Intriguingly, the clouds tend to form a cell-like pattern over scales of tens of kilometres. Now researchers from the US, Israel and China have found the mechanism by which aerosols can affect whether these cells have an open or closed structure, altering their reflectivity. In the case of open cells, aerosol particles can even drive oscillations in the structure of the cloud and synchronize rainfall.
"Our study points to the important role of aerosol particles, through their control on precipitation, in determining not only the local properties of the cloud – such as its ability to rain, or how much energy it reflects to space – but also the large scale organization of the cloud system," Graham Feingold of the US National Oceanographic and Atmospheric Administration (NOAA) told environmentalresearchweb. "We have developed numerical models and remote sensing techniques that show that when aerosol concentrations become sufficiently low, and rainfall sufficiently widespread, the cloud can shift from a highly reflective, non-precipitating state (the closed-cellular state) to a much less reflective, precipitating, open-cellular state."
According to Feingold, scientists have known for decades that suspended aerosol particles can modify the magnitude of cloud reflectance – more particles favour brighter clouds and less precipitation. "But it is only in recent years that we have begun to appreciate that the aerosol, through its effects on precipitation, can determine the self-organized, cellular patterns that the system exhibits at scales of a few to hundreds of kilometres," he said.
Feingold and colleagues from NOAA; the Weizmann Institute in Israel; Pacific Northwest National Laboratory, US; and Peking University, China, found that, once movement of clouds due to wind had been accounted for, they could see the clouds' oscillating structure in satellite images.
The researchers believe that precipitation causes air to move downwards from clouds to the Earth's surface, where it spreads out and meets air from neighbouring cells that are experiencing the same phenomenon. This results in the formation of surface convergence zones and new clouds. When these new clouds produce rain, the downwards-moving air is in a different location. The result is an oscillating, self-organizing system with a characteristic cell size and precipitation frequency.
The findings should help with climate modelling. "Our work shows that an entire cloud system can re-organize itself into a completely different pattern if precipitation is allowed to form," said Feingold. "The result is much lower cloud cover and reflectance than a non-precipitating cloud system and less of a cooling effect on climate."
What's more, when clouds are in the open-celled state, their precipitation can become periodic and synchronized. Many natural systems exhibit synchronized, oscillatory behaviour, says Feingold, for example "the synchronous flashing of fireflies, chirping of crickets, or ticking of pendulum clocks". He reckons the precipitation synchronization is "yet another example of how local interactions between neighbouring components of a natural system can result in self-organization of the entire system".
Now the researchers plan to explore the conditions under which the open-cellular cloud system might return to a highly reflective, closed state. They'd also like to "expand our current thinking of aerosol-cloud interactions to synthesize our current focus on fundamental physical processes with a broader 'systems approach'".
The team reported its work in Nature.