A satellite image showing water-vapour concentration reveals an atmospheric river (yellow) streaming northeast across the Pacific Ocean. Credit: Darren Jackson/NOAA ESRL

Californians call it the Pineapple Express: a weather pattern that zips across the Pacific Ocean from Hawaii, delivering not baskets of tropical fruit, but buckets of rain and snow.

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In meteorological terms, the Pineapple Express is an atmospheric river, a narrow band of air that carries huge amounts of moisture. For the next six weeks, meteorologists will be plying the eastern Pacific by air and sea, in the hope of catching several atmospheric rivers barrelling towards the coast. It is the biggest push yet to understand these phenomena, which have received serious scientific attention only in the past decade.

Atmospheric rivers get their start over warm tropical waters; they then flow eastwards and towards the poles a kilometre or two above the ocean surface. They may stretch for thousands of kilometres, but are only a few hundred kilometres wide. When they hit land, they start to drop their moisture in torrential downpours or blizzards.

“When we have too many atmospheric rivers, floods can occur, and when we don’t have enough we gradually fall into drought,” says Marty Ralph, a meteorologist at the Scripps Institution of Oceanography in La Jolla, California, and a leader of the field campaign.

In Europe, atmospheric rivers affect mostly the western part of the continent, but they can be felt as far inland as Poland. In North America, the entire west coast is affected, and parts of the central and eastern United States occasionally feel the effects of atmospheric rivers that develop over the Gulf of Mexico. The moisture is often welcome, bringing up to half of the year’s water supply in affected areas1. A 2013 study found that as many as three-quarters of all droughts in the Pacific Northwest between 1950 and 2010 had been brought to an end by atmospheric-river storms2. California has been stricken by drought for years (see Nature 512, 121–122; 2014), but last month, an atmospheric river dropped enough rain to erase one-third of the water deficit of one major reservoir in just two days.

Climate change may bring stronger and more frequent atmospheric rivers, because the warmer the atmosphere is, the more water it can hold, says David Lavers, a meteorologist at Scripps who is not involved in the project. “The more you know about how the atmosphere behaves,” he says, “the better position you’re in to prepare for extreme events.”

The current US$10-million field campaign, called CalWater 2015, is a massive push to capture the physics of atmospheric rivers as they make landfall. A research vessel from the US National Oceanographic and Atmospheric Administration (NOAA), the Ronald H. Brown, is sitting several hundred kilometres off the California coast, waiting to position itself beneath an atmospheric river whenever one forms. The ship carries a suite of instruments operated by the US Department of Energy (DOE) that will peer upwards to study aerosol concentrations and other properties of the atmosphere once a river has formed.

Four aeroplanes — two belonging to NOAA, and one each to the DOE and NASA — will fly directly into the atmospheric river. They will release dropsondes, miniature parachutes carrying an array of instruments that record weather data as they descend.

One major goal is to work out how aerosols affect atmospheric rivers, says Kimberly Prather, an atmospheric chemist at Scripps and another leader of the project. A smaller field campaign in California between 2009 and 2011 suggested that contrary to expectations, aerosols served as nuclei for ice crystals to grow within atmospheric rivers and later produce snow3. “It’s Mother Nature’s way of seeding things,” says Prather.

CalWater 2015 will measure how aerosol particles interact with atmospheric rivers, both offshore and when they reach the coast. Supercomputer simulations will examine how airborne dust affects the amount and type of precipitation that eventually falls, says Ruby Leung, an atmospheric modeller with the Pacific Northwest National Laboratory in Richland, Washington.

That information should help water managers to predict what atmospheric rivers may bring. Reservoir engineers in northern California typically release precious water from their reservoirs in the winter, so as to have enough space behind the dam to cope with the threat of flooding. With better knowledge of when atmospheric rivers might arrive and how much water they might carry, says Ralph, engineers should be able to manage that winter release more effectively.