Use of hyperspectral sounders to retrieve daytime sea-surface temperature from mid-infrared radiances: Application to IASI

We propose a physically-based methodology for sea-surface skin-temperature (SST) retrievals from daytime spectra recorded by the hyperspectral Infrared Atmospheric Sounding Interferometer (IASI). The approach, which takes sun light into account, minimizes the differences between measured and computed radiances at numerous points within two windows centered near 3.7 and 4.0 μm. We demonstrate that, even when the solar contribution is large, with brightness temperatures increased by several tens of kelvin at the shortest wavelengths, very consistent SSTs are retrieved in both windows. Furthermore, the values obtained are, after correction for the cool-skin effect, in excellent agreement (<0.05 K on average) with the in-situ measurements of the depth temperature provided by nearby drifters. This opens renewed perspectives for daytime SST determinations from space, since the IASI mid-infrared windows used here provide SSTs that are, with respect to those retrieved around 9 and 11 μm, much less sensitive to errors in the computed radiative contribution of water vapor.