A study that looked at two-decades worth of California rainfall found that most of the state’s annual precipitation arrives via atmospheric rivers.
What are atmospheric rivers?
Atmospheric rivers are long, narrow bands of highly concentrated moisture in the atmosphere that can transport large amounts of water vapor from the tropics to other parts of the world. These rivers in the sky can be thousands of kilometers long and a few hundred kilometers wide, and they are responsible for a significant portion of the global transport of water vapor.
An atmospheric river is a narrow corridor or channel of concentrated moisture in the atmosphere that can transport an enormous amount of water vapor from the topics to other areas of the world. Nicknamed “rivers in the sky” because of the enormous movement of water over long distances, atmospheric rivers can be thousands of kilometers long and a few hundred kilometers wide.
The term “atmospheric river” was coined in 1994.
A damaging mid-latitude storm
When atmospheric rivers make landfall, they can produce heavy precipitation, strong winds, and flooding.
While atmospheric rivers can also bring drought relief to dry regions by delivering much-needed moisture, the precipitation overload can also wreak havoc by causing flooding, toppling trees, down power lines, and producing landslides.
The high winds and precipitin that arrive with atmospheric rivers make these storms some of the most damaging types of mid-latitude storms according to researchers. The mid-latitudes are the latitudes between the tropics and polar regions.
Why does California get so many atmospheric rivers?
California is located in a region where atmospheric rivers are common, and the state is particularly vulnerable to their effects due to its long, narrow shape and mountainous terrain.
California’s location along western coast of the United States means that the state is affected by atmospheric rivers that originate in the tropical Pacific Ocean. These atmospheric rivers tend to form along the east side of the subtropical high pressure system that is centered over the Pacific Ocean and are often directed towards the west coast of North America by the prevailing westerly winds.
The combination of moist air from the tropics and the high terrain of the Sierra Nevada mountains can lead to the formation of strong, moisture-laden atmospheric rivers that can bring heavy rain and snowfall to California.
In addition, California has a Mediterranean climate, which means that the state experiences wet winters and dry summers. As a result, almost all of the state’s annual rain and snowfall comes from atmospheric rivers and other winter storms. These storms are an important source of water for California’s agriculture, hydropower, and other water-dependent industries.
Satellite imagery of California’s atmospheric river
The firehose effect of atmospheric rivers are best visualized by looking at various satellite imagery of these storms as they move towards California. Earth observation satellites can collect information about moisture levels and wind speeds to illustrate the strength of atmospheric rivers.
In this image, produced with data collected by NASA’s Goddard Earth Observing System, Atmospheric Data Assimilation System (GEOS ADAS), visualizes the total precipitable water vapor in the atmosphere on January 4, 2023 (5:30 a.m. Pacific Standard Time) as the storm approached California.
The NOAA-20 weather satellite captured this true color image of the same storm system on January 4, 2023, at 1:20 p.m. Pacific Standard Time. This storm system also produced a condition called bombogenesis, which is a condition where the intensity of a midlatitude cyclone rapidly increases. When a storm, like this recent one in California experiences fall of central pressure of at least 1 millibar per hour for 24 hours this is referred to as a “bomb cyclone” by meteorologists.
Cassidy, E. (2023, January 5). Atmospheric river lashes California. NASA Earth Observatory. https://earthobservatory.nasa.gov/images/150804/atmospheric-river-lashes-california
Lamjiri, M. A., Dettinger, M. D., Ralph, F. M., Oakley, N. S., & Rutz, J. J. (2018). Hourly Analyses of the Large Storms and Atmospheric Rivers that Provide Most of California’s Precipitation in Only 10 to 100 Hours per Year. San Francisco Estuary and Watershed Science, 16(4).
Waliser, D., & Guan, B. (2017). Extreme winds and precipitation during landfall of atmospheric rivers. Nature Geoscience, 10(3), 179-183. https://doi.org/10.1038/ngeo2894