Earth Science

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REVIEW Flood or Drought: How Do Aerosols Affect Precipitation? Daniel Rosenfeld,1* Ulrike Lohmann,2 Graciela B. Raga,3 Colin D. O’Dowd,4 Markku Kulmala,5 Sandro Fuzzi,6 Anni Reissell,5 Meinrat O. Andreae7 Aerosols serve as cloud condensation nuclei (CCN) and thus have a substantial effect on cloud properties and the initiation of precipitation. Large concentrations of human-made aerosols have been reported to both decrease and increase rainfall as a result of their radiative and CCN activities. At one extreme, pristine tropical clouds with low CCN concentrations rain out too quickly to mature into long-lived clouds. On the other hand, heavily polluted clouds evaporate much of their water before precipitation can occur, if they can form at all given the reduced surface heating resulting from the aerosol haze layer. We propose a conceptual model that explains this apparent dichotomy. loud physicists commonly classify the characteristics of aerosols and clouds into “maritime” and “continental” regimes, where “continental” has become synonymous with “aerosol-laden and polluted.” Indeed, aerosol concentrations in polluted air masses are typically one to two orders of magnitude greater than in pristine oceanic air (Fig. 1) (1). However, before humankind started to change the environment, aerosol concentrations were not much greater (up to double) over land than over the oceans (1, 2). Anthropogenic aerosols alter Earth’s energy budget by scattering and absorbing the solar radiation that energizes the formation of clouds (3–5). Because all cloud droplets must form on preexisting aerosol particles that act as cloud condensation nuclei (CCN), increased aerosols also change the composition of clouds (i.e., the size distribution of cloud droplets). This, in turn, determines to a large extent the precipitation-forming processes. Precipitation plays a key role in the climate system. About 37% of the energy input to the atmosphere occurs by release of latent heat from vapor that condenses into cloud drops and ice crystals (6). Reevaporation of clouds consumes back the released heat. When water is precipitated to the surface, this heat is left in the atmosphere and becomes available to energize convection and larger-scale atmospheric circulation systems. be available for evaporating water and energizing convective rain clouds (7). The fraction of radiation that is not reflected back to space by the aerosols is absorbed into the atmosphere, mainly by carbonaceous aerosols, leading to heating of the air above the surface. This stabilizes the low atmosphere and suppresses the generation of convective clouds (5). The warmer and drier air thus produces circulation systems that redistribute the remaining precipitation (8, 9). For example, elevated dry convection was observed to develop from the top of heavy smoke palls from burning oil wells (10). Warming of the lower troposphere by absorbing aerosols can also strengthen the Asian summer monsoon circulation and cause a local increase in precipitation, despite the global reduction of evaporation that compensates for greater radiative heating by aerosols (11). In the case of bright aerosols that mainly scatter the radiation back to space, the consequent surface cooling also can alter atmospheric circulation systems. It has been suggested that this mechanism has cooled the North Atlantic and hence pushed the Intertropical Convergence Zone southward, thereby contributing to the drying in the Sahel (12, 13). Aerosols also have important microphysical effects (14). Added CCN slow the conversion of cloud drops into raindrops by nucleating larger number concentrations of smaller drops, which are slower to coalesce into raindrops or rime onto ice hydrometeors (15, 16). This effect was shown to shut off precipitation from very shallow and short-lived clouds, as in the case of

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The dominance of anthropogenic aerosols over much of the land area means that cloud composition, precipitation, the...
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