MODIS observation of aerosols and estimation of aerosol radiative forcing over southern Africa during SAFARI 2000

Abstract

MODIS provides almost complete global coverage daily. Aerosol optical thickness (AOT or τ a λ ) and other aerosol parameters are retrieved over land and ocean at a spatial scale of 10 km (level 2), then aggregated to a global grid of 1° spatial resolution on daily, weekly, and monthly time scales (level 3). The SAFARI 2000 ground‐based measurements of AOT from the Aerosol Robotic Network (AERONET) group of Sun photometers were used to compare MODIS over‐land AOT based on a spatiotemporal statistical technique. At low aerosol loading (when τ a 470 < 0.6), MODIS τ a λ values agree with those of AERONET at the blue wavelength (λ = 440, 470 nm). But, at higher wavelengths and with increasing aerosol loading, MODIS AOT values underestimate those of AERONET. There are also regional variations in validation accuracy. This AOT underestimation by MODIS during SAFARI 2000 is attributed to the application of a constant single‐scattering‐albedo (SSA or ω 0 ) value of 0.90 globally for smoke aerosol retrieval. Recent studies based on long‐term observations with AERONET Sun photometers suggest that lower ω 0 values of 0.88 and 0.84 at 440 nm and 670 nm wavelengths respectively would be more applicable over southern Africa (particularly over Zambia, where most of the measurements were based). A column climate model, with MODIS aerosol information as input, is used to calculate aerosol radiative forcing over the (southern Atlantic) Ocean part of the SAFARI 2000 region. For September 2000, the results show a forcing of −10 W/m 2 at the top of the atmosphere and approximately −26 W/m 2 at the terrestrial surface.