trends observed are negative. The trends in the ozone-sensitive channel reflect, for their part, the impact of the ENSO phenomena on the global tropospheric-stratospheric circulation known as Brewer-Dobson (Whitburn et al, 2020). circulation (BDC).…”

“…(Bottom) An example is given of a thermal infrared spectrum with the region of absorption of the six principal absorbers superimposed (H 2 O, CO 2 , O 3 , N 2 O, CO, CH 4 ). Taken from(Whitburn et al, 2020).…”