CW3E Publication Notice: Characterizing Drought in California: new drought indices and scenario-testing in support of resource management

CW3E Publication Notice

Characterizing Drought in California: new drought indices and scenario-testing in support of resource management

February 2, 2018

CW3E collaborator, Lorrie Flint (USGS), published a paper with co-authors Alan Flint (USGS), John Mendoza (Sonoma County Water Agency), Julie Kalansky (CW3E) and Marty Ralph (CW3E Director), on landscape drought in California and extreme drought scenario testing in the Russian River watershed. The paper, “Characterizing Drought in California: new drought indices and scenario-testing in support of resource management” was published in Ecological Processes.

The paper explores the use of new drought indicators which include precipitation, air temperature, soil water content, climatic water deficit and recharge/runoff. Climatic water deficit (CWD) is the potential evapotranspiration minus the actual and can be thought of the missing water from the landscape. To test these drought indicators during the 2012-2016 drought, the study uses the Basin Characterization Model (BCM), a hydrology model that has been well calibrated in the Russian River. In the Russian River, after the 2012-2016 drought, the extreme conditions over five long years had depleted soil storage and the shallow unsaturated zone to such an extent that water supply, calculated as recharge + runoff was still behind normal by 1 to 3 years by the end of water year 2016. CWD ranged from 0 in the deep river valley to 1–2 years behind normal in the higher elevations (Figure 4).

Figure 4. Calculation of Russian River watershed drought metrics of (a) water supply (recharge plus run-off) for years of missing water supply accumulated for drought water years 2012-2016 and (b) landscape drought (climatic water deficit) for years of accumulated landscape stress for drought water years 2012-2016.

The paper then explores two extreme drought scenarios, one acute scenario and one extended drought. Based on Russian River watershed stakeholder’s feedback during the 2014 and 2015, they wanted to understand impacts of a drought scenario that was worse than the then on-going drought. Incorporating this feedback, the extreme drought scenarios added the historical droughts of 1976-1977 and of the 1930s to 2011-2015 record to represent an acute and extended drought respectively. These scenarios were run through the BCM and also through Sonoma County Water Agency reservoir model tool, Res-Sim. The results of the two droughts were very different (Figure 7). For the more acute 1976/1977 drought, Lake Mendocino was dry for much of 1977, but precipitation in 1978 was enough to recover the system as shown the run-off/recharge being above the historical normal. However, the low water supply years of the 30s never drained Lake Mendocino, but by the extended warm, relatively low precipitation years that dried out the landscape resulted in the reservoir not recovering until the late 30s with a couple of above normal water supply years coinciding with lower CWD.

Figure 7. Simulated reservoir storage for two extreme drought scenarios, case 1 (2011-2015 +1976-1985) and case 2 (2011-2015+1928-1937) for Lake Mendocino and Lake Sonoma.

The paper concludes by highlighting the benefit of two drought indicators, one that reflects the direct natural water supply (run-off/recharge) and one that represents landscapes. As stated in the conclusion “Both indices combine to provide local and regional managers a sense of how serious an ongoing drought is and the likelihood of rapid recovery.”

The research for the paper was supported by NOAA Sectorial Application Program project, “Coping with Drought in the Russian River Watershed”, and Sonoma County Water Agency.

CW3E Publication Notice: Historical and Future Relationship Between Large Storms and Droughts in California

CW3E Publication Notice

Historical and Future Relationship Between Large Storms and Droughts in California

August 16, 2016

CW3E collaborator, Michael Dettinger (USGS), published a paper titled “Historical and Future Relationship Between Large Storms and Droughts in California” in the journal of San Francisco Estuary and Watershed Science. The paper demonstrates that the largest storms, defined here as the 95th percentile storms, drive the year-to-year variability of precipitation in California (Figure 4). Further analysis shows that atmospheric river (AR) counts explain 75% of the historical variation. Using a frequency analysis Dettinger shows that episodes of longer than annual droughts are also a function of the absence of the largest storms.

Figure 4B. Seasonality of monthly variances of Central Valley Catchment’s total precipitation and contributions from the wettest 5% of wet days and remaining days, water years 1916-2010.

Dettinger then extends the analysis to examine how the importance of large storms will change in the future using 10 climate models that the CA Department of Water Resources had identified as being the most representative of California climate. The analysis shows that the non-extreme precipitation days decrease in all models and an increase in the most extreme storms, the 95th percentile storms, in most models. Under RCP 8.5, a non-mitigated, business as usual greenhouse gas emission scenario, 88% of the change in variance is a result of a change in variance in the wettest days (Figure 12A). This indicates that the largest storms contribute to the increase in year-to-year variability of precipitation. The paper concludes that fluctuations of these largest storms historically, and likely to a greater extent in the future, cause droughts and will continue to act as drought busters.

Figure 12A. Projected RCP8.5 changes in variance of water-year contributions of precipitation from the wettest 5% of wet days, remaining wet days, covariance of the two and total precipitation (all days) from 1951-2000 to 2046-2095, in climate-change projections by ten climate models

The research was in support of the NOAA Sectorial Application Program project, “Coping with Drought in the Russian River Watershed”, and a cooperative agreement with Sonoma County Water Agency.

CW3E Graduate Student Selected for National Water Center Summer Institute Fellowship

CW3E Graduate Student Selected for National Water Center Summer Institute Fellowship

August 3rd, 2018

Graduate Student, Maryam Asgari Lamjiri, was selected as a research fellow in the 2018 National Water Center Innovators Program Summer Institute. The program took place at the National Water Center in Tuscaloosa, Alabama from June 10 through July 27, 2018. Maryam, along with two other students (Sean Matus, University of Illinois at Urbana–Champaign; Kelly Flint, San Diego State University), worked on a project entitled ‘Using Dimensionless Scaling Parameters as Decision Metrics in a Heterogeneous Hydraulic Routing Scheme’ with the goal of increasing accuracy and robustness while decreasing computational costs of the hydraulic routing scheme used in the National Water Model. Students interacted with leaders in the field of hydrology, hydraulics, and applied mathematics. They were introduced to new tools and concepts and formed relationships with other fellows who will potentially be future colleagues. The project was presented at the capstone event on July 27, 2018, and the report will be posted here.

National Water Center Summer Institute. From left to right: Maryam Asgari Lamjiri, Kelly Flint, and Sean Matus

CW3E Tables at the Ecologik Program Summer Science Experience

CW3E Tables at the Ecologik Program Summer Science Experience

August 13, 2018

CW3E’s Anna Wilson hosted an interactive table display at the annual Ecologik Project Summer Science Experience hosted by the National Park Service at Cabrillo National Monument in San Diego, CA on August 3rd, 2018. Overall, the Ecologik Project (a collaboration between Cabrillo National Monument and the San Diego Central Library) is designed to connect the next generation of park stewards to the natural resources and science of Cabrillo National Monument, and provide the tools and context to empower the 21st century of environmental stewards in meaningful and relevant ways. The Summer Science Experience provides the opportunity for underrepresented young girls (ages 9-16) to explore careers in the natural and technical sciences. The last main day of the Summer Science Experience featured a cross section of female scientists from many disciplines, including neuroscience, biology, ecology, meteorology, and more, presenting an interactive slice of one aspect of their work to rotating small groups of program attendees.

The CW3E table showcased the Center’s research on atmospheric rivers (ARs) and their role in water resources and hydrology, including both providing beneficial water supply, and causing hazards such as floods and droughts. Attendees learned about the term “Atmospheric River” by viewing satellite animations of the associated clouds and precipitation during an AR event and referencing the narrow river-like transport of water vapor from the tropics using SSMI visualizations. The CW3E table featured a number of state-of-the-art observing tools, such as a rain gauge, soil moisture and temperature sensors, a radiosonde, and a dropsonde. Attendees were able to directly interact with CW3E scientist, Dr. Wilson, and with various types of instrumentation, gain an understanding of ARs and their impact on daily life, and learn about what research in atmospheric science, hydrology, and environmental engineering is like.

Ecologik Summer Science Experience attendees learning about how rain gauges work to measure precipitation brought by atmospheric rivers.

5th Annual Workshop on Forecast-Informed Reservoir Operations for Lake Mendocino

5th Annual Workshop on Forecast-Informed Reservoir Operations for Lake Mendocino

31 July – 2 August, Scripps Seaside Forum, La Jolla, CA

Over 70 experts from multiple disciplines and organizations came together for the fifth annual FIRO workshop, which was held at Scripps Institution of Oceanography (SIO), UC San Diego from 31 July-2 August 2018. This workshop was hosted jointly by Sonoma Water and CW3E. Opening remarks were provided by Mike Norman, Director of the San Diego Supercomputer Center and Grant Davis, General Manager of Sonoma Water. It was organized by the FIRO Steering Committee, co-chaired by CW3E’s Marty Ralph and Sonoma Water’s Jay Jasperse. There were representatives from organizations including the US Army Corps of Engineers (USACE), California Department of Water Resources (CA DWR), National Oceanic and Atmospheric Administration (NOAA), US Geological Survey (USGS), Desert Research Institute, US Bureau of Reclamation (USBR), Sonoma Water, Orange County Water District (OCWD), Yuba County Water Agency, UC Davis and CW3E.

During the workshop, participants shared recent updates on FIRO activities including progress towards a major deviation request at Lake Mendocino, a new decision support system (DSS) for the Russian River, research results and applications to the Lake Mendocino FIRO Final Viability Assessment, as well as enhanced monitoring and modeling efforts. A key topic was establishment of a FIRO effort on Prado Dam in southern California in a very populated basin, which builds on lessons from Mendocino and is identifying specific challenges unique to Prado. The meeting began with a roadmap of the Final Viability Assessment. Throughout the meeting there was a focus on the transferability of FIRO. The meeting concluded with highlights of important scientific advancements to advance FIRO and a discussion about how to apply the lessons learned at Lake Mendocino to other watersheds in California and beyond. A poster session was held to share some recent research findings relevant to FIRO. In short, the goals of year-3 of the 5-year FIRO Workplan are on track to be met, including work supporting the Final Viability Assessment.

Lake Mendocino FIRO is summarized at

Contacts: F. Martin Ralph (CW3E Director; and J. Jasperse (Sonoma Water Chief Engineer;

Group photo of FIRO Workshop attendees.

CW3E Publication Notice: The Gauging and Modeling of Rivers in the Sky

CW3E Publication Notice

The Gauging and Modeling of Rivers in the Sky

August 8, 2018

The first publication using dropsonde observations from the “Atmospheric River Reconnaissance” Program has just appeared in Geophysical Research Letters (Lavers et al. 2018). For a brief description of the AR Recon Program, which is co-led by CW3E’s Director, Marty Ralph, and NCEP’s Vijay Talapragada, with partners from NRL, ECMWF, NCAR and other universities, please see This paper also represents the first product from an interagency team focused on step-by-step exploration of the impact of the dropsondes through data assimilation and modeling tests.

It is titled “The Gauging and Modeling of Rivers in the Sky.” The analysis was led by ECMWF’s David Lavers, a leader in AR research and applications from Europe, and formerly a Postdoctoral Scholar at CW3E. The other authors are Mark Rodwell, David Richardson, and Florian Pappenberger from the European Centre for Medium-Range Weather Forecasts (ECMWF), James Doyle and Carolyn Reynolds from the Naval Research Laboratory, Vijay Tallapragada of NOAA/NCEP/Environmental Modeling Center, and CW3E director Marty Ralph.

The research undertook a diagnostics study into how well the ECMWF Integrated Forecasting system represents atmospheric rivers (ARs). Using observations from 319 dropsondes released during the AR Reconnaissance 2018 (AR Recon) campaign, the structure of ARs was shown to be well captured in the model, but the short range water vapor flux errors were ~22% of the mean observed flux. These errors are most related to uncertainties in low-level winds near the top of the planetary boundary layer (Fig. 1). The study hypothesizes that improved initialization of the forecasts, and thus water vapor flux forecasts, may be possible via extra observations of low level winds and water vapor from proposed future targeted airborne dropsonde campaigns and space-based observations.

Figure 1. (Figure 4 from Lavers et al., 2018): Uncertainty in IVT forecasts. (a) Scatterplot of the IVT in the 25 Ensemble of Data Assimilations background and observed realizations at the 319 dropsondes (n=7975). The linear correlation, mean error (ME; forecast-observed), standard deviation of the background forecasts (SDBG), standard deviation of the perturbed observations (SDOB), and the root mean square error (RMSE) are given. The 1:1 line is given in black, the linear regression line is in red, and the second degree polynomial line is in blue. (b) The relative change in IVT forecast standard deviation (%) compared to the forecast when replacing the forecast specific humidity q or winds uv at 925, 850, and 700 hPa levels with the unperturbed observation or unperturbed analysis value from the ensemble forecast system.

Lavers, D.A., M.J. Rodwell, D.S. Richardson, F.M. Ralph, J.D. Doyle, C.A. Reynolds, V. Tallapragada, and F. Pappenberger, 2018: The Gauging and Modeling of Rivers in the Sky. Geophysical Research Letters. doi:10.1029/2018GL079019.

CW3E Publication Notice: A 22-Year Climatology of Cool Season Hourly Precipitation Thresholds Conducive to Shallow Landslides in California

CW3E Publication Notice

A 22-Year Climatology of Cool Season Hourly Precipitation Thresholds Conducive to Shallow Landslides in California

July 31, 2018

CW3E collaborator Nina Oakley, along with CW3E director Mary Ralph and others recently published a study in the American Meteorological Society journal, Earth Interactions. The study is titled “A 22-Year Climatology of Cool Season Hourly Precipitation Thresholds Conducive to Shallow Landslides in California” and explores hourly rainfall observations throughout the state through the lens of precipitation as a trigger for shallow landslides.

Storm-triggered shallow landslides are a common hazard in California’s complex terrain. Where they mobilize into debris flows, they can have devastating impacts on life, property, and infrastructure. Most landslide-susceptible slopes require some amount of antecedent precipitation before intense rainfalls may generate the pore pressures thought to trigger shallow landslides. Post-wildfire debris flows, like the 9 January 2018 event in Montecito that claimed 23 lives, can be triggered by a host of other processes that involve direct runoff from hillslopes. All of these types of mass movements require moderate to high intensity rainfalls whose intensities exceed local infiltration rates. Our research is thus relevant to a broad category of geologic processes (landslides, runoff, runoff-generated debris flows, etc.) that threaten California’s welfare. There are implications of this work beyond landslides; results may inform reservoir operations, floodplain management, and public safety.

Historic records of landslide occurrence in California document many events within populated areas such as the San Francisco Bay Area and Transverse Ranges of southern California. There are far fewer records from outside these regions. One hypothesis for the distribution of observed landslides is that some areas of California do not frequently experience rainfall of sufficient intensities and durations to trigger shallow landslides. We tested this hypothesis by documenting where hourly rainfall in the cool season (October-May) in California met or exceeded published values thought to trigger landslides after a seasonal antecedent precipitation threshold has been met. The thresholds examined were 5, 10, 15 and 20 mm h-1 as well as 7.5 mm h-1 for 3h, and 5 mm h-1 for 6h. We also evaluated the relationship between these events and atmospheric rivers.

We develop an hourly precipitation data set for this analysis by applying quality control to a 147 Remote Automated Weather Stations (RAWS; across California over a 22-year period. Our analysis shows that stations with exposure to southerly atmospheric flow in the Coast and Transverse Ranges experience high numbers of over-threshold events at all intervals, coincident with areas with the greatest number of observed landslides. Our results also highlight areas where over threshold events frequently occur but landslide observations are lacking; one such region is the northwestern Sierra Nevada. In the regions experiencing the highest number of over-threshold events, approximately 60-90% (depending on threshold examined) are associated with atmospheric rivers. The number of over threshold events varies greatly both within a season and inter-annually, though the greatest frequency tends to coincide with the climatological wettest month of the year. Individual storm events commonly It is feature multiple over-threshold events, especially at lower intensities or multi-hour thresholds.

Our study brings together atmospheric scientists and geologists. Co-authors include Jeremy Lancaster and Stefani Lukashov from the California Geological Survey, Ben Hatchett and Shawn Roj form the Desert Research Institute/Western Regional Climate Center, and John Stock from the US Geological Survey. This study was supported by the California Department of Water Resources, the National Oceanic and Atmospheric Administration’s Climate Program Office through the California Nevada Applications Program, and the National Aeronautics and Space Administration through the Nevada NASA Space Grant Consortium.

Landslide observations and frequency of intense precipitation. Left: Shallow landslides compiled by the California Geological Survey are shown as black dots. Geomorphic provinces are overlain on the map. The four provinces most pertinent to the study are labeled. Right: Marker size indicates the number of precipitation events exceeding 15 mm h-1 during the October-May period 1995-2015 at each RAWS station. Marker color indicates the percentage of total 15 mm h-1 events associated with atmospheric river conditions.

Oakley, N.S., J.T. Lancaster, B.J. Hatchett, J. Stock, F.M. Ralph, S. Roj, and S. Lukashov, 2018: A 22-Year Climatology of Cool Season Hourly Precipitation Thresholds Conducive to Shallow Landslides in California. Earth Interact., 22, 1–35, ,

CW3E Presents at UCSD’s U.S.-China Future Leaders Summer Program

CW3E Presents at UCSD’s U.S.-China Future Leaders Summer Program

July 26, 2018

CW3E’s Anna Wilson recently gave a presentation introducing the center to high school students attending the U.S.-China Future Leaders Summer Program. This program, held at UCSD’s School of Global Policy and Strategy, was co-run by the School’s 21st Century China Center and the Global Leadership Institute, and led by award-winning high school teacher Kristen Druker. The program design is meant to foster problem-solving and decision-making skills, critical thinking about global policy issues and diplomacy and conflict resolution techniques. The students participate in game-based role-playing focused on global challenges that concern the U.S. and China, particularly on the topics of clean energy and climate change.

Hearing about how CW3E is working to reach our goals (to revolutionize the physical understanding, observations, weather predictions, seasonal outlooks and climate projections of extreme events in Western North America, including atmospheric rivers, the North American summer monsoon and their impacts on floods, droughts, hydropower, ecosystems, and the economy) reinforced the lessons they are learning in the curriculum. The CW3E presentation focused on several core areas of the center: atmospheric rivers; Forecast Informed Reservoir Operations (FIRO), and climate science. Discussion with the students throughout the presentation dealt with many topics, including policy implications of scientific findings and climate change mitigation and adaptation strategies. CW3E is grateful to have had the opportunity to interact with the intelligent and enthusiastic students.

CW3E Publication Notice: Global Evaluation of Atmospheric River Subseasonal Prediction Skill

CW3E Publication Notice

Global Evaluation of Atmospheric River Subseasonal Prediction Skill

July 26, 2018

CW3E collaborators Michael DeFlorio (NASA/JPL), Duane Waliser (NASA/JPL), and Bin Guan (UCLA), along with CW3E director Marty Ralph and colleague Frederic Vitart of the European Centre for Medium-Range Weather Forecasts (ECMWF), recently published a paper in Climate Dynamics titled Global Evaluation of Atmospheric River Subseasonal Prediction Skill (DeFlorio et al., 2018).

The study uses the number of AR days occurring over a week-long period (Fig 1), to quantify global atmospheric river (AR) prediction skill using Subseasonal to Seasonal (S2S) Project hindcast data from ECMWF. The study found that ECMWF forecast skill is higher than a reference forecast in several subtropical and midlatitude regions up to a three-week (15-day to 21-day) lead time. The forecast skill is affected during particular phases of climate mode variability, including the El Niño-Southern Oscillation, Arctic Oscillation, Pacific-North America teleconnection pattern, and Madden-Julian Oscillation (Fig 2). In particular, forecast skill is generally improved at week two during MJO Phase 8 conditions. The results highlighted in this paper provide information to support improved subseasonal forecasts.

Figure 1. Figure (1a-d) from DeFlorio et al. (2018): Global climatology of atmospheric river (AR) occurrence (number of AR days per week; hereafter AR1wk occurrence) for ERA-I and ECMWF in November-December-January-February-March (NDJFM; a,c) and May-June-July-August-September (MJJAS; b,d) during the period 1996-2015. The week-1 (0d-6d) lead time forecast window is chosen. Blue rectangles in a) denote area-averaged regions selected for analyses in subsequent figures.

Figure 2. Figure (13a,b) from DeFlorio et al. (2018): Composite forecast skill of AR1wk occurrence (number of AR days per week) anomalies during particular MJO phases (red) over the North Pacific/Western US regions in NDJFM during the period 1996–2015. Mode and region combinations with large (~ 95% confidence) skill differences between particular MJO phases and the all-days case are shown. Red and black horizontal lines represent 95 and 5% quantile values for a bootstrapped distribution of the particular MJO phase and all days case, respectively, re-sampled 1000 times with replacement.

DeFlorio, M., D. Waliser, B. Guan, F.M. Ralph, and F. Vitart, 2018: Global evaluation of atmospheric river subseasonal prediction skill. Climate Dynamics, early online release,

CW3E Welcomes Dr. Tom Corringham

CW3E Welcomes Dr. Tom Corringham

July 16, 2018

Dr. Tom Corringham joined CW3E as a Post-Doctoral Scholar in July of 2018 after completing his PhD in Economics at the University of California San Diego, under advisors Richard Carson and Dan Cayan.

At UCSD, Tom worked with LeRoy Westerling, now at UC Merced, and Barbara Morehouse at the University of Arizona, to determine the value of climate forecasts and information in wildland fire management in the western United States. With Dan Cayan, Tom studied the effect of El Niño on flood damages, finding that known large-scale hydrologic effects of ENSO variability largely carried over to economic impacts of damaging flood events in the western US.

Tom is currently working with Dan Cayan, Sasha Gershunov, and Marty Ralph to assess the costs of atmospheric rivers in terms of damaging floods in the western US, using data from the National Flood Insurance Program. Preliminary results suggest that atmospheric rivers are responsible for a significant proportion of total flood damages in the western US, with individual events causing over $1b in damages approximately once every five to ten years over the past 40 years.

At CW3E, Tom will be studying the economic impacts of extreme climate events in California and the western US. He intends to broaden his research to explore the economic aspects of AR forecasts in the context of water management and Forecast Informed Reservoir Operations (FIRO). He is also interested in climate impacts on agriculture, energy, human health, and ecosystem management.