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Atmospheric Sciences & Global Change
Research Highlights

October 2010

Shifting Showers

Researchers demonstrate how honeycomb clouds exhibit self-organization

Honeycomb clouds
Marine stratocumulus, honeycomb, clouds have open cells (sky in the middle) and closed cells (cloudy in the middle). Credit: Jeff Schmaltz, NASA Enlarge Image

Results:  Like shifting sand dunes, some clouds disappear in one place and reappear in another. New work by a team of atmospheric researchers, including Hailong Wang of Pacific Northwest National Laboratory, shows why: Rain causes air to move vertically, which breaks down cloud walls, then builds them back up with moist, warm rising air. The air movement forms patterns in low clouds that remain cohesive structures even while shifting about the sky, due to a principle called self-organization. These findings were published in Nature.

Why it matters: Marine stratocumulus clouds—clouds that reduce the amount of solar energy absorbed in the ocean—cover vast areas of the world's oceans. These clouds adopt many different patterns, such as open cells (less reflective) and closed cells (more reflective), that can radically change the amount of sunlight reflected on Earth. The open-cell clouds, which look like honeycombs, have white sections that reflect sunshine back into space, and open sections that let energy through to warm the planet. By understanding how cloud patterns form and evolve, scientists can build better models for predicting weather and climate change.

Methods:  The team started with a computer model called the Weather Research and Forecasting model, which scientists developed at the National Center for Atmospheric Research and the National Oceanic and Atmospheric Administration (NOAA). Wang and others improved the model to study interactions of aerosols and low clouds. They simulated fields of honeycomb clouds sitting below one kilometer over the ocean. The team fed the clouds just enough aerosols to produce rain and create the expected honeycomb shapes.

Though the open-cell clouds always resembled a honeycomb, the individual cells deformed and reformed over a couple hours. To determine why, researchers took the open-cell clouds and examined air flow and rain along the cell walls in the model. In addition, they looked at satellite images of real clouds and observations made by shipborne instruments. The answer lied within evaporation of raindrops and its effect on air motion.

What's next: Scientists are continuing to research the marine cellular clouds, and the interactions between aerosols and clouds to answer questions such as: Where are the aerosols from? Will the cloud patterns shift if we change the atmospheric aerosols in a controlled manner? What are the implications for weather modification and relevant strategy for mitigating global warming?

Acknowledgments: This work was supported by:  Hailong Wang of Pacific Northwest National Laboratory, Graham Feingold and William Alan Brewer of NOAA Earth System Research Laboratory,

IIan Koren from the Weizmann Institute, and Huiwen Xue from the Peking University School of Physics.

Reference: Feingold G, I Koren, H Wang, H Xue, and WA Brewer. 2010. "Precipitation-generated oscillations in open cellular cloud fields." Nature 466:849-852. DOI: 10.1038/nature09314

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