The synoptic and optical characteristics of the harmattan　dust spells over Nigeria

摘要

The synoptic and optical characteristic of days during severe dust spells of December 1998, January 2000, February 2001, March 2003 and January 2005 at Ilorin (8°19'N, 4°20'E), Nigeria are investigated in this study. For example, during the severe dust storm of 5 January 2005 a strong synoptic-scale ridge developed which caused a shift in the position of the anticyclone over the desert region. These conditions led to a higher pressure gradient, and in turn resulted in the generation of dust at the 900 hPa level and wind speeds of up to ～13 m/s over the source region between Bilma (18°12'N, 12°55'E), Faya Largeau (17°55'N, 19°07'E) and Maiduguri (11°50'N, 13°09'E) during the dust spells. Spectral aerosol optical depth τ_(aλ), Angstrom parameters (0:440-500 nm and β) derived from sun-photometer measurements and horizontal visibility (km) were used to study aerosol optical characteristics during and after the winter "harmattan" dust days. It was found that the daily mean τ_(a 500 nm) during the dust episodes ranged between 0.31 and 3.28, with an average of 0.98 ± 0.50. τ_(aλ) showed a pronounced temporal trend, with maximum dust loading during intense dust haze as τ_(500 nm) increased from a daily average value of 0.68 ± 0.48 to values >2.0 during severe dust spells. Daily mean spectral variations of the Angstrom exponent α show striking changes especially at high τ_(aλ), when coarse mode aerosol dominate the accumulation mode aerosol. During the dust spells of 1998-2005 average horizontal visibility was reduced to as low asrn1.66 ± 0.34 km at Ilorin with an increase in average τ_(a　500　nm) and β to ～2.06 ± 0.48 and 1.87 ± 0.49, respectively. However, for cases with strong biomass burning aerosol τ_(a 500 nm) = 0.67 with α and β = 1.02 ± 0.08 and 0.31 ± 0.06, respectively. In some cases, smoke particles mix with desert dust to produce intermediate values of α. A trajectory analysis at three different levels (200, 500 and 1000 magl) during a severe dust and biomass burning episode were further used to explain the transport of aerosols to Ilorin, Nigeria.