Delta science: Atmospheric rivers govern California’s rainfall

Rainfall patterns in California differ from any other place in the United States According to the U. S. Geological Survey (USGS), around 30 to 50 percent of average annual precipitation occurs in the form of a few large storms, or rapid series of storms.

The storms that come into Northern California are unlike the hurricanes that create havoc along the Gulf states and the Atlantic seaboard. The vortex of wind energy around the eye of a major hurricane can expand in a circle hundreds of miles wide, with accompanying ocean wave surges, causing catastrophic losses to life and property.

In contrast, the large Pacific storms are in the form of fast-moving atmospheric rivers. They are more linear, longer and narrower. However, they too can cause severe damage.

Scientists study atmospheric rivers

The Delta Stewardship Council is receiving the findings of the latest scientific research on these atmospheric rivers to include in their policy determinations.

At the Oct. 25 Delta Stewardship Council meeting, Lead Scientist Peter Goodwin, Ph.D, Hydraulic Engineering, gave an account of the seventh biennial Bay Delta Science Conference that was held Oct. 16-18 at the Sacramento Convention Center. Around 1,000 scientists, managers and policymakers attended. Goodwin was the featured speaker at the conference Town Hall meeting on “Building the Delta Science Plan.”

The Delta Stewardship Council lead scientist is director of the Delta Science Program. He is also in charge of the Delta Independent Science Board, and works with agency scientists and the whole scientific community.

Goodwin reminded the council that the concept of atmospheric rivers was first brought to their attention by the previous lead scientist, Clifford Dahm, Ph.D., Aquatic Ecology. The Delta Science Program sponsored two Brown Bag luncheon presentations in the summer of 2011 on the topic, with Dahm in attendance.

At one of the Brown Bag meetings, Michael Dettinger, from the U.S. Geological Survey (USGS), Scripps Institute of Oceanography in La Jolla, described atmospheric rivers as “narrow corridors of concentrated moisture in the atmosphere. Within these corridors, moisture in the air flows like a river in the sky. The corridors are approximately 150-200 miles across, can be thousands of miles long, and are responsible for most of the horizontal transport of water vapor outside of the tropics.” They move with the weather and are present somewhere on the earth at any time.

The Pineapple Express is one type of strong atmospheric river. A Pineapple Express will bring warm moisture from the tropics near Hawaii to the U. S. West Coast, especially to Northern California.

Most atmospheric rivers are relatively weak, supplying beneficial rain or snow. A giant, fast-moving storm, or series of storms, banging into the Sierra Nevada watersheds, can produce extreme rainfall and flooding. NASA research measures the “rivers in the sky” as being able to transport enough water vapor in one day, on average, to flood an area the size of Maryland one foot deep, or about seven times the average daily flow of water from the Mississippi River to the Gulf of Mexico.” The result is disastrous damage to life and property, disruption of travel and widespread power outages.

Dettinger led one of the sessions at the conference, called “Atmospheric Rivers, Levees and Floodplain Ecology in the Bay Delta system.” He was joined by Joan Florsheim, Ph.D, UC Davis research geologist, and Martin Ralph, Ph.D, research meteorologist, National Oceanic and Atmospheric Administration (NOAA), and several others.

Dettinger said that understanding the timing, location and features of atmospheric rivers will aid in the future management of California’s water resources, restoration activities and flood risks. California is implementing a world-class observation network to better track and forecast atmospheric rivers.

Unmanned aerial research

Improved understanding has resulted from more than a decade of scientific studies. Using satellite tracking provides horizontal data. Airplanes and radar tracking provide vertical data.

Some of the data comes from a mission conducted jointly by NOAA and NASA to evaluate unmanned air vehicles (UAVs) for future science and operational requirements within the agencies related to oceanic and atmospheric research, climate research, marine sanctuary mapping and enforcement, nautical charting, and fisheries assessment and enforcement.

Five flights were completed cooperatively with General Atomics Aeronautical Systems Inc. in 2005 in the Altair Integrated System Flight Demonstration Project.

The Altair is a high-altitude, long-endurance UAV built and operated by General Atomics. The Altair flew out of Vandenberg Air Force Base on the Central Coast near Lompoc and Edwards Air Force Base in the Mojave Desert 90 miles north of Los Angeles. The longest flight was more than 18 hours.

The Altair payload included remote and on-site instruments for measurements of ocean color, atmospheric composition and temperature, plus a surface imaging system.

In March 2011 NASA flew three more research missions out of the NASA Dryden Research Center at Edwards Air Force Base in a NOAA-sponsored Winter Storms and Pacific Atmospheric Rivers experiment.

NASA’s unmanned air vehicle, Golden Hawk, flew long-duration flights (20, 24 and 25 hours) over the Pacific Ocean to gather data on how atmospheric rivers form and behave and to evaluate the use of unmanned, high-altitude aircraft for conducting offshore monitoring of atmospheric river to aid in future weather predictions.

Scientific research application to water management

Goodwin talked about the relevance of science information to water management. Some of the questions for the future are how much the average annual temperature will change over an extended period, how much snowpack will accumulate at the high elevations of the Sierra Nevada watersheds, and the question of whether warm Pineapple Express storms in early spring will melt the snowpack and cause major unseasonable drainage, leading to early flooding and summer drought.

Goodwin called it a “no-win situation” for water managers. “If you get it wrong, if you keep too much water in the reservoirs, then there’s the risk of overtopping and catastrophe. If you let too much water out, the water is not there for future use. So this (advanced forecasting system) is really making a major contribution.”

Potential consequences to ecological management

He also referred to the relationship between atmospheric rivers and X2. X2 is the distance from the Golden Gate Bridge to where salinity near the bottom of the water drops to 2 percent. It is an important measurement used in monitoring Delta and longfin smelt, Bay shrimp and other estuarine species.

The line shifts east and west depending on whether the flow of fresh river water or ocean salt water dominates. In the wet years of 1998 and 2006, X2 was 45 kilometers, or 26 miles, east of the Golden Gate. In the consecutive dry years of 2007 and 2008, it reached 90 kilometers, or 56 miles.

Another study Goodwin brought up was done on levee breaks in the Central Valley from 1951 to 2006. The study showed that 81 percent (104 out of 128) of “well-dated levee breaks have been AR (atmospheric river) driven.” Normal snowmelt accounted for 15 percent. The remaining levee breaks involved other factors.

The scientists also looked at 55 years of Yolo Bypass outflows, using data from water years 1956 to 2010, and flooding lasting up to 28 days. Flooding in the Yolo Bypass usually occurs from March to May.

This study demonstrates that inundated floodplains are key nurseries and cafeterias for Bay-Delta fisheries and ecosystems. Feeding is greater on the floodplain than in the Sacramento River. However, an extended period of flooding is necessary for the benefits to develop.

Dettinger said that the combined research on atmospheric rivers indicates that they are projected to become more intense and 10 to 15 percent more common in the 21^st century, with notable new extremes. More intense storms and higher snowlines will result in greater flood risks.

Council discussion

Patrick Johnston focused on the likelihood of future levee breaks. He asked, “Is this a key variable, and is that (major atmospheric river storms) widely viewed as the major external threat to levees?

Goodwin responded, “I think the findings say that most people would have expected that these levee failure events would be snowmelt dominated. What this is saying is that’s not the case. It’s these Pineapple Expresses–newer-type events. In terms of disaster preparedness, those are the types of events that we certainly need to take a lot of care about. This new forecasting system that’s being put in place will allow perhaps greater warning of the risk when we should be particularly vigilant.

Johnston inquired whether there was any dissent to the findings. Goodwin said there was none. “They were looking at the data,” he said.

Johnston: “Is there any other place to look for dissenting opinion or to resolution of any controversy? The reason I ask is that some of the testimony here by engineers who work on levees in the Delta, and others, make an argument that the levees are growing stronger every day, and that the threats are manageable, and perhaps have taken this into account, and perhaps not.”

Goodwin: “The information that was presented at the conference was looking more at the types of events when they occur rather than the cause of the actual levee failures, because there is a whole suite of possible factors that go into this. This was focused more on what are the events that cause it. Chris Knopp (executive officer) commented that it seemed the engineering community, and the actual risks of the levees themselves, were very underrepresented in the conference. And it’s probably something that we may encourage more actually in the next event. There was the other conference about three or four weeks before that was looking specifically at levee stability.”

Don Nottoli said he would like to see engineers and scientists give cross-disciplinary perspectives. “Talk about saturation,” he said. “Are levee failures due to overtopping, seepage or other issues?”

He referred to Johnston’s point and said that one of the things that has been expressed at the Delta Stewardship Council and other forums is that improvements are being made that will help strengthen, raise and modify levees that will hopefully guard against not only events we know about, but also ones that might be anticipated. In a Pineapple Express situation, it is not just the rainfall, but the combination of the warm rainstorm and the snowpack melting at lower elevations that creates significant flooding. “Are they looking at the predictability of that?”

Goodwin responded that these events are significant. He said that he research reported did not go down to the details, but was a “big-picture process.” He added, “There is a lot of attention being focused on that right now. The research group at UC Merced with Roger Bales is looking at snow dynamics and the effects of rain on snow. This was mainly looking at what kind of mechanisms should we be aware of, particularly if we can’t rely so much on the past to predict the future.”

Delta Stewardship Council Science Plan

Goodwin has been assigned the task of leading development of a Science Plan for the Delta Stewardship Council. At the end of his report, he described his vision of the Science Plan under the theme, “One Delta-One Science,” with many programs.

The Oct. 25 Delta Stewardship Council meeting was held at the Ramada Inn & Suites in West Sacramento. Chairman Phil Isenberg, Vice Chairman Randy Fiorini, and council members Hank Nordhoff, Patrick Johnston and Don Nottoli attended. Gloria Gray was absent.