August 12, 2010 | 5
A thin, patchy cloud cover with clusters that seem to continuously form, dissipate and reemerge might not be just flowing at random. When these cloud fields appear over the ocean at least, their cycles and patterns are in fact quite regular, and new research explains how precipitation keeps an accumulation of these clouds knit closely together. Deciphering the surprisingly rigid order of these common, lacey clouds can go a long way toward creating more solid meteorological and climate models.
The physics of clouds have continued to perplex scientists—and hobby gazers—for decades. But high-powered computer modeling has allowed a new look into the complex dynamics that occur within—and among—clouds.
"Clouds organize in distinct patterns that are fingerprints of myriad physical processes," Graham Feingold, of the National Ocean and Atmospheric Administration’s System Research Laboratory, Chemical Sciences Division in Boulder, Co., said in a prepared statement. And these marine clouds seem to have a physical language all their own. "Cloud fields organize in such a way that their components ‘communicate’ with one another and produce regular, periodic rainfall events," said Feingold, who is also the lead author of the new study, in the August 12 issue of Nature (Scientific American is part of Nature Publishing Group).
General cloud theory has held that cloud structure was largely determined by temperature change, warming versus cooling shifts. But the new work joins a building body of literature pointing to precipitation as a driving factor in determining what shape clouds take and how they behave.
The marine stratocumulus clouds seem to be operating on a classic Rayleigh-Benard convection principle, in which moisture flows between two horizontal planes, in this case, the low clouds and ocean surface. When the bottom gets warm enough, the lower moist air rises, pushing cooler air down. If the surface on the bottom is evenly heated and exchanges are allowed to flow freely, the surface on top will divide into a honeycomb of hexagons—a series of Y-shaped intersections—a pattern that is optimal for heat transfer. In the messy world of uneven ocean temperatures and sea breezes, the resulting clouds manifest in less-than-perfect honeycomb formation, but the pattern still seems to be there, the researcher noted. And after adjusting photos for wind speed and other perturbations, Feingold and his team saw the clear hexagonal forms.
The researchers discovered that these cloud formations establish equilibrium of cover, reforming new cells after old ones empty as the result of a shower.
The falling rain creates a downward draft. When this encounters the surface of the ocean, it pushes out to the sides, runs into other drafts and rises as heated, humidified air, forming a new batch of clouds and restarting the rain cycle. "As old precipitation zones dissipate, new ones are generated through local interaction between cells," the researchers noted in the study.
By running this dynamic through computer models, augmented by algorithms for real-world mixers such as airborne particles known as aerosols that can affect rain-drop formation, the research team was able to follow cloud formation—and deformation—as the digital stratocumulus clouds mimicked real ones. The simulated cycle had the clouds dissipating and re-forming over the course of a couple hours. The group then verified their findings against photographs of such cloud cover and with laser and radar readings from a boat.
"Together, these analyses demonstrated that the rearrangement is a result of precipitation, and that clouds belonging to this kind of system rain almost in unison," Feingold said. "Rain keeps the oscillating, open honeycomb pattern in motion."
The researchers point to this as an example of self-organization, a function that is seen throughout science, in flocks of birds, laser emission and even free-market economics. "The fundamental principle of self-organization is that a system-wide order emerges from local interactions or competition between system components," the researchers explained in their paper.
The new computer analyses allowed researchers to hone in on the important role aerosols play in determining the presence of this light, shifting cloud cover. The group found that more aerosols in the air tended to lead to larger, thicker clouds that produced less rain. And less rain means less honeycomb-patterned oscillation, fewer open "holes" in the cloud cover—and thus more reflected light from the sun and a cooler surface temperature.
"The pattern of the clouds affects how much of the sun’s energy gets reflected back into space," Hailong Wang, of the Pacific Northwest National Laboratory in Richland, Wash., and coauthor on the new paper, noted in a prepared statement. "Being able to simulate these clouds in computer models, we gain more insights into the physics behind the phenomenon. This will help us to better interpret measurement in the real atmosphere and represent these clouds in climate models."
Until then, sky gazers can linger longer to try to piece together the conversing clouds.
Image of honeycomb clouds courtesy of NASA