Observations of Saharan Aerosols: Results of ECLATS Field Experiment. Part II: Broadband Radiative Characteristics of the Aerosols and Vertical Radiative Flux Divergence

Abstract

The results presented in this paper are a part of those obtained during the ECLATS experiment The broadband radiative characteristics of the Sahelian aerosol layer and the vertical radiative flux divergence within the dust layer were determined both from in situ measurements and Mie calculations. In situ measurements of the aerosol layer's reflectances and transmittances of solar radiation led to aerosol single-scattering albedos close to ω_A∼0.95. Measurements of the 8–14 μm radiances led to an optied depth by unit of volume of dust in a vertical column C_A∼0.34 μm⁻¹. Mie calculations assuming the aerosol refractive index published by Carlson and Benjamin for solar radiation and that measured by Volz for the atmospheric window, showed good agreement with observations. The ratio of infrared to visible optical thickness was δ_A(8–14 μm)/δ_A (0.55 μm)∼0.1, instead of 0.3 as calculated by Carlson and Benjamin. This discrepancy is attributable to differences in size distributions assumed. The radiative budget of the Sahelian aerosol layer was determined for clear and dusty conditions. The additional aerosol shortwave heating was as much as 5 K day⁻¹ for δ_A(0.55 μm) = 1.5 and with the sun overhead, whereas the additional cooling was close to 1 K day⁻¹. As a consequence of the large temperature discontinuity at the surface, important infrared heating at the surface layer was observed. The rather large differences between the aerosol optical properties reported here and those previously reported in the literature are due to different aerosol size distributions; therefore the present paper stresses the importance of careful determination of the size distributions.