Researchers embark on an ambitious project to unravel how precipitation comes to drought-stricken California.

“3-2-1, DROP! Buoy away.” Todd Richards releases the high-tech float from its plastic tube, and it plummets away from the belly of the P-3 research plane. Jostled by heavy turbulence and lashed by rain, the plane battles onward through the monster storm off the California coast, its four turboprop engines roaring.

Scientists hope studies of conveyor belts of moisture called atmospheric rivers will elucidate the role that the storms play in the California coast’s all-important rainfall patterns. Image courtesy of Shutterstock/Galyna Andrushko.

The sensing buoy, meanwhile, has fallen 2,500 meters and landed in the Pacific Ocean about 100 miles west of Sacramento. As the buoy sinks beneath the waves, it takes the temperature of the water to a depth of 400 meters, all the while pinging the measurements back to the plane.

Richards plans to drop about a dozen of these buoys during the P-3′s 6-hour flight, along with roughly 30 weather reconnaissance devices called dropsondes that parachute slowly through the air measuring pressure, temperature, humidity, and wind speeds. The data from these buoys and dropsondes will help the researchers study the top-to-bottom mechanics of this storm in unprecedented detail.

This flight is part of an ambitious experiment called CalWater2, whose goal is to understand exactly where California’s rain and snow comes from. By understanding the inputs of storms, they will, they hope, better understand the outputs. “Precipitation is a really tough problem to forecast,” says Marty Ralph, a research meteorologist at University of California, San Diego (UCSD) and coleader of CalWater2. “On average, heavy rainfall events are predicted to be about half as strong as they actually end up being. It’s just a very hard thing to get right.”

But by gathering precise data on atmospheric chemistry, meteorological interactions, cloud physics, ocean conditions, and a slew of other physical features of storms, CalWater2 aims to provide unprecedented insight into “atmospheric rivers.” These supersoaker storms act as conveyor belts of moisture that can deliver up to half of a region’s annual precipitation, and which last for days or even weeks at a time. CalWater2 also will explore the role these atmospheric rivers play in the coast’s all-important rainfall patterns.

For a state such as California, where drought conditions are beginning to look like the new normal, improving weather and climate forecasting models is a crucial way to help planners and policy makers manage the state’s water resources better and more sustainably. The work is already paying off: Over the past few months, team scientists have released findings and meteorological datasets that can be used to hone atmospheric models and forecasts. “Better forecasts will definitely benefit California and help us manage water,” says Michael Anderson, California's state climatologist, who is already looking forward to folding CalWater’s discoveries into the models he uses to provide climate information and predictions to state agencies. But these insights also stand to help track and understand atmospheric rivers flowing not just in California but in locales around the globe.

Rivers in the Sky CalWater2’s data-gathering expedition required a veritable scientific flotilla. The P-3 research plane, operated by the National Oceanic and Atmospheric Administration (NOAA) and affectionately named Miss Piggy, was not alone on its February 5, 2015 mission. Another P-3 called Kermit the Frog accompanied her. Also assisting were a Department of Defense G-1 plane that serves as a flying chemical laboratory, able to measure the interactions of cloud−aerosol particles within storms, and a high-altitude NASA ER2 plane that is essentially a U2 spy plane adapted for environmental research. Down in the Pacific, NOAA’s research vessel Ron Brown coordinated operations. “This is what we are calling our ‘max mission,’” says Ryan Spackman, program manager at NOAA Earth System Research Laboratory. “For the first time, we’re going to get a full suite of measurements, of the meteorology, the chemistry, and the aerosols associated with these events.” CalWater2 is the second phase of a large, multiagency, multidisciplinary experiment to understand the key factors that control the timing, location, and quantity of precipitation in the western United States. The first sampling campaign (www.esrl.noaa.gov/psd/calwater/overview/calwater1.html), which ran from 2009 to 2011, revealed some of the basics about the structure and behavior of large storms, such as the role of dust in promoting snowfall and the influence of winds on precipitation. Armed with new sets of questions and research objectives—including the planned and ongoing development of tools to support decisions during extreme storms—CalWater2 began in 2014 and will continue until early 2018. During the 2015 field campaign, the team collected data from 56 research flights, releasing 443 dropsondes and 148 buoys. Researchers do not yet fully understand how atmospheric rivers arise, nor what determines whether a weak storm will develop into a monster. However, it is clear that, for rain to fall, the storm needs aerosol particles to act as seeds for cloud droplets. “We’re trying, basically, to unravel what seeds a cloud, how much water you get out of a cloud, how much snow you get out of a cloud,” says Kim Prather, atmospheric chemist at UCSD and coleader on the experiment. During the first CalWater campaign, Prather’s laboratory found that different kinds of aerosols, such as dust, smoke, or sea spray, affect how much it rains or snows. Her research showed that the nature of the aerosol particle can change precipitation levels by as much as 40% (1). The star of the precipitation show is dust—sometimes from as far away as the Sahara desert or the Middle East—which is able to trigger more snowfall than any other type of particle studied (2). “It was like a switch,” Prather says. “On cold days when there was more dust in the air, there was more snowfall.” Between February 22 and March 11, 2009, Prather and her team spent several days coordinating measurements between several sites in or near the Sierra Nevada Mountains, and in coastal sites near San Francisco Bay (3). “On days when there wasn’t [any dust], the cloud would just be sitting there … it might give you a little drizzle.” Her team recently published work (4) showing that, somewhat to their surprise, sea salt in ocean spray is also able to seed ice particles. “That’s pretty important for climate science, as most of the water that we get on the ground starts as ice, frozen in clouds,” says Doug Collins, a postdoctoral researcher in Prather’s laboratory. “A lot of climate models have ignored sea spray particles as sources of frozen cloud droplets, but we're finding sea spray has an important contribution.” Still unclear, however, is the size of that contribution compared with terrestrial dust. The team has also seen pollution play the opposite role, by suppressing instead of fostering precipitation (5). This occurs because pollutant particles tend to be so small that the water droplets that gather around them have trouble gaining enough mass to fall out of the clouds. Right now, Prather says, essentially all of the models that predict California's weather and climate ignore the role that aerosol particles play in affecting rainfall. She hopes this will soon change. “Incorporating the aerosol component into weather forecast models, so we can predict when we’re going to have too much water or flooding, is really, really important so that we can prepare for those situations.” It might make the difference between needlessly releasing water out of a reservoir for flood control, as the law dictates, and keeping that water for California’s long, dry summers. With 95% of the state categorized as in “severe drought status” during much of last summer, according to the state’s official drought monitor, every drop counts (www.californiadrought.org/drought/current-conditions). From a chute in a NOAA P-3 research plane, electronics technician Todd Richards drops a buoy that will enable scientists to collect a profile of ocean temperatures in the water below.

Wet and Wild Aside from aerosols, the quantity of moisture in the atmospheric rivers directly affects, not surprisingly, how much precipitation falls from the sky. Miss Piggy’s flight at an altitude of 2,500 meters took her through the base of the atmospheric river, where most of the moisture is located. “We’re taking some of those experimental data,” says Ralph, “and we’re creating a weather model tailored to the atmospheric river problem.” Generally, Ralph says, precipitation prediction models underestimate heavy rainfall events by about half. That’s partly because most numerical models don’t represent the clouds, the aerosols, or the vertical air motions that create and control the conditions for precipitation. Using atmospheric river data to develop more detailed models can make those estimates more accurate. At Ralph’s laboratory, one key discovery from CalWater2 has been that atmospheric rivers contain almost twice as much moisture as they’d previously been given credit for (6). On average, Ralph says, a cross-section of a strong atmospheric river carries 27 times the amount of water flowing through a similar cross-section of the Mississippi River. After measuring 17 examples of atmospheric rivers, and combining data gathered between 2005 and 2015, researchers found that atmospheric rivers transported an average of 450 million liters of water every second. In the strongest atmospheric river, flow peaked at 800 million liters per second—enough to fill 320 Olympic-sized swimming pools. The temperature of the atmospheric river’s moisture, and of the ocean beneath it, may also play an important role in fueling the strength of the storm. “For regular hurricanes, water temperature definitely is a major factor,” says Richards. Although atmospheric rivers are not hurricanes, they may share similarities, such as gaining power from warm ocean water. Like hurricanes, atmospheric rivers also bring strong winds that can down trees and torrential rain that can trigger floods. Although the more advanced weather models being developed with the help of CalWater2 are not ready yet for prime time, state climatologist Anderson fully expects that they will be, in a few years. “We recognize now the role that atmospheric rivers play in our water supply and in floods,” he says. It’s vital, he adds, to understand the nuances between a storm that offers a useful water supply and one that poses a flood threat. However, research advancements don’t quickly translate into improved tools for federal and state officials. “It’s not easy to first understand the science and then fold it into operational activities,” says Anderson. “There are quite a few steps in between … particularly when you're identifying something like the importance of aerosol particles that you hadn't accounted for previously.”