What is the Outlook for California’s Atmospheric Rivers?

Caitlin Dempsey

Updated:

The recent record-breaking rainfall totals already for the 2023-2024 wet season in California are largely due to a series of storms bringing precipitin in a form known as atmospheric rivers.

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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. Nicknamed “rivers in the sky”, atmospheric river events 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.

The term “atmospheric river” was coined in 1994.

An atmospheric river occured between 9th and 12th of Dec. 2014 over the Pacific Ocean and Southwest US. Image: NASA.
An atmospheric river occured between 9th and 12th of Dec. 2014 over the Pacific Ocean and Southwest US. Image: NASA.

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 trigger landslides.

The high winds and precipitation that arrives 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.

How much rain fell in Southern California during the early February 2024 winter storm?

The recent winter storm that brought a series of atmospheric rivers to both Northern and Southern California over the course of several days in early February 2024 is an example of the intensity of precipitation that these types of storms bring.

Overall, the February 4 to 6, 2024 winter storm brought record breaking amounts of rain to the Southern California area. Parts of Los Angeles such as the neighborhood of Bel Air received over 13 inches of rain during this period.

The map below visualizes average rainfall rates per hour obtained from satellite-based estimates provided by IMERG, which stands for Integrated Multi-Satellite Retrievals as part of the Global Precipitation Measurement mission, which collects detailed information on precipitation patterns around the globe.

A map showing a gradient of yellow (light rain) to deep red (heavy rain) for mm per second around the Los Angeles area during a winter storm.
Map showing the average rates of rainfall in the afternoons for the Los Angeles area based on satellite data between February 4 and 6, 2024. Map: NASA.

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. A study published in 2018 in San Francisco Estuary and Watershed Science looked at two-decades worth of rainfall data and found that the majority of California’s rain arrives due to atmospheric rivers.

By examining rainfall data from 176 weather stations in California over 20 years and checking this against times when atmospheric rivers were present, the study demonstrated how rain patterns vary across California and how they relate to atmospheric rivers.

Researchers also determined that a significant portion of the yearly rainfall in California comes from just a few heavy rainfalls. In the southern part of the state, 10 to 40 hours of rain can make up more than half of the year’s total rain. In the northern part, it takes 60 to 120 hours of rain to do the same. According to this research, just one big storm can bring 10% to 30% of a year’s rain to places across California.

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 is 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.

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.

A map showing moisture levels carried by an atmospheric river over the West Coast including California. High moisture levels are a dark blue-green and low moisture levels are brown.
An atmospheric river approaching California. High moisture levels are a dark green-green color and low moisture levels are brown and tan. Image: NASA, 5:30 a.m. Pacific Standard Time on January 4, 2023, public domain.

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 January 4, 2023 event makes landfall, California experiences a fall of central pressure of at least 1 millibar per hour for 24 hours — this is referred to as a “bomb cyclone” by meteorologists.

A true color satellite image showing an atmospheric river off the coast of California.
Satellite image taken January 4, 2023, at 1:20 p.m. of an atmospheric river affecting California. Image: NOAA-20 satellite, public domain.

What does Research indicate about the future of atmospheric rivers in California?

Researchers from UCLA looked at how the intensity of atmospheric rivers is expected to increase with climate change. Published in Science Advances in 2020, the study analyzed specific historical and future heavy rainfall events, focusing on atmospheric rivers.

The study’s findings indicate a projected increase of 10 to 40% in total rainfall from these storms, with the most substantial relative increases occurring in valleys and on the lee sides of mountains. Furthermore, the researchers modeled an even more intense rise in the intensity of maximum hourly rainfall, surpassing what would be expected based on the Clausius-Clapeyron equation, which describes how air can hold more water vapor as it warms. According to the authors of the study, about 85% of the increased volume of water moisture in atmospheric rivers can be attributed to warmer air temperatures.

A second published paper from 2022 on megafloods in California, also led by the same two researchers from UCLA, echoed this finding by noting that the main reason behind the expected increase in heavy rainfall in the state is due to atmospheric river events during the cooler months are getting stronger.

An intensification of the integrated vapor transport, a measurement of the intensity of atmospheric river events, brings with it an increase in the risk of flash flooding as well as elevated landslide risk and debris flow.

References

Cassidy, E. (2023, January 5). Atmospheric river lashes California. NASA Earth Observatory. https://earthobservatory.nasa.gov/images/150804/atmospheric-river-lashes-california

Huang, X., Swain, D. L., & Hall, A. D. (2020). Future precipitation increase from very high resolution ensemble downscaling of extreme atmospheric river storms in California. Science advances6(29), eaba1323. DOI: 10.1126/sciadv.aba1323

Huang, X., & Swain, D. L. (2022). Climate change is increasing the risk of a California megaflood. Science advances8(31), eabq0995. DOI: 10.1126/sciadv.abq099

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 YearSan Francisco Estuary and Watershed Science16(4).

Waliser, D., & Guan, B. (2017). Extreme winds and precipitation during landfall of atmospheric rivers. Nature Geoscience10(3), 179-183. https://doi.org/10.1038/ngeo2894

This article was originally published on January 6, 2023 and has since been updated.

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About the author
Caitlin Dempsey
Caitlin Dempsey is the editor of Geography Realm and holds a master's degree in Geography from UCLA as well as a Master of Library and Information Science (MLIS) from SJSU.

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