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[pulsing music]

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Title: A 3D View of an Atmospheric River from an Earth System Model

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Features in Earth’s atmosphere,

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spawned by the heat of the Sun and the rotation of the Earth,

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transport water and energy around the globe.

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Clouds and precipitation shown here are from NASA’s MERRA-2 reanalysis,

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a retrospective blend of a weather model

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and conventional and satellite observations.

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Within the mid-latitudes, winds move clouds from west to east.

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Within the tropics easterly trade winds converge

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along the equator to create a moisture rich cluster

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of clouds, convection, and precipitation called the

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intertropical convergence zone, or ITCZ.

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Disturbances in its flow transport immense amounts of moisture

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and energy from the tropics to the poles.

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Studies have shown that atmospheric rivers account for the vast majority

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of the poleward transport of water vapor.

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The American Meteorological Society defines an atmospheric river

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as “a long, narrow, and transient corridor of

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strong horizontal water vapor transport that is typically associated

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with a low-level jet stream ahead of the cold front of an extratropical cyclone.”

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A common measure for the strength of an atmospheric river

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is the integrated water vapor transport, or the amount of moisture

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that is moved from one place to another by the flow of the atmosphere.

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The blue shading shown here gives a three-dimensional view

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of the water vapor transport.

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Tropical moisture is pulled in from the ITCZ

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and in this example, converges with other flows to form an atmospheric river.

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The feature then travels towards the west coast of the United States

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as a sub-class of atmospheric rivers commonly referred to

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as the “pineapple express” due to its origin near Hawai’i.

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The atmospheric river is guided

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guided by the semi-permanent sub-tropical high pressure off the coast of California

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and the Baja Peninsula as well as

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the Aleutian low in the Gulf of Alaska.

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The pressure gradient between the clockwise flow of the Californian high

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and the counterclockwise flow of the Aleutian low funnel the atmospheric moisture into a narrow corridor.

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The more intense the pressure gradient is,

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the stronger the winds are that transport the water vapor.

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Extreme rainfall has also been associated with the more intense gradients.

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Much of the moisture stays close to the surface

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but the rising motion of the low pressure to the north

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results in the air cooling, condensing the water vapor into a liquid.

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Precipitation over the ocean falls along the feature’s cold front on its northern side.

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Another way that air can rise

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and condense into precipitation is through orographic lift.

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When air encounters the mountains

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along the west coast of the United States, it is forced upwards.

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The rising air becomes saturated, causing rain and snow to fall,

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particularly on the windward side of the mountain.

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The flow of air continues eastward, depleted of its moisture.

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The precipitation that falls because of atmospheric rivers

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is important for the hydrologic cycle in the western United States.

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The winter buildup of the snowpack provides valuable freshwater resources.

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Despite being beneficial at times,

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, atmospheric river induced precipitation can also be destructive.

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The occurrence of extreme atmospheric river precipitation events,

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such as the one that occurred in this example,

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can result in widespread flooding and mudslides.

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Atmospheric rivers are not unique to the west coast of North America

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and occur around the globe, including New Zealand,

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Greenland, Antarctica, and the Middle East.

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The study of global phenomenon such as atmospheric rivers over the past four decades

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decades is made possible through NASA’s MERRA-2 reanalysis,

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, a spatially and temporally consistent blend of satellite

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and conventional observations with a numerical model.

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With a dataset that provides hourly information

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around the globe since 1980,

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there is still much that can be learned about Earth’s atmosphere

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and the transport of water and energy around the globe.

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