A series of “atmospheric rivers” have impacted California over the past week. Such storms are common in this region during the cold season. Atmospheric rivers can be quite devastating in terms of flooding rainfall and extreme winds. However, they can also serve as “dryness relief.” According to federal agencies, 30-50% of total precipitation (rain and snow) in the western United States is produced by river-related atmospheric storms. Although this may be an unfamiliar term to many in the media and the general public, scientists have known them for some time. Unfortunately, like the term “Polar Vortex”, atmospheric rivers have been misrepresented in the media and in many public accounts.

    I tried to ignore it, but the atmospheric scientist in me couldn’t let go as I continued to see atmospheric rivers in mainstream outlets called “fire pipes, plumes or funnels” of precipitation. It reminded me of the days when people described the polar vortex as some kind of arctic hurricane. With the reach of traditional and social media, long-standing weather terms like Polar Vortex, derecho, cyclone bomb, and atmospheric river have entered the public lexicon. Inevitably, some of them have been mischaracterized.

    The National Oceanic and Atmospheric Administration website defines atmospheric rivers (RA) as “relatively long, narrow regions in the atmosphere – like rivers in the sky – that carry most of the water vapor out of the tropics”. The key words in this definition are “water vapour.” The NOAA website goes on to say, “When atmospheric rivers make landfall, they often release this water vapor as rain or snow.” The most powerful ARs can carry up to 15 times the average flow of water leaving the mouth of the Mississippi River.

    They are called rivers because they are characterized, usually using satellite data, as ribbons of water vapor about 250 to 375 miles wide. The image below reveals the AR which impacted the west coast on January 4, 2023. It is revealed by high values ​​of precipitable water, the total amount of water in a column if all the water vapor was condensed in liquid form. NASA’s Earth Facebook page notes: “A tropical Pacific moisture plume interacted with a rapidly strengthening low pressure system over the northeast Pacific, producing a storm that caused floods, knocked down trees and knocked down power lines. The low-pressure system strengthened rapidly over 24-hour periods in a process called “bombogenesis.” In my Satellite Weather course at the University of Georgia, we had a solid lecture on atmospheric rivers and how they are identified using humidity and motion information obtained from satellite.

    In 2019, a team of scientists released a new scale to assess the impact and strength of atmospheric rivers. The UC San Diego press release stated, “It ranks atmospheric rivers from 1 to 5 and creates the categories ‘weak,’ ‘moderate,’ ‘strong,’ ‘extreme,’ and ‘exceptional.’ water associated with the atmospheric river over a period of 1-2 days is the parameter used to categorize the storm A unique aspect of this scale compared to the Saffir-Simpson (hurricanes) or Fujita (tornadoes) scales is that it transmits the nuance of whether AR is potentially beneficial or destructive.The full scale can be seen at this link.Recent storms have ranged from Level 3 to Level 5, according to reports.

    Yes, storms ultimately produce significant precipitation on landfall through meteorological processes, orographic elevation associated with mountains, etc. However, the basis for describing atmospheric rivers is the water vapor in the plume. Hopefully this essay will help refine the description of atmospheric rivers in the future.

    By the way, you might hear the term “Pineapple Express”. What is that? NOAA’s Ocean Service website says it’s a “well-known example of a strong atmospheric river…moisture builds up in the tropical Pacific around Hawaii and can hit the shores western United States and Canada with heavy rain and snow.” But not all ARs are the Pineapple Express. ARs can be found all over the world. Our research at the University of Georgia even revealed the impact of ARs in the southeastern United States.

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