A SIMPLE SPECTRAL SOLAR IRRADIANCE MODEL FOR CLOUDLESS MARITIME ATMOSPHERES

A simple spectral atmospheric radiative transfer model specific for oceanographic applications begins with spectral extraterrestrial solar irradiance corrected for earth-sun orbital distance. Irradiance is then attenuated in passing through the atmosphere by Rayleigh scattering, ozone, oxygen, and water vapor absorption, and marine aerosol scattering and absorption, and is finally reduced by reflectance at the air-sea interface. The model is an extension of the continental aerosol model of Bird and Riordan, modified to include maritime aerosol properties, irradiance transmittance through the air-sea interface, and atmospheric absorption at very high spectral resolution (1 nm). Atmospheric optical constituents and the surface reflectance are functions of the local meteorological conditions, imparting flexibility to the model to reproduce the spectral irradiance under a variety of maritime atmospheres. The model computes irradiance at or just below the ocean surface at high spectral resolution in the range 350-700 nm, i.e. within the range required for photosynthetically available radiation (PAR) calculations. It agrees spectrally with observed surface spectral irradiances to within +/- 6.6% (rms) and as integrated PAR to within +/- 5.1%. The computed spectral irradiance is useful as an input to bio-optical models in the ocean, to phytoplankton growth and primary production models, and in remote-sensing applications.