Penetration of solar radiation in the upper ocean: A numerical model for oceanic and coastal waters

Abstract

For studies of heat transfer in the upper ocean, the vertical distribution of solar radiation (ESR) in the shortwave domain plays an important role. In earlier studies, a sum of multiple exponentials was used to describe the vertical transmittance of ESR. For those exponential terms, an attenuation coefficient for each term is assigned, and those attenuation coefficients are assumed to be vertically constant. Furthermore, those attenuation coefficients are, empirically modeled as functions of chlorophyll concentration (Chl) to cope with varying water properties of the oceans. Since attenuation coefficients of ocean waters are generally determined by components more than Chl alone, we developed a generalized model bypassing the use of Chl on the basis of extensive numerical simulations. In this new model, vertical transmittance of ESR is separated into two exponential terms, with one for the contributions of wavelengths below 700 nm (the visible domain, EVIS) and one for wavelengths above 700 nm (the infrared domain, EIR). An attenuation coefficient is assigned for each of the two exponential terms. Unlike earlier models, these attenuation coefficients vary with depth as expected. Furthermore, the attenuation coefficient for the visible domain is modeled as a function of water's absorption and backscattering coefficients. Since absorption and backscattering coefficients of the world oceans can be adequately derived from observations of ocean color, the model we developed provides an effective and adequate means to describe the three-dimensional variation of both EVIS and ESR in the upper layer of the world oceans.