Spectral solar irradiance under Harmattan conditions

Results of spectral solar radiation measurements at Ile-Ife (7 degrees-30'N, 4 degrees 31'E; South Western Nigeria) for two non-consecutive months between December 1991 and February 1992 are presented. The Angstrom atmospheric turbidity coefficients beta, beta(0.5) (0.5 referring to 0.5 mu m) and a for the wavelength interval 350-1038 nm were derived from a series of spectroradiometric measurements at Ile-Ife. The values of the turbidity parameters, beta and beta(0.5) are high with averages of 1.06 and 1.38, respectively. The average value of a was about 0.5. Classifying the Harmattan season into periods with ''moderate'' characteristics (background Harmattan) and periods with intensive spells, the mean background conditions had values ranging from 0.99 (at 350 nm) to 0.53 (at 862 nm) while Harmattan spell conditions had higher turbidities varying from 1.95 at 350 nn to 1.51 at 862 mm. There were severe reductions in the direct solar irradiance at Ile-Ife during this period due to the strong attenuating effects of the Harmattan dust. On the other hand, a significant increase in the diffuse irradiance was observed which, in all the cases examined during the period, exceeded the direct irradiance. The parameters beta and beta(0.5) were found to decrease when the intertropical discontinuity (ITD) moved North of a station during the Harmattan period and to increase when it migrates South of it. The coefficient ct, on the other hand, increased with the Northward movement of the ITD and decreased when it moved southwards.