An Observation-based Estimation of Dust Aerosol Net Direct Radiative Effects over the Tropical Northeast Atlantic and Sahara Desert
Abstract
Dust aerosols play an important role in the radiative energy budget of Earth-Atmospheric system. They participate in Earth's energy budget directly by absorbing and scattering radiation, which is so called direct radiative effect (DRE).
Many previous studies have focused on studying the DRE of dust in the shortwave (SW) domain. However, dust aerosols are large enough in size to have significant longwave(LW) DRE. Overall, the DRE of dust aerosols remains uncertain in magnitude and even sign in recent assessments. Recently, we integrate recent aircraft measurements of dust physical properties with satellite retrievals of aerosol and radiative fluxes to quantify the dust SW and LW DRE 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 TOA to be -49.7±7.1 W/m2/AOD and -36.5±4.8 W/m2/AOD based on AOD from MODIS and CALIOP, respectively. We then perform various sensitivity studies with recent measurements of dust particle size distribution, refractive index, and particle shape distribution to determine how the dust physical 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 properties we further estimate the diurnal mean dust TOA and surface DRESW in the region is approximately -10 W/m2 and -26 W/m2, respectively, of which 30% is canceled out by the positive DRELW. The next objective of this study is to extend the abovementioned study to estimate net DRE of dust aerosols over Sahara Desert. For cloud-free and dust-laden conditions, we could get SW and LW upward fluxes from CERES observations. Then we use the Rapid Radiative Transfer Model to compute both SW and LW upward flux in dust-removed conditions at TOA. The difference in upward flux between RRTM simulations and CERES observations is so-called semi-observation-based dust DREs.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFM.A21I2810Z
- Keywords:
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- 0305 Aerosols and particles;
- ATMOSPHERIC COMPOSITION AND STRUCTUREDE: 3305 Climate change and variability;
- ATMOSPHERIC PROCESSESDE: 3322 Land/atmosphere interactions;
- ATMOSPHERIC PROCESSESDE: 1622 Earth system modeling;
- GLOBAL CHANGE