Impact of dust aerosols on Earth-atmosphere clear-sky albedo and its short wave radiative forcing over African and Arabian regions

Abstract

Satellite‐ (MODIS onboard TERRA satellite) measured regional dust distribution has been used in this paper to examine the effect of dust aerosols on top of the atmosphere (TOA) albedo and it was found they have significant impact, especially over deserts, where surface reflection is high. With increasing dust load, TOA albedo was found to decrease significantly and it was observed that over deserts, TOA albedo becomes less than that at the surface (as much as 15% in near infrared) due to absorption by dust. The temporal variations of aerosol properties were examined at three representative locations: northern Africa (10° N–20° N; 20° E–30° E) and Saudi Arabia (19° N–24° N; 53° E–58° E) (both of which are influenced heavily by dust); and southern Africa (10° S–20° S; 20° E–30° E) (where the influence of dust is seasonal). Over northern Africa, optical depth was maximum (~0.6) during the June–August period and lower (0.3–0.6) during the rest of the year. The aerosol fine mode fraction (AFMF) was, in general, low (~0.05), except in the June–August period, when the AFMF shot up to 0.4. Over southern Africa, optical depth was maximum (~0.5) during the September–October period, lower (~0.3) in December–March, with the minimum (~0.1) during April–June. The AFMF was, in general, high (~0.9) throughout the year, and minimum (~0.6) during the May–July period. Over Saudi Arabia, optical depths and AFMF showed opposite trends, which is typical of regions influenced heavily by dust. Optical depths as high as ~0.8 (AFMF ~0.3) were observed during June–July. The regional average aerosol radiative forcing at these locations was estimated by integrating the measured (column) aerosol properties (from AERONET) into satellite (MODIS) data and then incorporating them into a radiation model. The radiative forcing at the TOA over northern Africa and Saudi Arabia was in the range of +2 to +4 W m⁻², whereas that estimated over southern Africa was in the range of −1 to +2 W m⁻². While TOA forcing was nearly zero (due to large surface reflectance), the radiative forcing at the surface was large. The radiative forcing at the surface over northern and southern Africa was in the range of −15 to −45 W m⁻², whereas that over Saudi Arabia was in the range of −30 to −70 W m⁻². The atmospheric absorption estimated for northern and southern Africa translates into a lower atmospheric heating rate of 0.4 to 1.2 K day⁻¹, and the same for Saudi Arabia was in the range of 0.8 to 2.2 K day⁻¹.