The trade winds blow westward across Earth in the tropics and have been exploited by sailors like Christopher Columbus since humans took to the seas. The winds influence much of the planet’s weather and climate and are famous for the low, shallow cumulus clouds they produce. The tiny droplets of water in these fluffy white clouds form around small soluble aerosol particles in the atmosphere. In recent decades, scientists have begun to apply very highly resolved, but computationally intensive, models of the fluid dynamics describing the interaction of a population of clouds to evaluate how anthropogenic aerosols influence the cloud life cycle.
Previous work, conducted as early as the 1970s, has suggested that more aerosols in the atmosphere could make clouds brighter—reflecting more light and keeping the planet cooler. Adding more particles to the atmosphere increases the number density of cloud droplets, and as the concentration of droplets increases, the size of each droplet decreases. Smaller droplets have more surface area per volume and thus absorb less light—and reflect more back into space.
Recently, however, scientists have begun to question whether the role of aerosols is more complex than they initially thought. Using computer models, Seifert et al. investigate how changing the density of moisture droplets within trade wind cumulus clouds impacts their reflectivity, development, and organization. The results of the team’s simulations show a three-stage progression in which small, isolated clouds initially form over the ocean; a small subset of these develops into larger, thicker clumps that eventually reach a stable equilibrium.
According to the simulations, increasing the droplet number density (as would happen with increased amounts of aerosolized particles) leads to deeper clouds during the equilibrium stage. A few deeper clouds help mix drier air down into the cloud layer and reduce the humidity. This causes the much larger population of smaller clouds to evaporate faster. The authors demonstrate that the loss of cloud cover compensates for any increased brightness caused by the smaller droplet size, meaning overall reflectivity doesn’t change much. The team concludes that the discovery of these macroscopic effects of increasing droplet size—which is quite the opposite of what is parameterized in many climate models—hints that current models are overestimating the cooling impact that aerosols have on the planet. (Journal of Advances in Modeling Earth Systems (JAMES), doi:10.1002/2015MS000489, 2015)
—David Shultz, Freelance Writer
Citation: Shultz, D. (2016), Aerosols make cumulus clouds brighter but shorter lived, Eos, 97, doi:10.1029/2016EO045611. Published on 10 February 2016.