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 DRESW and DRELW, 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 DRESW efficiency

at the TOA to be -49.7±7.1 W m−2 AOD−1 and

-36.5±4.8 W m−2 AOD−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 DRESW and DRELW 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 DRESW in the region

of −10 W m−2 at TOA and −26 W m−2 at the surface, respectively, of

which ∼ 30 % is canceled out by the positive DRELW. This

yields a net DRE of about −6.9 and −18.3 W m−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.