Toward an Observation Based Estimate of Dust Net Radiative Effects in Tropical North Atlantic Through Integrating Satellite Observations and In SituMeasurements of Dust Properties
Abstract. In this study, we integrate recent aircraft measurements of dust microphysical and optical properties with satellite retrievals of aerosol and radiative fluxes to quantify the dust direct radiative effects on the shortwave (SW) and longwave (LW) radiation (denoted as DRESW and DRELW, respectively) at both the top of atmosphere (TOA) and surface in the tropical North Atlantic during summer months. Through linear regression of CERES measured TOA flux versus satellite aerosol optical depth (AOD) retrievals under cloud free and dust laden atmospheric conditions, we estimate the instantaneous DRESW efficiency at the top of the atmosphere (TOA) to be 49.7+ 7.1 W/m 2 /AOD and 36.5+ 4.8 W/m 2 /AOD based on AOD from MODIS and CALIOP, respectively. The corresponding DRESW at TOA is 14.2+ 2.0 W/m 2 and 10.4+ 1.4 W/m 2 respectively. We also estimate the instantaneous DRELW at TOA to be between +2.7+ 0.32 W/m 2 to +3.4+ 0.32 W/m 2 based on the difference between computed dust free outgoing longwave radiation (OLR) and CERES measured OLR. We then perform various sensitivity studies with 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 abovementioned 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 and optical properties we further estimate the diurnal mean dust DRESW efficiency of 28 W/m 2 /AOD at TOA and 82 W/m 2 /AOD at surface. The corresponding TOA and surface DRESW in the region is approximately 10 W/m 2 and 26 W/m 2 respectively, of which ~30% is canceled out by the positive DRELW. This yields a net DRE of about 6.9 W/m 2 and 18.3 W/m 2 at TOA and surface, respectively. Our study suggests that the LW flux contains useful information of dust particle size, which could be used together with SW observation to achieve more holistic understanding of the dust radiative effect.