A sensitivity study on radiative effects due to the parameterization of dust optical properties in models

Most of the dust models underestimate the load of the large dust particles, consider spherical shapes instead of irregular ones, and have to deal with a wide range of dust refractive index (RI) to be used. This leads to an incomplete assessment of the dust radiative effects and dust-related impacts on climate and weather. The current work aims to provide an assessment, through a sensitivity study, of the limitations of models to calculate the dust direct radiative effect (DRE) due to the underrepresentation of its size, RI and shape. We show that the main limitations stem from the size and RI, while the shape plays only a minor role, with our results agreeing with recent findings in the literature. At the top of the atmosphere (TOA) close to dust sources, the underestimation of size issues an underestimation of the direct warming effect of dust of ~18 - 25W/m2, for dust aerosol optical depth (DOD) of 1 at 0.5 μm, depending on the solar zenith angle (SZA) and RI. The underestimation of the dust size in models is less above the ocean than above dust sources, resulting in an underestimation of the direct cooling effect of dust above the ocean by up to 3 W/m2, for AOD of 1 at 0.5 μm. We also show that the RI of dust may change its DRE by 80 W/m2 above the dust sources, and by 50 W/m2 at downwind oceanic areas, for AOD of 1 at 0.5 μm at TOA. These results indicate the necessity of including more realistic sizes and RIs for dust particles in dust models, in order to derive better estimations of the dust DRE, especially near the dust sources and mostly for studies dealing with local radiation effects of dust aerosols.