The effect of smoke, dust, and pollution aerosol on shallow cloud development over the Atlantic Ocean. Proc Natl Acad Sci USA

Hebrew University of Jerusalem, Yerushalayim, Jerusalem, Israel
Proceedings of the National Academy of Sciences (Impact Factor: 9.67). 09/2005; 102(32):11207-12. DOI: 10.1073/pnas.0505191102
Source: PubMed


Clouds developing in a polluted environment tend to have more numerous but smaller droplets. This property may lead to suppression of precipitation and longer cloud lifetime. Absorption of incoming solar radiation by aerosols, however, can reduce the cloud cover. The net aerosol effect on clouds is currently the largest uncertainty in evaluating climate forcing. Using large statistics of 1-km resolution MODIS (Moderate Resolution Imaging Spectroradiometer) satellite data, we study the aerosol effect on shallow water clouds, separately in four regions of the Atlantic Ocean, for June through August 2002: marine aerosol (30 degrees S-20 degrees S), smoke (20 degrees S-5 degrees N), mineral dust (5 degrees N-25 degrees N), and pollution aerosols (30 degrees N- 60 degrees N). All four aerosol types affect the cloud droplet size. We also find that the coverage of shallow clouds increases in all of the cases by 0.2-0.4 from clean to polluted, smoky, or dusty conditions. Covariability analysis with meteorological parameters associates most of this change to aerosol, for each of the four regions and 3 months studied. In our opinion, there is low probability that the net aerosol effect can be explained by coincidental, unresolved, changes in meteorological conditions that also accumulate aerosol, or errors in the data, although further in situ measurements and model developments are needed to fully understand the processes. The radiative effect at the top of the atmosphere incurred by the aerosol effect on the shallow clouds and solar radiation is -11 +/- 3 W/m2 for the 3 months studied; 2/3 of it is due to the aerosol-induced cloud changes, and 1/3 is due to aerosol direct radiative effect.

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Available from: Ilan Koren
    • "In addition to the profound direct impact of aerosols on the radiation budget of the earthatmosphere system, the development of clouds in a polluted environment can also significantly affect the radiation budget, and changes in cloud properties can have an influence on precipitation (e.g., Flossmann et al., 1985; Andreae et al., 2004; Kaufman et al., 2005a; Andreae and Rosenfeld, 2008; Jiang et al., 2011; Gu et al., 2012). Aerosols play a critical role in the process of cloud formation, although the absorbing and non-absorbing aerosols affect clouds differently (Kaufman and Koren, 2006). "
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    ABSTRACT: Atmospheric aerosols influence the earth’s radiative balance directly through scattering and absorbing solar radiation, and indirectly through affecting cloud properties. An understanding of aerosol optical properties is fundamental to studies of aerosol effects on climate. Although many such studies have been undertaken, large uncertainties in describing aerosol optical characteristics remain, especially regarding the absorption properties of different aerosols. Aerosol radiative effects are considered as either positive or negative perturbations to the radiation balance, and they include direct, indirect (albedo effect and cloud lifetime effect), and semi-direct effects. The total direct effect of anthropogenic aerosols is negative (cooling), although some components may contribute a positive effect (warming). Both the albedo effect and cloud lifetime effect cool the atmosphere by increasing cloud optical depth and cloud cover, respectively. Absorbing aerosols, such as carbonaceous aerosols and dust, exert a positive forcing at the top of atmosphere and a negative forcing at the surface, and they can directly warm the atmosphere. Internally mixed black carbon aerosols produce a stronger warming effect than externally mixed black carbon particles do. The semidirect effect of absorbing aerosols could amplify this warming effect. Based on observational (ground- and satellite-based) and simulation studies, this paper reviews current progress in research regarding the optical properties and radiative effects of aerosols and also discusses several important issues to be addressed in future studies.
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    • "Several researchers have used different methods for estimation of direct radiative forcing (DRF) due to aerosols from satellite retrievals (Kaufman et al., 2005; Christopher et al., 2006; Bellouin et al., 2008). Kaufman et al. (2005) and Christopher et al. (2006) estimated an annual average DRF of −1.4 Wm −2 , over clear-sky ocean; the former used satellite retrievals combined with aerosol radiative forcing efficiencies while the latter used a combination of aerosol retrievals and broadband-flux measurements from satellite instruments . Yu et al. (2004) used a combination of Georgia Tech/Goddard Global Ozone Chemistry Aerosol Radiation and Transport (GOCART) model simulations and satellite observation to estimate global DRF and found that the weaker dust absorption increases the TOA cooling by 0.4 Wm −2 . "
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    • "Alpert et al. (1998) discussed the response of the atmospheric temperature field to the radiative forcing of Saharan dust over the North Atlantic Ocean. Dust particles over the Atlantic Ocean may essentially influence tropical cloud systems and precipitation (Kaufman, Koren, et al. 2005; Johnson, Shine, and Forster 2004; Min et al. 2009; Ben- Ami, Koren, and Altaratz 2009; Feingold et al. 2009; Rosenfeld, Rudich, and Lahav 2001). "
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