A lookup table-based method for estimating sea surface hemispherical broadband emissivity values (8-13.5 μm)

Abstract

Sea surface hemispherical broadband emissivity (BBE, 8-13.5 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.