Most of California’s Yearly Rainfall Arrives Via Atmospheric Rivers

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A study that looked at two-decades worth of rainfall data has found that the majority of California’s rain arrives due to  atmospheric rivers.

Furthermore, about 10-30% of the rainfall measured at locations across the state comes from one major storm.  

The study analyzed rainfall rates at 176 weather stations across California for this study.

What are Atmospheric Rivers?

Atmospheric rivers are long, thin bands of moisture that travel through the sky like a river, transporting water vapor from the equator to the poles.

Along the West Coast, warm moist air from Hawaii is the source of atmospheric rivers during the rainy season.

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.

These “rivers in the sky” are typically 250 to 375 miles (400 to 600 kilometers) broad and convey as much water vapor as around 25 Mississippi Rivers.

Extreme precipitation events (which can produce severe floods) are mostly caused by atmospheric rivers in numerous mid-latitude, westerly coastal locations around the world, notably the west coasts of North America, Western Europe, and North Africa.

Rainfall is Different Between Northern and Southern California

Rainfall is distinctly different between northern and southern California, with the line demarcated at 37.5°N.  

Northern California averages about twice as many rainfall events as southern California.  Researchers found that, on average, there are 25 – 45 rainfall events each year in Northern California.  

Of those, about 40% – 50% are atmospheric rivers which contribute an average of 79% of extreme-rainfall along the northern coast and 76% in the norther Sierras.

Median characteristics of rainfall events including (A) annual numbers, (B) event-total rainfall, (C) event-duration. Figure: Lamjiri et al., 2018
Median characteristics of rainfall events including (A) annual numbers, (B) event-total rainfall, (C) event-duration. Figure: Lamjiri et al., 2018

Southern California tends to also experience lower median rainfall totals than northern California (10–14 mm versus 10–22 mm per event) as well as shorter rainfall durations (<5–11 hours versus 10-14 hours for northern California).

Atmospheric rivers contribute to 68% of extreme-rainfall accumulations in southern California.

The study

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).

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