Polygons are widespread in nature: Drying mud may crack into many-sided blocks, and bees shape honeycomb into regular, six-sided cells. Hexagons also appear in broad sheets of clouds across parts of Earth's oceans, and now a team of researchers has used a network approach to analyze why. Their work promises to help scientists to find more accurate descriptions of clouds in computer models of weather and climate change.
Large decks of stratocumulus clouds self-organize into honeycomb-like patterns. "These types of clouds cool the planet by reflecting solar radiation but their description in climate models is still rather crude", said lead author Franziska Glassmeier. She found that she could use a relatively simple mathematical model to re-create the cloud patterns, which are shaped in nature by a complex interplay of physical processes.
The new paper, co-authored by NOAA scientist and CIRES Fellow Graham Feingold, is published today in the journal Proceedings of the National Academy of Sciences. The work was supported in part by the CIRES Innovative Research Program.
Since the first satellite images in the early 60s, scientists have recognized that stratocumulus clouds--carpet-like, low clouds often draped across large sections of subtropical oceans-- look like imperfect honeycombs. Sometimes the cells are "closed," with cloudy areas in the cells surrounded by cloud-free rings; and sometimes they are "open," with cloud-free cells surrounded by cloudy rings. The pattern constantly changes as cells emerge, disappear, and re-arrange.
The researchers ran highly detailed simulations of clouds to capture the precise air movements that create these patterns: in general, where air moves up it creates cloudy regions, and where it descends, cloud-free regions form. They then applied a mathematical technique called Voronoi tessellation to translate air movements into a network of polygonal tiles. The simple mathematical model developed by Glassmeier and Feingold re-creates this pattern. "It is like creating a dynamic mosaic with specific rules that allow one to replace different patches with a new set of tiles over and over again", Glassmeier explains. Their model offers a fundamental explanation for the structure and dynamics of stunning stratocumulus cloud decks.
And perhaps more importantly, the network analysis technique can help to produce more accurate clouds in computer models. "Clouds still represent a significant uncertainty in our climate projections", said Feingold. "Our hope is that this novel cellular network approach will lead to new ways of looking at the cloud parameterization problem".
CIRES is a partnership of NOAA and the University of Colorado Boulder.