Solar irradiance levels play a key role for energy exchange and primary production in marine polar environments. Because the ocean is typically covered by sea ice and snow for several months of the year, theoretical and experimental knowledge of radiation levels at various depths in the snow, ice, and ocean is essential. The photosynthetically active radiation (PAR, 400 nm < A < 700 nm) drives the pelagic primary production in general, and the amount of PAR reaching the bottom of the sea ice is of crucial importance for the growth of ice algae, which accounts for between 5% and 30% of the total plankton production in the polar regions (Legendre et al., 1992; Wheeler et al., 1996). The wavelength dependence of UV radiation ( A < 400 nm) is required to assess potential damage and inhibition of primary production induced by UV radiation (Neal et al., 1998). Finally, the amount of radiation that penetrates the snow and ice determines the rate of energy absorption, melting of sea ice, and warming of the upper ocean (Perovich and Maykut, 1996; Zeebe et al., 1996).
Here we provide a brief description of the CASIO Discrete-Ordinate Radiative Transfer (CASIO-DISORT) model, which treats transfer of solar radiation in the CASIO system based on the theory provided above and its implementation by use of the discrete-ordinate method. Note that in the absence of sea ice, the CASIO system defaults to the CAO (coupled atmosphere-ocean) system.
In summary, the CASIO-DISORT method works as follows:
(1) The atmosphere and the ice-ocean media are treated as two adjacent slabs separated by an interface across which the real part of the refractive index changes from matmr = 1 in the atmosphere (including a possible snow layer at the bottom, i.e., on top of the ice) to mocnr = 1.31 (1.33 in the absence of ice) in the ice-ocean medium.
(2) Each of the two slabs is divided into a sufficient number of horizontal layers to adequately resolve vertical variations in their inherent optical properties.
(3) The reflection and transmission through the interface between the atmosphere and the ice-ocean are computed by Fresnel's equations, and the reflection and refraction of a beam at the interface follow the reflection law and Snell's law, respectively.
(4) The radiative transfer equation is solved separately for each layer in the atmosphere and in the ice-ocean using the discrete-ordinate method.
(5) The solution is completed by applying boundary conditions at the top of the atmosphere and the bottom of the ocean, as well as appropriate radiance continuity conditions at each layer interface in the atmosphere, in the ice-ocean media, and at the atmosphere-ice-ocean interface (where Fresnel's equations apply).
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