August 12, 2010 - Findings has implications for climate change, a new NOAA study has found that rain clouds form synchronous patterns in which individual clouds in a large cloud field respond to signals from other clouds, much like chirping crickets or flashing fireflies on a summer night. The study, published online in the journal Nature also has significant implications for our understanding of climate change research.  

This research, led by Graham Feingold of NOAA’s Earth System Research Laboratory in Boulder, Colo., shows for the first time that interactions between certain types of neighboring clouds can result in synchronized rain patterns within a large cloud system.

“Clouds organize in distinct patterns that are fingerprints of myriad physical processes,” Feingold explained. “Precipitation can generate fascinating honeycomb-like patterns that are clearly visible from satellites. Cloud fields organize in such a way that their components ‘communicate’ with one another and produce regular, periodic rainfall events.” 

How does this synchronization come about? Falling rain cools the air as it descends. This creates downward air currents. These downdrafts hit the surface, flow outward and collide with each other, forming updrafts. The air flowing up creates new clouds in previously open sky as older clouds dissipate. Then the new clouds rain, and the oscillating pattern repeats itself. 

“Once precipitation ensues and an open structure has formed, it is difficult to revert the cloud field to a closed-cell, or overcast state,” Feingold said. “Rain keeps the oscillating, open honeycomb pattern in motion, which allows more sun to reach Earth’s surface.” 

The scientists say that their findings point to a significant influence of particulate matter, or aerosols, on the large-scale structure of clouds and therefore on climate change. Scientists have long known that aerosols can influence local rain formation and block solar energy from reaching the Earth’s surface—for an overall surface cooling effect. 

However, until recently, the scientific community has not considered the self-organization that result from these effects. Computer simulations for this study indicate that high aerosol concentrations favor the formation of large, dense cloud fields with less open space and less rain. This creates a more reflective cloud pattern and cooling of the surface. Low particulate levels in computer models resulted in rain and the open honeycomb structure with an oscillating pattern. The open honeycomb structure in a large cloud field lets more sunlight reach the surface, and hence results in surface warming.