Cloud Cellular Communication

No, I’m not talking about the Internet or the latest mobile phone. Apparently clouds are “teleconnected”. Two press releases were made the yesterday on the same subject, both are presented here. Note to climate scientists, try adding this to GCM’s.

NOAA scientists uncover oscillating patterns in clouds

Finding has implications for climate change

For all who have ever lain on their backs and gazed at clouds adrift in the blue: A new NOAA study has found that clouds “communicate” with each other, 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.

“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.”

While the discovery of synchronized behavior in clouds is one of many recent findings on self-organization in nature, the study also examines how suspended particles, or aerosols, in the atmosphere can influence these patterns and be a factor in climate change.

The team, which also includes Ilan Koren of the Weizmann Institute, Hailong Wang of Pacific Northwest National Laboratory, Huiwen Xue of Peking University, and Alan Brewer of NOAA, used satellite imagery to identify cloud systems with a “cellular, almost honeycomb-like structure.” In such systems, thick clouds form the walls of the honeycomb, and cloud-free zones form the open cells between the walls. The team also observed that these cellular structures constantly rearrange themselves, with cloud walls dissolving and open cells forming in their place, while walls form where open cells once existed.

Using computer models, the scientists reproduced this rearrangement or oscillation of the cloud honeycomb pattern, and identified the driving factor – rain. Next, they analyzed scanning laser measurements from a ship cruising under cloud systems to verify their model results.

“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.

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 results 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.

“Our work also suggests that we should expand our thinking about interactions between aerosols and clouds,” Feingold said. “Integrating our current focus on fundamental physical processes with broader studies on system dynamics could give us a more complete understanding of climate change.”

NOAA’s mission is to understand and predict changes in the Earth’s environment, from the depths of the ocean to the surface of the sun, and to conserve and manage our coastal and marine resources. Visit us on Facebook.

On the Web: NOAA Earth System Research Laboratory: www.esrl.noaa.gov

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Rain contributes to cycling patterns of clouds

Researchers demonstrate how honeycomb clouds exhibit self-organization

RICHLAND, Wash. — Like shifting sand dunes, some clouds disappear in one place and reappear in another. New work this week in Nature shows why: Rain causes air to move vertically, which breaks down and builds up cloud walls. The air movement forms patterns in low clouds that remain cohesive structures even while appearing to shift about the sky, due to a principle called self-organization.

These clouds, called open-cell clouds that look like honeycombs, cover much of the open ocean. Understanding how their patterns evolve will eventually help scientists build better models for predicting climate change. This is the first time researchers have shown the patterns cycle regularly and why.

“The pattern of the clouds affects how much of the sun’s energy gets reflected back into space,” said atmospheric scientist Hailong Wang of the Department of Energy’s Pacific Northwest National Laboratory, a coauthor on the study led by physicist Graham Feingold at the National Oceanic and Atmospheric Administration.

“We’ve teased out the fundamental reasons why the open-cell clouds oscillate. 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 measurements in the real atmosphere and represent these clouds in climate models,” Wang said.

In addition, this is the first time researchers have shown that open-cell clouds follow the principles of self-organizing systems — they spontaneously form dynamic, coherent structures that tend to repair themselves and resist change. Such clouds join other self-organizing networks such as flocks of birds, shifting sand dunes or bubbles in boiling water.

Convection Imperfection

Open-cell clouds are low, flat clouds that look like a quilt to someone looking down from an airplane. The quilt patches are frames of cloud that are clear in the middle, similar to a honeycomb. These honeycomb clouds develop from atmospheric convection, which is air movement caused by warm air rising and cold air falling.

The white parts of the honeycomb clouds reflect sunshine back into space, but the open spaces let energy through to warm up the planet. Because these clouds cover a lot of the ocean, climate scientists need to incorporate the clouds into computer models.

The simplest explanation for their appearance is what is known as Rayleigh-Benard convection. This classic form of convection can be seen between two horizontal, flat plates separated by a thin liquid layer: Heat up the bottom and warm liquid rises, pushing cold liquid near the top downward. The updrafts and downdrafts mold the liquid into vertical walls. If the bottom heats uniformly, the flow causes the top surface to break up into hexagonal cells, looking like a honeycomb. A honeycomb structure, it turns out, is one of the most effective way to transfer heat.

This occurs on a large scale in our atmosphere from the surface up to a couple kilometers (less than two miles). But the earth’s ocean is not a uniform surface and it doesn’t warm the atmosphere evenly from below. That’s one reason why open-cell clouds do not organize into perfect hexagons.

Also, the atmosphere is much more complex than a laboratory experiment. Other factors interfere with this type of convection such as aerosols, tiny particles of dirt around which cloud drops form. The number of aerosols determines the size of cloud drops and whether to form rain. To test the role of aerosols and rain, the international team led by Feingold at NOAA’s Earth System Research Laboratory in Boulder, Colo., used computer simulations and satellite images to explore how open-cell clouds develop and oscillate.

Shifting Showers

First, the team started with a computer model called the Weather Research and Forecasting model, which a team of scientists developed at the National Center for Atmospheric Research in Boulder, Colo. and NOAA. Wang and others improved upon it to study interactions of aerosols and low clouds.

For this study, they simulated fields of honeycomb clouds sitting below one kilometer (about 3/4 of a mile) over the ocean, where they are known as marine stratocumulus clouds. The team fed the clouds with just enough aerosols to produce rain and create the expected honeycomb shapes.

Though the open-cell clouds always looked like a honeycomb, the individual cells deformed and reformed over a couple hours. To determine why they changed in this way, the team took the open-cell clouds and examined air flow and rain along the cell walls.

Strong updrafts coincided with the presence of the thick vertical walls, the scientists found. Over time, however, these regions accumulated enough water to rain, which caused downdrafts. When adjacent downdrafts approached the ocean surface, they flowed outward and collided — air converged and formed new updrafts. The air in the downdrafts cooled off initially by evaporation of raindrops, but warmed up again near the ocean, starting the updraft cycle again but shifted over in space.

This cycling of falling rain, downdrafts and updrafts caused cloud walls and their cells to disappear but reappear somewhere else in the field. The honeycomb-structure of the clouds remained, but cells shifted in space. The authors call these shifts oscillations in open cells.

The Real World

The team then looked at satellite images of real clouds. They used pictures of cloud fields at different times and corrected for them being blown about by wind flowing horizontally. Over time, they saw bright white spaces replaced by dark empty ones, and again replaced by bright whiteness. The team’s computer model had replicated these oscillating light-dark cycles.

Wind and rain measurements also supported the simulation. Instruments on a ship on the ocean measured wind up to one kilometer high. The data showed outflows from rain in different parts of the sky collide at the ocean surface and flow back up. Instruments that measured precipitation showed periodic rainfall that coincided with the shifting cloud pattern.

Taken together, the set of experiments showed that rain causes open-cell clouds to form spontaneously, oscillate in the sky and resist change in the overall pattern. These are three characteristics of complex systems that self-organize and form a cell structure, such as flocks of birds or bubbles on a boiling surface.

Reference: Graham Feingold, Ilan Koren, Hailong Wang, Huiwen Xue, and Wm. Alan Brewer, Precipitation-generated oscillations in open cellular cloud fields, Nature, August 12, 2010. DOI 10.1038/nature09314 (http://www.nature.com/nature/index.html).

This work was supported by NOAA, the Cooperative Institute for Research in Environmental Sciences and PNNL.

Pacific Northwest National Laboratory is a Department of Energy Office of Science national laboratory where interdisciplinary teams advance science and technology and deliver solutions to America’s most intractable problems in energy, national security and the environment. PNNL employs 4,700 staff, has an annual budget of nearly $1.1 billion, and has been managed by Ohio-based Battelle since the lab’s inception in 1965. Follow PNNL on Facebook, LinkedIn and Twitter.