Assessment of Downwelling Surface and TOA Solar Radiation Fluxes in Cameroon using a Parameterized Solar Radiative Transfer Model in a Molecular Atmosphere

摘要

This study presents surface and atmospheric solar radiation fluxes results obtained in Cameroon using a combination of geographical considerations and the parameterized radiative transfer model called CLIRAD-SW. This model is a plane parallel radiative transfer model developed in 1999 by Minh-Dah Chou and J. Max Suarez from the NASA GSFC. It derives surface solar radiation, upward and downward fluxes within the atmosphere along with local heating rate from calculated atmospheric transmissivities and reflectivities using the two stream adding method. The aerosol optical thickness, single-scattering albedo, and asymmetry factor are specified input parameters to the radiation model. Following Klein, new average solar days are proposed. The average solar day is that day which has the extraterrestrial radiation closest to the average for the month. To assess the performances of our method, we compared the model results with the January and February (zero Octa nebulosity) groundbased measured global radiation data obtained using Eppley PSP Pyranometers for the Banyo, Poli and Garoua stations. Strong correlations (99%) were observed suggesting that the method is appropriate for retrieving solar irradiation even in case of a turbid atmosphere. Following this overall agreement, a one year hourly surface and TOA solar fluxes are derived for the Yaounde station. Similar monthly averaged fluxes were also derived for the ten meteorological stations across the country. The model has also been used to infer the upward and downward diffuse and direct fluxes within the molecular atmosphere of Yaounde and Garoua which are representative of the whole Cameroon solar climate.