Net radiative effects of dust in the tropical North Atlantic based on integrated satellite observations and in situ measurements

Abstract

In this study, we integrate recent in situ measurements with satellite retrievals of dust physical and radiative properties to quantify dust direct radiative effects on shortwave (SW) and longwave (LW) radiation (denoted as DRE(SW) and DRE(LW), respectively) in the tropical North Atlantic during the summer months from 2007 to 2010. Through linear regression of the CERES-measured top-of-atmosphere (TOA) flux versus satellite aerosol optical depth (AOD) retrievals, we estimate the instantaneous DRE(SW) efficiency at the TOA to be −49.7±7.1Wm⁻²AOD⁻¹ and −36.5±4.8Wm⁻²AOD⁻¹ based on AOD from MODIS and CALIOP, respectively. We then perform various sensitivity studies based on recent measurements of dust particle size distribution (PSD), refractive index, and particle shape distribution to determine how the dust microphysical and optical properties affect DRE estimates and its agreement with the above-mentioned satellite-derived DREs. Our analysis shows that a good agreement with the observation-based estimates of instantaneous DRE(SW) and DRE(LW) can be achieved through a combination of recently observed PSD with substantial presence of coarse particles, a less absorptive SW refractive index, and spheroid shapes. Based on this optimal combination of dust physical properties we further estimate the diurnal mean dust DRE(SW) in the region of −10Wm⁻² at TOA and −26Wm⁻² at the surface, respectively, of which ∼ 30% is canceled out by the positive DRE(LW). This yields a net DRE of about −6.9 and −18.3Wm⁻² at TOA and the surface, respectively. Our study suggests that the LW flux contains useful information on dust particle size, which could be used together with SW observations to achieve a more holistic understanding of the dust radiative effect.