ESRL Quarterly Newsletter - Winter 2009


ESRL scientists study California’s water future in a changing climate

Water security is never far from the mind of any decision maker in the U.S. West— but it’s not a terrorist attack on a dam or canal that triggers the most anxiety, it’s climate change. This fall, California Gov. Arnold Schwarzenegger called his state’s water troubles a “holy water war” pitting “north versus south, California versus the feds, rural versus urban....” And well known challenges— the vicissitudes of weather, periodic drought, and a growing demand for water from a growing population—could be exacerbated by climate change, the governor told a state water committee this fall.

Rainy California coast.

In that fraught setting, ESRL scientists helped launch CalWater this fall, a major mission to better understand how climate change could affect California’s water resources, through changes in rain and snowfall patterns. The project involves researchers from a wide variety of institutions, including the University of California, the California Department of Natural Resources, the state’s Air Resource Board, the Scripps Institution of Oceanography, and others; and it is funded by NOAA, Scripps, and the California Energy Commission.

“There’s evidence from modeling studies that climate change could affect the pattern and intensity of precipitation in North America,” said Marty Ralph (Physical Sciences Division). “We want to understand exactly how that could play out across the state of California, but our results could be applicable to other parts of the country as well.”

ESRL’s involvement in CalWater is two-fold, Ralph said: ESRL scientists will study how tiny particles of air pollution called aerosols affect the amount and location of rainfall in California; and how changes in the frequency and intensity of atmospheric rivers affect extreme rainfall, water supply, and flood risk in the state. Both will help assess the performance of regional climate models, which decision makers rely upon to understand the future of water resources in the state.

CalWater and HMT instrument sites.

Christopher Williams (Physical Sciences Division and CIRES) is helping to lead the aerosol research. Scientists understand, generally, that aerosols from air pollution can affect climate, cloud formation and behavior, Williams said: High levels of aerosol can make some clouds less likely to drop rain or snow, for example, and can lead to clouds that reflect more or less sunlight back into space, cooling or warming the surface. But how those behaviors modify precipitation processes within clouds is not yet well understood, creating uncertainty in global and regional climate models.

Some researchers have proposed that air pollution has already changed the pattern of rainfall in the state, letting water-rich clouds travel over the Sierra Nevada mountains into Nevada before dropping their rain, leaving California with less. Williams and his colleagues suspect other factors, distinct from aerosol effects, may be at play, too, such as the “Sierra barrier jet” that can form along the Sierra Nevada. The scientists will use sophisticated instruments and techniques—from ground-based radars and airborne instruments to chemical “fingerprinting” of particles found in falling rain and snow—to gather data across California for the next several years, to build a more accurate picture of the relationship between air quality, water resources, and climate.

Since climate models try to represent the physical processes that occur in clouds, better understanding from ESRL research will create more accurate projections as California experiences a changing climate, Williams said.

Atmospheric rivers also play a major role in California’s water resources. These moisturerich “rivers” of air can sweep up from the Pacific, dumping rain on California’s coast and snow in the mountains. In earlier work in the state, ESRL researchers and colleagues have estimated that atmospheric rivers deliver as much as 50 percent of the state’s precipitation every year, and are responsible for most of the major flood events.

ESRL scientists suspect that some of the uncertainty in climate model projections of California’s future is the fault of poor representation of atmospheric rivers in the models. CalWater’s measurement-intensive campaign—involving atmospheric river and climate observatories on the coast of California and inland—should help, said Gary Wick (Physical Sciences Division). Those observatories include sophisticated radars, raindrop disdrometers, GPS-meteorology instruments, and others that will let Wick and his colleagues record the structure and variability of atmospheric river systems at sea and as they move onto land, where they are transformed by regional topography and weather systems.

Because atmospheric rivers events in California can end droughts, fill reservoirs, and trigger dangerous flooding, Wick said, it’s critical to better understand how climate change could affect the systems. The findings will be relevant for policy makers considering decisions about public investments in flood control and water storage projects, including new reservoirs.

Emerging science: Snow-level radar and HMT’s legacy project

ESRL scientists have developed an inexpensive radar system that can detect and monitor snow level during winter storms. Because snow level can determine how much of a particular mountain basin will experience rain versus snow, it is an important predictor of snowpack levels and streamflow amount and timing- key data for water supply managers and for flood control.

Snow-level radar.

ESRL’s new snow-level radars have been installed at two sites in California, with seven more to follow in the next few years. With 18 other snow-level detecting radars deployed across the state this winter, these new systems will serve climate missions in the CalWater project and a legacy of the HMT-West project, the California Department of Water Resources EFREP (Enhanced Flood Response and Emergency Preparedness) program.

HMT’s legacy project, which also involves other state-of-the-art monitoring equipment, modeling efforts and decision support tools, is intended to help California deal with the challenges of an aging water infrastructure, increased standards for urban flood protection, and the impacts of climate change.

The snow-level radars record cloud and precipitation information every 35 seconds, and report snow level every 10 minutes. These radars will help determine if there are long-term trends in snow level, and will assist forecasters in providing more accurate and timely forecasts of winter storms. Data from the radars, combined with CalWater measurements, will also provide critical data scientists need to improve regional climate models. Better modeling can help water managers prepare for California’s future under climate change.


CalWater draws from another major ESRL effort focused on California—HMT-West—taking advantage of some of the custom instruments, sites, and knowledge developed earlier by HMT (NOAA’s Hydrometeorology Testbed Program).

SkyWater radar.

Measuring water in the air Another key tool for CalWater is the Physical Sciences Division’s scanning “SkyWater” radar, which is being set up north of Sacramento, to monitor the interaction between landfalling atmospheric rivers and the Sacramento barrier jet for two winters. The jet is not included in climate models, and detailed observations from the Sacramento instruments will help scientists assess how its absence from models affects projections of California precipitation.

Weather forecasters know that conventional instrument suites aren’t always enough to accurately predict hazardous weather. Deadly storms can sneak in literally below radar, and runoff models sometimes don’t capture an imminent flood. HMT, which involves ESRL scientists and research products, is developing, prototyping, and infusing new science and technology into the daily operations of the National Weather Service and its River Forecast Centers. ESRL researchers have helped design and place custom instrument packages (from disdrometers and radars to weather instruments launched on balloons) and run experimental models in regions of California that are prone to winter floods.

The knowledge and tools generated through HMT are aiding in the design of climate-focused field studies, such as CalWater, and are providing new methods for regional climate monitoring, such as soil moisture measurement methods and systems that are useful for NIDIS pilot studies.

Isidora Jankov, Steve Albers, and colleagues in the Global Systems Division are doing the modeling. The team gathers all available observations— from HMT-West’s custom instrument suites to conventional meteorological data gathered in both California and Washington regions—and uses it to fire up forecast models.

“See that blue? That’s enhanced moisture flux moving toward the coast,” Jankov said, pointing to her computer screen last month. “That’s going to hit Seattle tomorrow, but today, it’s hitting Vancouver. They’re going to get some heavy rain.”

The modelers are producing some of the longest-lead-time, high-resolution forecasts ever for the California and Pacific Northwest regions, Jankov and Albers explain, and they are mixing-and-matching different physics, dynamics, and initialization schemes to create ensembles. (Model ensembles are widely recognized as being able to produce more skillful forecasts than single, or “deterministic” model runs).

Jankov and Albers say they are still working closely with the HMT and CalWater teams, including the River Forecast Centers, and the National Weather Service, to figure out what combinations of model components and ensemble methods will produce the best precipitation forecasts. The researchers are especially interested in accurately forecasting when and where rain turns to snow, or vice versa. “This is very helpful information for the water managers who need accurate runoff predictions,” Albers said.