DISCUSSION: Atmospheric rivers are long, narrow corridors in the atmosphere through which vast amounts of water vapor are transported. The strongest atmospheric rivers can carry up to 15 times more water than what flows through the mouth of the Mississippi River. These rivers are best depicted in satellite images that show water vapor in the atmosphere, but can also sometimes be seen via cloud cover. For example, the clouds extending from the bottom-left corner of the satellite image above (from NOAA’s GOES West satellite) into the west coast of North America represents an atmospheric river from 14 February 2019. Atmospheric rivers often make landfall along the west coasts of continents and can bring much-needed rainfall. They can also bring torrential rains that cause flooding/landslides and may be associated with high winds and associated damage.
Traditionally, a quantity known as integrated vapor transport (IVT) has been used to assess the intensity of atmospheric rivers and to assess their impacts. IVT incorporates measures of both water vapor and winds. Thus, it is possible that two atmospheric rivers have the same values of IVT, but one river has high moisture combined with relatively low wind speeds, while the other river has relatively low moisture with high wind speeds. According to the IVT measure, both these atmospheric rivers have the same intensity, but they can have very different impacts.
Scientists at Stanford University separated IVT values into their water vapor and wind components for historical atmospheric rivers. They then used this information to classify the rivers into four categories: wet atmospheric rivers have high moisture with low wind speeds, windy rivers have relatively low vapor contents with high wind speeds, wet and windy atmospheric rivers have both high vapor content and high wind speeds, and neutral rivers have average values of moisture and wind speeds. The Stanford University scientists studied the impacts of the four types of atmospheric rivers and somewhat surprisingly found that windy rivers tend to result in higher precipitation totals in the western United States than wet rivers. It might be expected that atmospheric rivers with higher moisture contents would bring more rain/ snow. However, in order to get precipitation, moisture-laden air must be forced to rise and cool. Windy atmospheric rivers may force more air up the slope of the mountains in the western United States causing more of the moisture in the river to precipitate out. Research is continuing to try to better forecast each type of atmospheric river in order to better anticipate and prepare for particular impacts of a given event.
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©2020 Meteorologist Dr. Ken Leppert II