Abstract
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.
Highlights
Mineral dust is the most abundant atmospheric aerosol component in terms of dry mass (Choobari et al, 2014; Textor et al, 2006)
Focusing on our computations first, we note that as expected the most reflective dust based on the combination of AERONET particle size distribution (PSD) and Colarco-SW refractive index leads to the largest DRESW efficiency (−70.5 W m−2 aerosol optical depth (AOD)−1), while the least reflective dust based on the combination of Fennec-Saharan air layer (SAL) PSD and Optical Properties for Aerosols and Clouds (OPAC) refractive index yields the smallest DRESW efficiency (–30.6 W m−2 AOD−1)
As expected, the combination of Fennec PSD– OPAC-LW yields the best simulation of the dust DRELW, at 3.0 W m−2, in comparison with the result derived from the CERES outgoing longwave radiation (OLR) observations and Rapid Radiative Transfer Model (RRTM) dust-free OLR computation with ancillary data provided by the CERES–CALIPSO– CloudSat–MODIS (CCCM) product (i.e., +3.4 ± 0.32 W m−2 based on the CERESCALIPSO combination)
Summary
Mineral dust is the most abundant atmospheric aerosol component in terms of dry mass (Choobari et al, 2014; Textor et al, 2006). A prominent example is the CERES–CALIPSO– CloudSat–MODIS (CCCM) product developed by Kato et al (2011), which has become a popular dataset for studying the radiative effects of clouds and aerosols and for evaluating GCMs. The present study is inspired and motivated by the latest measurements of the microphysical and optical properties of dust, namely the in situ dust PSD from the Fennec field campaign (Ryder et al, 2013a, b) and the dust LW refractive index from Di Biagio (2014, 2017), as well as recent studies (e.g., Kok et al, 2017) suggesting that cooling effects of dust are overestimated in most climate models due to the underestimation of dust size.
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