A full physics algorithm to retrieve nighttime sea surface temperature with IASI: Toward an independent homogeneous long time-series for climate studies

Abstract

A fully physically-based algorithm is presented and used to retrieve nighttime sea-surface (skin) temperatures (SSTs) from spectra recorded by the Infrared Atmospheric Sounder Interferometer (IASI). The main advantage of the proposed approach, based on radiative transfer calculations, is to provide a dataset totally independent of any in-situ measurement or model. The SSTs are retrieved at the IASI spot resolution (clear sky) for the three Metop platforms, today leading to a continuous record over more than 13 years, which is planned to be extended for at least another decade. Five atmospheric transparency windows centered at 3.8, 4.0, 4.7, 9.0 and 11 μm have been used in the retrieval. Results are compared with in-situ temperatures provided by the buoys network as well as with other satellite datasets. This first analysis enables to draw the following three main conclusions. First, we confirm the interest of using short wavelengths around 4 μm which enables accurate estimates characterized by a reduced bias with respect to those around 9 and 11 μm, thanks to significantly weaker sensitivities to errors in the modeling of the water vapor contribution to the atmospheric absorption and emission. Second, we show that the observed dependences of the skin-depth temperature difference on various observed-scene parameters (wind speed, air-sea temperature difference, humidity) are consistent with predictions of theoretical and empirical models, as well as with those from other SST products. Finally, the overall averaged bias between our retrieved values converted to depth temperatures and those provided by in-situ measurements is −0.04 K (median: −0.024 K), with a standard deviation (SD) of 0.38 K (Robust SD = 0.25 K). The bias is thus well below the 0.1 K required for a SST product usable for climate studies and extremely stable over more than a decade. Then, we investigate the consistency between the three generations of IASI on-board Metop-A, -B, and -C over their overlap periods (2013–2020 for Metop-A and Metop-B, and 2019–2020 for Metop-C) by comparing the associated retrieved monthly-averaged SSTs over 1°x1° grids. The mean differences obtained are lower than 0.02 K, with a SD of 0.3 K consistent with the natural variation of the SST within a month. This demonstrates the high stability of the SSTs retrieved using spectra recorded from the IASI suite and opens the possibility to generate long time series for climate studies. This is tested in a preliminary investigation of the SST anomalies between 2008 and 2020, with the successful detection of short-term (El Niño and La Niña) changes as well as of a global warming trend of about +0.3 K/decade quantitatively consistent with other observations.