The earth receives virtually all of its energy from the sun in the form of electromagnetic radiation. This radiation is absorbed, reflected, and scattered by the earth”s surface, the ocean, and the atmosphere. The absorbed radiation is transformed into heat and other forms of energy, and eventually it is returned to space as low-temperature terrestrial radiation. It is clear that radiation is of fundamental importance to atmosphere-ocean interaction. There exists an adequate body of literature on the subject from an introductory treatment by Fleagle and Businger (1980) to specialized monographs by Kondratjev (1969), Liou (1980), and Goody and Yung (1989). Here it will suffice to introduce the basic concepts and focus on the applications to the air-sea interface. Radiation in the atmosphere and ocean comes from all directions simultaneously. The radiation energy per unit time coming from a specific direction and passing through a unit area perpendicular to that direction is called the radiance, I. The irradiance, Fi, or radiant flux density, is the radiant energy that passes through a unit horizontal area per unit time coming from all directions above it. Therefore where θ is the zenith angle and dω is an infinitesimal solid angle. The cos θ reflects the projection of the horizontal unit area into the direction from where I comes. The limits 0 and 2π of the integral reflect the hemisphere of directions above the unit area. When the radiance is independent of direction it is called isotropic. Equation may then be integrated to yield The irradiance from below the unit area is also called exitance and is denoted by Fe. The net irradiance, Fn, is defined by For isotropic radiance, the net irradiance Fn = 0. The fluxes are positive when upward and negative when downward. The interactions between radiation and matter may take various forms. They include refraction, reflection, scattering, diffraction, absorption, and emission. All these interactions are described by the theory of electromagnetic waves (e.g., Panofsky and Phillips, 1962). The full theory will not be developed here, but a number of basic and useful relations will be introduced to describe the characteristics of the interactions mentioned previously.
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