Abstract
Four radiometric models are compared to study the Angstr?m turbidity coefficient over Gharda?a (Algeria). Five years of global irradiance measurements and space data recorded with MODIS are used to estimate . The models are referenced as for Dogniaux’s method, for Louche’s method, for Pinazo’s method, for Gueymard’s method and by for MODIS data. The results showed that and are very close as the couple and . values are between them. Results showed also that all Angstr?m coefficient curves have the same annual trend with maximum and minimum values respectively in summer and winter months. Annual mean values of increased from 2005 to 2008 with a slight jump in 2007 except for . The city environment explains it since the urban aerosols predominate over all other types during this period. The jump in 2007 is attributed to the ozone layer thickness that undergoes the same behavior. Some models are then more sensitive to this atmospheric component than others. The occurrence frequency distribution showed that , , , and had their maximum recurrent values near 0.03, 0.07, 0.10, 0.09 and 0.02 respectively. The cumulative frequency distribution revealed also that and yielded maximum “clean to clear” conditions with respect to others while and had the minimum. The opposite was observed on the same pairs with regard to “clear to turbid” and “turbid to very turbid” conditions. Louche’s model gave middle values of sky conditions comparing to the other models.
Highlights
The atmospheric turbidity is responsible of the attenuation of solar radiation reaching a local area of the Earth surface under cloudless sky conditions
The results showed that βGyem and βPinz are very close as the couple βDog and βmodis . βLouch values are between them
Since good measurement of solar radiation is strongly dependent on Earth atmosphere state, so it is important to quantify the effect of its constituents where solar irradiance is measured
Summary
The atmospheric turbidity is responsible of the attenuation of solar radiation reaching a local area of the Earth surface under cloudless sky conditions. Since good measurement of solar radiation is strongly dependent on Earth atmosphere state, so it is important to quantify the effect of its constituents where solar irradiance is measured. The Angström turbidity coefficient which is commonly used [2]. The Angström coefficient β has typical values that vary between 0 and 0.5. Several models may be used to estimate β from broadband measurements of solar irradiance and meteorological data when spectral measurements are not available. We will investigate the Angström turbidity coefficient of a semi-arid region in Algeria with the widely used broadband models. Energies (URAER, Ghardaïa) in the south of Algeria from 2005 to 2008 and those obtained from space measurements during the same period
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