Radiative effects of aerosols in sub‐Sahel Africa: Dust and biomass burning

Abstract

Information on radiative fluxes that reach the ground is needed in numerous areas of climate research. On a global scale, such information is obtainable only from satellites. Top of the atmosphere satellite observations during clear and cloudy sky conditions have been found useful for inferring, among others, information on aerosol optical depth (AOD) and cloud optical depth (COD). These are important elements for estimating surface radiative fluxes. Satellite retrievals of AOD are based on the assumption that the aerosols are of a specific type. In certain climatic regions, dust aerosols and those from biomass burning are injected into the atmosphere simultaneously and are distributed distinctly in the vertical. In this study, it is demonstrated that in such scenarios inaccurate assumption on the vertical distribution of the aerosols can lead to errors in AOD estimates from satellites and, subsequently, in the inferred shortwave (SW) radiative fluxes that reach the surface. The magnitude of such errors can be as high as 80 W m−2. The frequency of these mixed situations has not as yet been documented, and it is possible that on global scale the impact is small. Using available records of observed SW radiative fluxes, their reduction (“dimming”) at numerous locations has been reported. Some relate this reduction to aerosol effects. The findings of this study have implications for the ability to assess such “dimming” from satellites in areas where the vertical structure of aerosol distribution needs to be accounted for.