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The low rain clouds known as marine stratocumulus are generally found off the west coast of continents along the subtropical belts, sometimes extending for thousands of kilometers over the oceans. They form amazingly organized systems. Satellite images of these cloud systems reveal a puzzling pattern: a nearly perfect grid of cloud cells with hexagonal shapes. Some parts of these cloud fields are formed of closed cells – which look, in images, like fluffy white beehives. Other areas of the fields contain open cells, in which the clouds concentrate at the edges of the cell boundaries. Such cloud systems can persist for many hours.
Scientists at the Weizmann Institute, the NOAA Earth System Research Laboratory, the Pacific Northwest National Laboratory and Peking University have now shed new light on this long-standing mystery, revealing some of the basic physical principles that control these systems. But their findings may have other implications as well: The bright cloud cover of the closed cells cools the planet by reflecting radiation back into space, while the open ones reflect much less. Understanding the dynamic process that creates these open and closed patterns may help researchers to ascertain how human activity could affect their cooling capacity.
“A similar phenomenon to these cloud cells,” says Dr. Ilan Koren of the Weizmann Institute’s Environmental Sciences and Energy Research Department (Faculty of Chemistry), “is water boiling, in which the temperature drop from the pot’s heated base to the upper surface causes the water to rise and fall in columns. This rolling, cell-like pattern, known as Rayleigh-Bénard convection, was described over a hundred years ago.” In both cases, cells are generated when the bottom is hotter than the top and heat is transferred within the system.
As in many cases of Rayleigh-Bénard convection, cloud cells appear to be well organized and to obey a strict overall pattern. Koren and Dr. Graham Feingold of the NOAA lab in Colorado studied the role of rain in the open marine stratocumulus systems, watching how these systems progress in satellite images over periods of hours and days, as well as developing models to uncover their underlying organizing principles. They found a new feedback loop – one in which rain plays a central role – that forces the cells to oscillate between two states.
Their model begins with clouds forming on the edges (the walls) of the open cells as seawater evaporates and rises. Eventually, the water turns to rain, which then generates the opposite dynamic of that which created the clouds in the first place. In the first stages of the cloud formation, the energy that is released as the rising vapor condenses enhances and amplifies the updraft. Later, however, once the rain starts to fall and evaporate below the cloud base, a cooling effect takes over.
This cooling effect forces the air to sink, evaporating the parent cloud along the way. The new downdraft zone forces the air around it to be lifted, creating the next generation of clouds in a location that was previously the empty center of the parent cell.
In this way, all the cells are linked, and thus the various cells’ oscillations will be synchronized with one another. A single cell cannot act on its own: It needs the “permission” and collaboration of all the neighboring ones. Koren: “Like the sound of many hands clapping in rhythm, or the synchronized flashing of fireflies on a summer night, such communication creates a self-organized system that oscillates in a coherent way.” Though the individual cells alternately clear up and cloud over, the system as a whole persists for days at a time, leading the scientists to refer to “dynamic stability” in the cloud cover.
In his previous research, Koren investigated the role of tiny particles called aerosols – both natural and man-made – in cloud formation and precipitation. Because aerosols affect drop size and thus rainfall, he believes that the dynamics of marine stratocumulus systems could be especially vulnerable to the changes brought about by man-made particles in the atmosphere. This, in turn, could affect how well they reflect sunlight and cool the Earth.
Dr. Ilan Koren's research is supported by the Yeda-Sela Center for Basic Research. Dr. Koren is the incumbent of the Benjamin H. Swig and Jack D. Weiler Career Development Chair in Perpetuity.
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