Friday, February 23, 2018
Rivers in the sky

Rivers in the sky

By Kenneth Lang, Sea Grant Knauss Fellow at NOAA's Office of Oceanic and Atmospheric Research

Yes, there are rivers in the sky!  Atmospheric rivers, to be exact, are narrow bands of moisture that regularly form above the Pacific Ocean and flow towards North America’s west coast, drenching it in rain and packing it with snow.   These rivers, which transport more water than the Amazon or the Mississippi, have a far-reaching impact - even on the food you may be eating today.

With the January 14 sailing of NOAA’s largest ship, the Ronald H. Brown, a major investigation of atmospheric rivers named CalWater 2015 is now underway.

Measuring from sea

Measuring from sea

Scientists aboard NOAA Ship Ronald H. Brown will measure wind, temperature, and the air pressure within atmospheric rivers while sailing from Hawaii to California. (NOAA)
“Improving our understanding of atmospheric rivers will help us produce better forecasts of where they will hit and when, and how much rain and snow they will deliver,”  said Chris Fairall, the chief of Weather and Climate Physics at NOAA’s Office of Oceanic and Atmospheric Research Earth System Research Laboratory in Boulder, Colorado.

While atmospheric rivers impact the entire West Coast, they are particularly important for California, where they can deliver half of the state’s total annual precipitation in just a few storms.

California produces nearly half of all the fruits, vegetables and nuts grown in the United States, requiring an enormous amount of water.  If it were its own country, California would be the eighth largest economy in the world – larger than Italy or Russia. Interruptions to the water supply impact not only the agriculture, industry, wildlife, and citizens of the state, but the nation as a whole.

With California currently facing one of the most severe droughts on record, the mission could not be more pressing: atmospheric rivers are collectively drought breakers - though they can also cause serious and dangerous flooding.

The Pineapple Express

Atmospheric river animation

Atmospheric river animation

This animation shows an atmospheric river rolling into the California coast last February 2014 during earlier NOAA and partner research. (NOAA)
TV  weather forecasters in California in the 1960s noticed the atmospheric rivers traveling from an area near Hawaii to the West Coast and dubbed the phenomenon the Pineapple Express.

“But the atmospheric rivers sometimes flow north towards Alaska, bypassing California altogether.  Or they may arrive in California but with much less precipitation than usual,” said Fairall.  Fifteen years ago scientists began to study these changes and how to better anticipate them.

One question CalWater 2015 is investigating is the degree to which precipitation from atmospheric rivers is affected by aerosols.  Aerosols, in scientific parlance, do not refer to spray cans but to particles suspended in the air, such as dust or pollen or pollution.  Moisture in the air condenses around such particles and this is what forms clouds and rain.

Scientists are learning that not all particles produce the same kind of precipitation under the same conditions.  For example, particles blown from the deserts of Africa via the jet stream may have more precipitation forming qualities than local man-made pollutants, which may actually decrease precipitation.  Some particles may also be better at forming snow versus rain, a critical difference in the Sierras where the snowpack serves as California’s biggest reservoir.

CalWater 2015 is also investigating what proportion of the water falling on California originated at the rivers’ “source” in the tropics, and how much was picked up from the ocean along its 3,000 mile route.

“I was quite surprised by some early analyses that indicate the rivers may actually be picking up very little water along the way.  This is one of the things I’ll be measuring at sea on the Ron Brown,” said Fairall.

An Armada’s effort

Flying through rivers in the sky

Flying through rivers in the sky

Scientists aboard NOAA hurricane hunter aircraft will measure wind, air pressure and moisture while flying through atmospheric rivers. (NOAA)
CalWater 2015 will simultaneously employ, in addition to NOAA Ship Ronald H. Brown, two highly specialized NOAA aircraft, the Lockheed WP-3D Orion and Gulfstream IV-SP “hurricane hunters”; NASA and Department of Energy aircraft; NOAA satellites; roughly 100 ground stations; and hundreds of weather balloons, all being deployed into the line of storm activity of two to three anticipated atmospheric rivers.

NOAA Ship Ronald H. Brown’s commanding officer, Capt. Robert A. Kamphaus, explains that, while it may seem counterintuitive to put a ship in the path of a winter storm, “the Brown is a very seaworthy and capable platform with a great crew.”  The crew will also closely watch National Weather Service forecasts.  “I’m confident we will meet the project objectives while keeping the ship and all aboard safe,” said Kamphaus.

Environmental intelligence

The goal of CalWater 2015 is the same as the mission of NOAA: to provide environmental intelligence.  This intelligence will be used by water resource managers in many fields, including those involved with agriculture and flood control.

The risk of severe flooding from atmospheric rivers can be significant, and environmental information can save lives, property, and money.  With severe weather forecasting, every hour counts.

“Flooding, besides being a serious life and limb hazard, has other impacts, such as mudslides, transportation issues, sewage overflowing into the ocean…It even affects the management of salmon and steelhead.  Better forecasts derived from the data we are collecting this month will make it easier to manage all of it,” said Fairall.

The CalWater 2015 field campaign runs from January 14, 2015 to mid-March 2015.  Leading agencies are NOAA; Scripps Institution of Oceanography; NASA; the Department of Energy; the Naval Research Laboratory; and the State of California.  Other partners include CIRES, the Cooperative Institute for Research in Environmental Sciences, and STC, Science and Technology Corporation. CalWater 2015 evolved out of an ongoing NOAA Earth System Research Laboratory project called the Hydrometeorology Testbed.

For more information, please contact Monica Allen, director of public affairs for NOAA's Office of Oceanic and Atmospheric Research, at 301-734-1123 or by email at




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