Research Highlight: The Most Studied Atmospheric River in History

On Feb. 5, a narrow channel of water vapor drifting across the sky a mile over the Pacific Ocean aimed straight at Northern California. Going out to meet it were a research vessel and four research aircraft flying at different altitudes. Back on land, several dozen ground stations waited for the storm, instruments trained to look into clouds and see what is invisible to the eye.

The size of the research mission made it the most intensive study ever of a weather phenomenon that governs more than any other single event in nature – even earthquakes – what makes California California. California’s economy and its ability to sustain its vital agricultural productivity, environmental health, vineyards, suburbs, parks, and golf courses are directly affected by how the state obtains and uses water. Channels of water vapor in the sky, known as atmospheric rivers, like the one that struck on Feb. 5 are the fire hoses of precipitation that give large areas of the state up to half of their water. When atmospheric rivers strike, the water sometimes comes in flood-triggering bursts that cannot be controlled and sometimes the water doesn’t come at all.

The goal of the $10-million CalWater 2015 multidisciplinary field campaign is to understand why atmospheric rivers do what they do and to probe how aerosol particles from different sources influence clouds and precipitation processes. CalWater scientists want to determine under which conditions aerosols impact the amount of snow and rain that ultimately falls during atmospheric rivers and other storms. A critical goal is to distinguish the effects of local pollution aerosols, natural aerosols such as dust and sea spray, and pollution transported from other continents including Asia and Africa on clouds and precipitation processes in California.

For the two studies, Scripps Institution of Oceanography at UC San Diego, NOAA, the Department of Energy, NSF, California Department of Water Resources, NASA, and several other agencies have contributed equipment, meteorologists, atmospheric chemistry experts, and climate modelers to see atmospheric rivers in all dimensions. For tools, the experts have at their disposal lasers, mass spectrometers, radar, and other instruments on ships, aircraft, ground stations, and even the International Space Station to measure the conditions that control how much water vapor is in an atmospheric river and what its potential is to produce precipitation when it makes landfall.

“This is a real milestone for those of us involved in West Coast weather research and prediction over the last 20 years,” said Marty Ralph, a CalWater 2015 mission scientist and leader of the Center for Western Weather and Water Extremes at Scripps of the campaign. “Nothing of this scope has ever happened.”

The Feb. 5 atmospheric river pounced days after it was announced that San Francisco went through an entire January without rain for the first time in its history. In fact, it was the driest January ever recorded in Northern California. The news underscored not just how vital the precipitation the atmospheric river could produce was but also how important it is for scientists to understand how successful the storm was in letting go of it.

Aerosols are tiny natural particles like sea salt, dust, and biological matter kicked up from the oceans and land and human-produced particle pollution such as the soot from diesel exhaust. They are the building blocks of clouds. Water vapor clings to these bits, sometimes in sufficient mass to fall from the sky as rain or snow. The success of clouds in producing precipitation depends in large part on the nature of the aerosols that are seeding them.

The awareness of the importance of aerosols in clouds and overall climate has advanced significantly in the past 20 years. In that same timespan, scientists have identified and defined atmospheric rivers, patterns sometimes represented by Pineapple Express blasts on the West Coast but found throughout the world.

CalWater 2015 is part of an ongoing long-term study that began with a precursor called CalWater 1, which ran from 2009-2011.  That campaign and early results from this year’s have established that “local” aerosol sources are not the only important source. In fact, researchers found in the first campaign that some of the cloud-seeding aerosols that determined whether California got rain or not were dust and soot particles from Africa and Asia that made their way across the Pacific Ocean borne on winds at high altitudes before finally falling to earth mixed in with rain on California windshields and snow on Sierra Nevada slopes. CalWater 1 also helped quantify that between 40 and 50 percent of all precipitation comes during atmospheric river episodes. The scientists also concluded that human-made pollution particles appear to be capable of suppressing that precipitation at least some of the time.

"In 2011, we saw the first hints at a synergy between meteorological conditions such as atmospheric rivers and the transport of specific aerosol types which can change the precipitation efficiency," said Prather.  "This laid the groundwork for CalWater 2015."

Just two weeks into CalWater 2015, which formally launched on Jan. 14, researchers already made findings that suggest that international cooperation is as important as local pollution control in maintaining the global climate to which society has become accustomed. In late January, the researchers flew into a weak atmospheric river over the Pacific. Even with the aircraft flying at low altitudes, the particle analyzers on board could detect no particles of sea salt or any of the other aerosols they would expect to see being kicked into the atmosphere.

It turned out that a layer of warm air between the ocean just off the West Coast and the atmospheric river overhead acted as a barrier preventing any particles from the ocean from rising into the storm clouds. The effect of local aerosols off California’s coast or from land was nil and thus the aerosols from other continents were the only ones to influence what the clouds would do with the moisture they carried. It has been established that aerosols are key in creating clouds but this was evidence of the reverse, that meteorological forces control what types of aerosols make it into clouds.

Evidence suggests that atmospheric rivers need a certain amount of strength to overcome such local barriers. It will take time and data analysis to understand the full importance of this finding, but CalWater mission scientist Kim Prather said it suggests at first blush that air pollution control efforts in California or anywhere, while logically focusing on local sources, might only be addressing part of what influences local rainfall. It behooves each country to care about what other countries upwind of it are adding to the mix of aerosols in the sky.

“California is focused on sources of pollution on the ground but the sources of pollution above that could be quite different. You hear about how atmospheric rivers have all this water available but that on average, only about 20-30 percent falls, and this can vary from storm to storm,” said Prather, a distinguished chair in atmospheric chemistry with appointments at Scripps Oceanography and the Department of Chemistry and Biochemistry at UC San Diego, “but what we are only beginning to appreciate is how much aerosols affect the efficiency of the removal of precipitation from clouds.”

Between CalWater 2015 and a companion study called the ARM Cloud Aerosol Precipitation Experiment (ACAPEX), researchers use a total of four research aircraft flying through major storms while a ship outfitted with additional instruments cruises below.

Into mid-February, instrument teams will gather data from the NOAA research vessel Ronald H. Brown and two NOAA, one DOE, and one NASA research aircraft when weather forecasters see atmospheric rivers developing in the Pacific Ocean off the coast of California. NASA will also provide remote sensing data for the project. Study data are also being used to calibrate two new instruments installed on the International Space Station, a light detection and ranging (LIDAR) unit and an instrument for measuring wind speeds at the ocean surface. CalWater 2015 is one component of a larger second stage of the overall project called CalWater 2, which will continue through 2017.

CalWater mission scientist Allen White, a research meteorologist and acting chief of the Water Cycle Branch in NOAA’s Earth System Research Laboratory in Colorado, said that among other things the study represents a forward-thinking investment by collaborators, especially the California Department of Water Resources, which contributed heavily to the creation of a network of more than 100 meteorological stations around the state.

“You have an insurance policy,” White told a group of reporters gathered at McClellan Airfield in Sacramento at a Feb. 3 open house for media. “No other state in the country has invested in this kind of observation system so you should be very proud.”

Ralph added that the size of the investment into CalWater and the number of research centers involved, 10 in all, indicate the magnitude of the importance of understanding a phenomenon on which California and the West depend.

“To see the kind of team we have, the facilities we have, is really an amazing story about cooperation, partnership, perseverance, and the power of science to make a difference for society,” Ralph said.

Related Image Gallery: The Most Studied Atmospheric River in History