A Lookup Table-Based Method for Estimating Sea Surface Hemispherical Broadband Emissivity Values (8–13.5 μm)

Abstract

Sea surface hemispherical broadband emissivity (BBE,(8)(9)(10)(11)(12)(13).5 µm) is a vital parameter for calculating surface radiation budgets. Such data are currently unavailable. This paper proposes a lookup table-based method for retrieving sea surface hemispherical BBE values. The physically-based sea surface emissivity model of Wu and Smith, together with the optimal refractive index, were used to generate hemispherical BBE values under wind speeds ranging from zero to 50 m/s. A lookup table of hemispherical BBE values as a function of wind speed was established and used to retrieve sea surface hemispherical BBE values under foam-free conditions. The accuracy of the estimates of hemispherical BBE was 0.003, given a wind speed of zero. The foam effect was explicitly considered. After incorporating the foam effect, hemispherical BBE was expressed as a linear function of the hemispherical BBE values of sea water and foam, weighted by the fraction of foam coverage. With this method, we have produced an hourly sea surface hemispherical BBE product with a resolution of 10 km and global coverage that covers the period from 2003 to 2005, using wind speed data from Modern-Era Retrospective analysis for Research and Applications (MERRA)-2.

The Global Land Surface Satellite (GLASS) Product Suite

“…Efforts are being made to generate more land, atmospheric and oceanic climate data records (CDRs) from different satellite observations. Many algorithms for generating these products have been developed, including those for top-of-atmosphere (TOA) shortwave albedo (Song et al 2018b;Wang and Liang 2016;Wang and Liang 2017), ocean shortwave albedo (Feng et al 2016; and longwave emissivity (Cheng et al 2017b), land surface NDVI (Xiao et al 2015a;Xiao et al 2017b), fractional snow cover (Chen et al 2018), surface air temperature (Rao et al 2020), and forest above-ground biomass (Yang et al 2020a).…”

A Monte Carlo Emissivity Model for Wind-Roughened Sea Surface

“…The hemispherical broadband emissivity is an important parameter for calculating the sea surface energy balance. Cheng et al [6] developed a lookup-table based algorithm for estimating hemispherical broadband emissivity using the SSE model of Wu and Smith [10]. Even the Wu and Smith model cannot reproduce well the observations of Smith et al [27] and Branch et al [29].…”

“…Different authors have published various complex refractive indices for pure water and sea water [25,30]. Larger discrepancies occur in these indices [6]. More laboratory or field measurements should be deployed to obtain high quality data of the complex refractive index.…”

“…Currently, the whitecap coverage product and laboratory measured emissivity of whitecaps or foam are available [35,36]. The effect of foam is incorporated in the retrieval sea surface broadband emissivity empirically [6]. More studies should be conducted to study the emissivity and coverage of whitecaps.…”

“…The Earth’s energy imbalance (EEI) drives global warming [3,4], which can be best estimated from changes in the ocean heat content (OHC), complemented by radiation measurements from space [5]. However, the spectral variation as well as the directionality of SSE are ignored in calculating the sea surface energy balance, which will certainly affect the accuracy of the estimate of the sea surface radiation budget [6,7,8]. Additionally, current operational SST products cannot meet the desired goal of 0.3 K [2,9].…”