Atmospheric river
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What is an atmospheric river?
An atmospheric river is a long, narrow band of concentrated moisture in the troposphere that transports water vapor and heat from the tropics to higher latitudes. They are components of extratropical cyclones with a low-pressure cell at their leading edges and are associated with low-altitude jet streams.
Atmospheric rivers provide over half the precipitation to coastal areas, but some can cause extreme precipitation and flooding that may last for days. When the leading edge of an atmospheric river passes over a coastal area, the river's path is forced upward, forming clouds and precipitation.
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How do atmospheric rivers form?
Atmospheric rivers are long, narrow bands of concentrated moisture in the troposphere that transport vast amounts of water vapor from tropical regions to higher latitudes. They form as part of extratropical cyclones, possessing a low-pressure cell at their leading edges and are associated with low-altitude jet streams ahead of a cold front.
As the leading edge of an atmospheric river moves over coastal areas, its path is forced upward by topography and warm air pockets. This causes the water vapor to lift and cool, resulting in cloud formation and precipitation. Mountain barriers can also block air flow, forcing moist air up slopes and causing precipitation on the windward sides.
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What kind of weather can an atmospheric river cause?
Atmospheric rivers can cause a range of weather conditions, from beneficial rain and snow to extreme and hazardous events.
Most atmospheric rivers are benign and deliver much-needed precipitation to drier regions, replenishing water supplies and supporting ecosystems. However, some can lead to extreme snowfall and rainfall, including thunderstorms, high winds, and flooding.
These extreme events can last for several days, overwhelming the soil's absorption capacity and local flood-control infrastructure. In mountainous coastal areas, they can also trigger landslides and mudslides, resulting in property damage, injuries, and loss of life.
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Where do atmospheric rivers usually occur in the world?
Atmospheric rivers are concentrated corridors of water vapor in the troposphere that transport vast amounts of water and heat from the tropics to higher latitudes. They are the primary source of water transport in the midlatitudes, providing over half the precipitation to coastal areas.
Most atmospheric rivers occur in the North Pacific, Atlantic, southeastern Pacific, and South Atlantic oceans, away from the tropics. On average, four to five atmospheric rivers are active in Earth’s atmosphere at any time, producing moderate rain and snow, but some can cause extreme precipitation and flooding.
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How do scientists detect and track atmospheric rivers?
Scientists use both in situ (on-site) and remote sensing methods to detect and track atmospheric rivers.
- Remote sensing uses instruments at a distance, with passive systems receiving emitted information and active systems emitting energy and recording its reflection.
- In situ measurements require direct contact with the subject of interest, using towers, tethered balloons, and surface data collection platforms.
These tools measure various meteorological parameters, including temperature, wind velocity, and moisture content. Doppler radars and lidars are also used, with Doppler radars estimating wind speed and direction. Aircraft, including commercial planes and research aircraft like the NOAA P-3, gather detailed atmospheric data, while ground-based detectors monitor lightning to map thunderstorm occurrences.
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Why are atmospheric rivers important for water supply and flood risks?
Atmospheric rivers are vital for the global hydrologic cycle, serving as major water vapor transporters from the tropics to higher latitudes. They provide over half the precipitation to coastal regions, essential for water supplies, vegetation, and maintaining ecosystems.
However, some atmospheric rivers cause extreme precipitation, leading to floods, high winds, landslides, and mudslides, especially in mountainous coastal areas. Climate change and rising air temperatures can increase the severity and frequency of these events, increasing the risk of flash floods.
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Atmospheric river, any long, narrow, and concentrated horizontal corridor of moisture in Earth’s troposphere. Such formations transport vast amounts of water vapor—at flow rates more than double that of the Amazon River—and heat from tropical regions near the Equator toward the middle and higher latitudes. They serve as the primary source of horizontal water transport in the midlatitudes, providing more than half of the precipitation to coastal areas in parts of Europe, North America, South America, New Zealand, and Southeast Asia and facilitating the movement of more than 90 percent of the world’s moisture from the tropics to the poles. Most atmospheric rivers can be found in the North Pacific, Atlantic, southeastern Pacific, and South Atlantic oceans away from the tropics, and they produce moderate amounts of rain and snow. However, some atmospheric rivers are responsible for extreme precipitation and flooding events that may last up to several days in some regions. An average of four to five atmospheric rivers are active in Earth’s atmosphere at any given time.
Characteristics
An atmospheric river varies in size, shape, and strength, appearing as a ribbon of moisture that may stretch more than 2,000 km (1,245 miles) long but somewhat less than 1,000 km (621 miles) wide across oceans. Such formations are components of extratropical cyclones that possess a low-pressure cell at their leading edges. They are associated with low-altitude jet streams—such as those that occur at heights of 2–3 km (1.2–1.9 miles)—that appear ahead of a cold front. Studies have shown that the intensity, location, and frequency of atmospheric rivers are affected by larger-scale climate phenomena, such as polar vortices, El Niño/Southern Oscillation (ENSO), and the Madden-Julian Oscillation (MJO).
As the leading edge of an atmospheric river passes over a coastal area, the river’s path is forced upward by topography and pockets of warm air. The water vapor entrained within the river lifts and cools, forming clouds and precipitation. Most atmospheric rivers are relatively benign phenomena that deliver much-needed rain and snow to dry areas, providing water to vegetation and helping to recharge snowpacks, reservoirs, and groundwater systems that contribute to the growth of all forms of life, the maintenance of biodiversity, domestic and industrial water supplies, agriculture, and hydroelectric power generation. However, some atmospheric rivers can produce extreme snowfall and rainfall events (including thunderstorms), which generate local high winds and flooding. Such events typically last about 20 hours, but some can continue over the course of several days, delivering steady, massive amounts of precipitation that accumulate so rapidly that the ability of the soil to absorb water and the capacity of local flood-control infrastructure are quickly overwhelmed. If such heavy precipitation events occur in mountainous coastal regions, such as those of California’s Pacific coast, they can also result in landslides and mudslides, which can lead to widespread property damage, injury, and loss of life (see also orographic precipitation).
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The AR scale
The effect of an atmospheric river in a given area is ranked by its duration and intensity according to the AR scale, which was developed by American meteorologists F. Martin Ralph, Jonathan J. Rutz, Jason M. Cordeira, and others in 2019. Atmospheric rivers are evaluated using a unit of flow measurement called integrated vapor transport (IVT), which accounts for winds and moisture quantities to determine total water volume carried by an atmospheric river across a line perpendicular to its flow. Rankings on the AR scale range from 1 to 5: AR-1 events produce an IVT greater than 250 kg of water vapor per meter (about 168 pounds per foot) per second, and each subsequent rank represents an increase in IVT of 250 kg per meter per second (500 kg per meter per second is needed to reach AR-2, 750 kg per meter per second is needed to reach AR-3, and so on).