Radiative properties and direct radiative effect of Saharan dust measured by the C‐130 aircraft during SHADE: 1. Solar spectrum

TLDR

The study used C‑130 radiation measurements and multiple retrieval methods to determine Saharan dust aerosol optical depth and its direct radiative effect. Measurements show Saharan dust has an optical depth of 1.48 ± 0.05 at 0.55 µm, a direct radiative effect of about –129 ± 5 W m⁻² (albedo increase factor 2.7 ± 0.1), and, with an imaginary refractive index of 0.0015 i, represents the strongest local and global DRE among aerosol types, with C‑130 and CERES results agreeing within 20 %.

Abstract

The physical and optical properties of Saharan dust aerosol measured by the Met Office C‐130 during the Saharan Dust Experiment (SHADE) are presented. Additional radiation measurements enable the determination of the aerosol optical depth, τ aer λ , and the direct radiative effect (DRE) of the mineral dust. The results suggest that the absorption by Saharan dust is significantly overestimated in the solar spectrum if standard refractive indices are used. Our measurements suggest an imaginary part of the refractive index of 0.0015 i is appropriate at a wavelength λ of 0.55 μm. Different methods for determining τ aer λ=0.55 are presented, and the accuracy of each retrieval method is assessed. The value τ aer λ=0.55 is estimated as 1.48 ± 0.05 during the period of heaviest dust loading, which is derived from an instantaneous DRE of approximately −129 ± 5 Wm −2 or an enhancement of the local planetary albedo over ocean of a factor of 2.7 ± 0.1. A comparison of the DRE derived from the C‐130 instrumentation and from the Clouds and the Earth's Radiant Energy System (CERES) instrument on the Tropical Rainfall Measuring Mission (TRMM) satellite is presented; the results generally showing agreement to within a factor of 1.2. The results suggest that Saharan dust aerosol exerts the largest local and global DRE of all aerosol species and should be considered explicitly in global radiation budget studies.