The above graphic (click to enlarge) summarizes the basics of atmospheric rivers. The most well-known example, as indicated above, delivers water vapor from near the equator to the western coast of north America, especially California. This phenomenon is the source of a large portion, if not a majority, of precipitation in these regions when moist air is forced upward over the Sierra Nevada mountains.
The formation of a specific atmospheric river event often occurs when a strong extratropical cyclone moves away from the equator toward higher latitudes, dragging warm equatorial air in its wake. This paper analyses the role such cyclones have in driving atmospheric rivers, focusing on a case study of a powerful February 2002 cyclone. This cyclone formed in the north Atlantic and moved northeastward, passing west of the British Isles before ultimately dissipating in the Barents Sea north of Scandinavia. At its peak intensity, it bottomed out at 935 mb, a pressure comparable to that of a category 4 hurricane!
The above shows the cloud pattern of the cyclone on an infrared satellite view (top) and a diagram showing total column water vapor (TCWV, bottom) in the same region. Unlike for hurricanes, most of the extratropical storm's water vapor is concentrated in a linear feature far from the center and extending south and west. The counterclockwise rotation of the large circulation in the northern hemisphere draws a long train of warm, tropical air northward, and it is here that the atmospheric river sets up.
In practice, meteorologists detect atmospheric rivers using satellite imagery, which can detect the transport of water vapor in the atmosphere, and the threshold for being an atmospheric river depends on the integrated water vapor transport (IVT) value in a given region. This is calculated by adding up the flow of moisture in different layers of the atmosphere. Using these tools, the frequency/severity of such events across the world has recently been analyzed.
The above map shows the percentage of severe surface precipitation events are associated with atmospheric rivers in different locations across the world over the period 1997-2014. Only precipitation events ranking in the top 2% for a given region are considered. Therefore, the map shows where atmospheric rivers have the greatest impact: mostly over the ocean, but also in portions of western North America, southern South America, central Asia, and eastern Australia. They also are associated with extreme wind events over many of the same regions. As a result, atmospheric rivers are responsible for many of the most damaging flooding events across the globe. Better understanding their formation and evolution will improve our ability to forecast and respond to flooding events in the future.
Sources: https://web.archive.org/web/20100610035058/http://paos.colorado.edu/~dcn/ATOC6020/papers/AtmosphericRivers_94GL01710.pdf, https://noaa.gov, https://journals.ametsoc.org/doi/pdf/10.1175/BAMS-D-14-00031.1, http://cw3e.ucsd.edu/wp-content/uploads/2018/02/WaliserGuan-ngeo2894.pdf
