Estimating the radiation environment in the Great Barrier Reef

Masiri, IO 2009 , 'Estimating the radiation environment in the Great Barrier Reef', PhD thesis, University of Tasmania.

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Abstract

There are a number of studies that have examined solar irradiance in the environment above and below the ocean surface. Unlike other previous studies, this thesis maps both surface and underwater irradiances using a combination of geostationary and Sea WiFS satellite data and surface measurements. Detailed estimates of broadband, PAR and UVB solar radiation, both above and below water, is obtained for the Great Barrier Reef region in northeast Australia. The study area covers the Coral Sea including the Great Barrier Reef (GBR), northeast of Queensland (10° S - 26° S, 142° E - 155° E). Geostationary meteorological satellite (GMS) data collected for periods between 1995 and 2005 are combined with the models to obtain broadband solar radiation at the surface. PAR and UVB underwater irradiance are estimated from the attenuation coefficients K$$_d$$ derived from the Sea WiFS instrument for the period of 1998-2005.
In developing algorithms, a look-up table (LUT), which contains senes of broadband surface solar radiation and the Earth-atmosphere reflectivity, is created from the Streamer radiative transfer model. The broadband Earth-atmosphere reflectivity α$$_{EA}$$ derived from the GMS data is used to estimate the surface solar radiation from the LUT algorithms. The attenuation coefficients K$$_d$$ for PAR, UVB and Sea WiFS wavebands are obtained from Monte Carlo models, and relationships between these Kd values are derived as a result. Using Sea WiFS-derived K$$_d$$ data, PAR and UVB underwater irradiance are estimated across the regions from the K$$_d$$ relationships.
Comparisons between the satellite-derived surface solar radiation and the measurements at Rockhampton and Cairns give good agreement with root-mean-square difference (RMSD) of 2.26 MJ-m$$^{-2}$$-day$$^{-1}$$ and 2.85 MJ-m$$^{-2}$$-day$$^{-1}$$ , respectively. The Sea WiFS-derived K$$_d$$ at six different wavebands obtained from Monte Carlo models are compared to the measurements at Heron Island, with a smallest RMSD value of 0.075m$$^{-1}$$.
Maps of solar radiation both above and below the water surface are provided for the study area. On average, strong solar radiation is exhibited in summer and autumn periods at latitude bands between 18° S and 26° S. The temporal information suggests there is a small increase in solar radiation of about 1 percent per decade. The underwater PAR and UVB irradiances along the GBR coastline at 2.5 m depth were in magnitude of ~300 W-m$$^{-2}$$ and ~120 mW-m$$^{-2}$$, respectively. Monthly distribution at 2.5 m depth underwater for PAR shows an outstanding feature throughout between September and February. There are patterns of high PAR irradiance throughout the summer months of 2001/2002 and 2002/2003. Results of the satellite-derived solar radiation are used to examine the 2002 coral bleaching events. A correlation between maximum solar radiation and the bleaching data are seen in summer months of 2001/2002.

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