August 7, 2012

Unknown Mechanism For Rare Twinned Rainbows Discovered

Brett Smith for redOrbit.com - Your Universe Online

Disney-funded research intended to better depict CGI rainbows stumbled onto a previously unknown mechanism that causes rare twinned rainbows, according to a report to be presented this week in Los Angeles at SIGGRAPH 2012, the International Conference on Computer Graphics and Interactive Techniques.

Twinned rainbows, unlike the more popular double rainbows, appear as two independent rainbow arcs that share a common base rainbow. It has often been known to appear in conjunction with a double rainbow.

"Everyone has seen rainbows, even double-rainbows, and they continue to fascinate the scientific community," said co-author Wojciech Jarosz, a researcher at Disney Research in Zürich. "Sometimes, when the conditions are just right, we can observe extremely exotic rainbows, such as a twinned rainbow. Until now, no one has really known why such rainbows occur."

The first studies of rainbows assumed raindrops acted as tiny prisms, simply refracting light as it enters or leaves the water drop. While this model works for single and double rainbows, it falls short when trying to explain supernumerary or twinned bows.

A more complicated model, the Lorenz-Mie theory, accounts for many different factors involved in the scattering of sunlight, however it can yield the wrong predictions and it limits the kinds of rainbows that can be explained as well.

One of the problems with the Lorenz-Mie theory regarding rain is that it assumes the drops are spherical. Because water drops become non-spherical as they get larger, this theory also falls flat.

In the latest study, the researchers turned to computer modeling to fill in the knowledge gaps surrounding how sunlight interplays with atmospheric water droplets to form rainbows.

The research team included University of California, Davis computer science professor Henrik Wann Jensen, who has also created special effects on films such as “Avatar.” Jensen began work on the project with the intent of finding out how accurately computers could illustrate the irregularities found in naturally occurring rainbows.

Using 3-D modeling technology, the team was able to accurately replicate the raindrops that cause rainbows. They found that non-spherical, hamburger-shaped type of raindrop they called "burgeroids" shift light in a slightly different direction, causing the twinning rainbow phenomenon.

"Sometimes two rain showers combine," Jarosz said. "When the two are composed of different sized raindrops, each set of raindrops produces slightly deformed rainbows, which combine to form the elusive twinned rainbow."

Software developed by the team was able to reproduce these conditions in simulation so that the results matched, for the first time, twinned rainbows seen in photographs. The team also modeled a wide array of other rainbows based on photographs.

“This goes beyond computer graphics,” Jensen said. “We now have an almost complete picture of how rainbows form.”

The researchers speculated that this newly developed technology could accurately render models of other atmospheric phenomena commonly studied by meteorologists and climate change researchers. It would allow these scientists to study atmospheric activity from the comfort and convenience of a research lab, rather than out in the field in the middle of a downpour.