CW3E Publication Notice: Quantification of the impacts of Atmospheric Rivers on precipitation over southern South America

CW3E Publication Notice

Quantification of the impacts of Atmospheric Rivers on precipitation over southern South America

October 15, 2018

Researchers from South America, Maximiliano Viale from IANIGLA-CONICET Argentina and Raul Valenzuela and Rene Garreaud from University of Chile, along with CW3E director Marty Ralph, recently published a paper in the Journal of Hydrometeorology titled “Impacts of atmospheric rivers on precipitation over southern South America” (Viale et al. 2018).

This paper quantifies the impact of atmospheric rivers (AR) on precipitation in southern South America. An AR detection algorithm was developed based on integrated water vapor transport (IVT) from six-hourly CFSR reanalysis data over a 16-year period (2001-2016). Along the Pacific (west) coast, AR landfalls are most frequent between 38°S and 50°S, averaging 35-40 days/year. This decreases rapidly to the south and north of this maximum, as well as to the east of the Andes. Landfalling ARs are more frequent in winter/spring (summer/fall) to the north (south) of ~43°S (Fig. 1).

Figure 1: Seasonal frequencies of landfalling ARs on the west coast of South America: a) Summer (DJF), b) Fall (MAM), c) Winter (JJA), d) Spring (SON). Units are expressed in the average number of days with ARs per year. Blue points are plotted on the coast for reference at 33.5ºS (latitude of the Santiago city), 43.5ºS (southern Chiloe Island), and 53.5ºS (latitude of Punta Arenas city).

ARs contribute 45%-60% of the annual precipitation in subtropical Chile (37°S-32°S) and 40%-55% along the midlatitude west coast (37°S-47°S, see Fig. 2). These values significantly exceed those in western North America, likely due to the Andes being taller. In subtropical and midlatitude regions roughly half of all events with top-quartile precipitation rates occur under AR conditions. Median daily and hourly precipitation in ARs are 2-3 times that of other storms. The results of this study extend knowledge of the key roles of ARs on precipitation, weather and climate in the South American region. They enable comparisons with other areas globally, provide context for specific events and support local nowcasting and forecasting.

Figure 2: (a) Fraction of annual total precipitation associated with AR conditions over the 2001-2016 period. Fractions are multiplied by 100 to express the results in percentage. AR fractions at each station site are calculated using daily rain dataset. Cross-barrier plots of the AR-Fraction for the (b) Subtropical, (c) Midlatitude, and (d) Austral zones. The limits of each zone are defined in the plan-view plot of the panel (a). The meridionally averaged west-east cross-sections of the topography (within the rectangle shown in panel a) are shown in panels (b)-(d) as a reference.

Viale, M., R. Valenzuela, R.D. Garreaud, and F.M. Ralph, 2018: Impacts of Atmospheric Rivers on Precipitation in Southern South America. J. Hydrometeor., Early Online Release,

UCAR/NCAR 2nd Annual visit to CW3E

UCAR/NCAR 2nd Annual visit to CW3E

October 12, 2018

For the second year in a row, CW3E had the pleasure of hosting visitors from NCAR and UCAR to discuss the development and implementation of West-WRF, which is the regional forecast model that CW3E is optimizing with a focus on extreme precipitation. The team from UCAR and NCAR included Bill Kuo, the Director of UCAR Community Programs, who helped lead the development of WRF, Chris Davis, the NCAR Associate Director of the Mesoscale and Microscale Meteorology (MMM) Laboratory, David Gill, computation expert, Jake Liu, data assimilation expert, and Wei Wang, modeling expert.

The two and half day visit began with technical meetings on near real time WRF modeling and data assimilation. The entire group convened on the morning of the second day. Marty Ralph, the CW3E Director, Bill Kuo, and Chris Davis began the large group meeting by discussing the collaborations that have developed since the meeting last year and providing context for this year’s meeting. Marty presented an update on CW3E which was then followed by presentations from the UCAR/NCAR group on research and program updates. After lunch there were more technical meetings on AR reconnaissance, WRF modeling best practices, hydrology, and the National Water Model. The day ended with Chris Davis giving a seminar on tropical cyclone wind structure and storm surge.

NCAR’s Chris Davis discussing tropical cyclone wind structure and storm surge.

The third and final day allowed for more technical one-on-one discussions on the efforts that are going on at both UCAR/NCAR and CW3E and how the centers can collaborate going forward. This included time for graduate students to inquire about research opportunities at NCAR/UCAR. The meeting built on the synergy that started last year with the visit and presented the groups with new opportunities for collaborations. The engagement and participation of UCAR and NCAR in supporting the technical development of one of its member institutions is greatly appreciated and we look forward to continuing this collaboration.

Distribution of Landfalling Atmospheric Rivers on the U.S. West Coast During Water Year 2018: End of Water Year Summary

Distribution of Landfalling Atmospheric Rivers on the U.S. West Coast During Water Year 2018: End of Water Year Summary

October 8, 2018

For a pdf of this information click here.

*Arrows on this map are placed where each atmospheric river was strongest over the coastline.




Link to a summary on the meteorological conditions that led to the Montecito, CA debris flow here

Analysis by Chad Hecht & F. Martin Ralph. This analysis is considered experimental. For questions regarding the data or methodology please contact Chad Hecht

CW3E Publication Notice: The Relationship between Extratropical Cyclone Strength and Atmospheric River Intensity and Position

CW3E Publication Notice

The Relationship between Extratropical Cyclone Strength and Atmospheric River Intensity and Position

October 4, 2018

CW3E postdoc Zhenhai Zhang, along with CW3E director Marty Ralph, and postdoc Minghua Zheng recently published a paper in Geophysical Research Letters, titled: The Relationship between Extratropical Cyclone Strength and Atmospheric River Intensity and Position. In their study, the extratropical cyclones (ECs) and atmospheric rivers (ARs) in 1979-2009 cool seasons (November-March) over the U.S. West Coast were identified using sea level pressure and integrated water vapor transport from NCEP Climate Forecast System Reanalysis. An EC/AR relative composite approach was employed to determine the association between EC and AR. The statistical relationship and the dynamical interaction between ECs and ARs over the U.S. West Coast were investigated.

The results of this study show that while 82% of ARs are associated with an EC, only 45% of ECs have a paired AR. The distance between the AR and an associated EC varies greatly. Roughly 20% of ARs occur without a nearby EC. Those ARs are usually close to a subtropical/tropical moisture source and include a southeastern anticyclone. AR intensity is only moderately proportional to EC strength. Neither the location nor intensity of an AR can be simply determined by an EC. An EC often intensifies the AR with stronger wind-driven meridional water vapor transport, while the southeastern anticyclone is also critical to the existence and intensity of AR. On the other hand, an AR can provide sufficient water vapor to enhance the precipitation and thus latent heat release, which contributes to EC intensification.

Figure 1: (a): the probability (colors, %) of AR occurrence around the composite EC center (black dot). (b): the count of AR IVT maximum points (colors) in 31 (1979-2009) cool seasons in a unit area (40,000 km2) around the composite EC center; the “x” indicates the positions of the exceptional AR IVT maximum points. (c): the EC center density (colors, number of centers per cool season per 50,000 km2) and the percentage (contours, %) of ECs associated with AR. (d): the AR frequency (percent of time steps with AR in a cool season, colors, %) and the percentage (contours, %) of ARs associated with EC. The blue box in (a-b) is used to define the association between EC and AR. The blue box in (c-d) is the U.S. West Coast domain.

Zhang, Z., Ralph, F. M., and Zheng, M., 2018: The relationship between extratropical cyclone strength and atmospheric river intensity and position. Geophysical Research Letters., 45, 164-176, https://

CW3E Publication Notice: Origins and Variability of Extreme Precipitation in the Santa Ynez River Basin of Southern California

CW3E Publication Notice

Origins and Variability of Extreme Precipitation in the Santa Ynez River Basin of Southern California

September 25, 2018

CW3E collaborator Nina Oakley published a paper with CW3E researchers Forest Cannon and Marty Ralph and National Weather Service meteorologists Eric Boldt and John Dumas. The paper examines the meteorological conditions that lead to wet years in the Lake Cachuma waterbasin. Lake Cachuma, a reservoir on the Santa Ynez River, provides water for over 280,000 residents and agricultural lands of Santa Barbara County, California. This area experiences high inter-annual precipitation variability, and is driven by the presence or absence of a few large precipitation events each year. We use daily precipitation observations from 1965 to 2017 to identify extreme precipitation events, defined as those exceeding the 90th percentile. We examine the role of these events, their associated synoptic patterns, and the El Niño Southern Oscillation (ENSO) in driving inter-annual precipitation variability in this basin.

Results show that on average, a wet year features three or more extreme events, a normal year 1–2 events, and a dry year 0–1 events. We categorize extreme events based on distinctive large-scale circulation features and also find that 74% of events are associated with atmospheric rivers. El Niño years tend to have a greater number of extreme events, though this relationship is not dependable. The reliance on just a few extreme precipitation events and diversity among these events highlights the challenges of seasonal prediction and resource management in this area. This novel approach to defining variability on a watershed scale can support ecological, geological, and hydrological studies as well as regional water resource management.

Figure Caption: Top panel shows contribution from ≥90th percentile events (orange portion of bars) and all other precipitation (blue bars) averaged across Santa Ynez River Basin Index stations for each season. The 90th percentile event count among the six stations for each season is given at the top of the bar. As storm count and contribution is averaged across the six Basin Index stations by season, fractional contributions and storm counts are present. The period of record mean precipitation is 648 mm. The mean contribution for non-extreme events is 343 mm and mean for extreme events is 305 mm. The bottom panel shows a 5-year running mean for total precipitation (black), precipitation from ≥90th percentile events (orange) and all other precipitation (blue). This demonstrates the dominant role of relatively infrequent extreme events on seasonal precipitation totals. *precipitation data in 2006 was missing from several stations.

Oakley, N.S., F. Cannon, E. Boldt, J. Dumas, and F.M. Ralph, 2018: Origins and variability of extreme precipitation in the Santa Ynez River Basin of Southern California, Journal of Hydrology: Regional Studies., 19, 164-176,

CW3E Publication Notice: ARTMIP-Early Start Comparison of Atmospheric River Detection Tools: How Many Atmospheric Rivers Hit Northern California’s Russian River Watershed?

CW3E Publication Notice

ARTMIP-Early Start Comparison of Atmospheric River Detection Tools: How Many Atmospheric Rivers Hit Northern California’s Russian River Watershed?

September 17, 2018

A team led by CW3E director Marty Ralph recently published a paper in Climate Dynamics titled ARTMIP-Early Start Comparison of Atmospheric River Detection Tools: How Many Atmospheric Rivers Hit Northern California’s Russian River Watershed? (Ralph et al. (2018)).

Many Atmospheric River Detection Tools (ARDTs) have now been developed; however, their relative performance is not well documented. This paper set out to address this gap in the simplest possible way – at a single location where unique in-situ observations of AR conditions could be used as well. The analysis team included the developers of many of the key ARDTs used most often today, and represent several key organizations studying ARs, i.e., NWS, NASA, NOAA/ESRL, CSU and Scripps/CW3E. As such, this study provides an early-start analysis that helps set the stage for the Atmospheric River Tracking Method Intercomparison Project (ARTMIP) community effort, which has been organized by an ad-hoc planning committee (Shields et al., 2018). In this work, we quantify the sensitivity of the diagnosed number, duration, and intensity of ARs to the choice of ARDT and to the choice of reanalysis data. Results did not vary much with reanalysis data set, regardless of very different resolutions (Fig. 1). However, the annual AR event count varied by about a factor of two (10-25 per year) depending on the ARDT. Average AR duration and maximum intensity varied by less than ±10%, i.e., 24 ± 2 h duration and 458 ± 44 kg m-1 s-1 maximum IVT (Fig. 2). This work provides an important foundation for researchers as they consider using various ARDTs and evaluate what the choice of ARDT might mean in terms of results. It also provides insight into why counts differ so much across methods, and shows that such differences are reasonable and should be expected.

Figure 1. (Figure 1 from Ralph et al., 2018): Map of the study region with the Bodega Bay ARO location marked in yellow. Grid center points and boxes for all of the reanalyses used in this study are presented in solid markers and shading (colored according to scale). Terrain represented by gray shading.

Figure 2. (Adapted from Figure 4 and Table 5 in Ralph et al., 2018): (left); Number of distinct AR events counted by each native reanalysis-based AR catalog at the grid cell containing BBY (see Figure 1) during water years 2005-2015. Dashed lines(*) indicate methods that identify ARs that persist for at least 12 hours with much higher AR strength thresholds. (right); AR event counts with ARDT characteristics. ARDTs are sorted by criteria. * indicates those catalogs that are designed to identify only stronger storms; ** indicates observational catalogs with significant missing data during some years; *** indicates catalogs using IWV alone.

Ralph, F. M., A. M. Wilson, T. Shulgina, B. Kawzenuk, S. Sellars, J. J. Rutz, M.A. Lamjiri, E. A. Barnes, A. Gershunov, B. Guan, K. Nardi, T. Osborne, and G. A. Wick, 2018: Comparison of Atmospheric River Detection Tools: How Many Atmospheric Rivers Hit Northern California’s Russian River Watershed? Clim. Dyn.,

CW3E Graduate Student Selected for a NASA Goddard Space Flight Center Internship

CW3E Graduate Student Selected for a NASA Goddard Space Flight Center Internship

September 14, 2018

NASA Goddard’s Earth Science interns Ada Shaw, Stephanie Lin, Tashiana Osborne, Haiden Mersiovsky, Nikita Mukherjee (not shown) (Photo Credit: NASA Goddard)

CW3E graduate student Tashiana Osborne was selected for a competitive NASA Internship this summer at the Goddard Space Flight Center in Greenbelt, Maryland. During her internship, she used MERRA-2 reanalysis products to examine the spatial variability and the year-to-year variability of rain and snow over the western U.S., with an emphasis on extreme precipitation events and the contribution of West Coast atmospheric rivers. In early August, she presented a scientific poster to the Goddard community showcasing results. She will continue this work as part of her Ph.D. dissertation.

A day of science research presentations at NASA Goddard (Photo Credit: Dr. J. Rice)

While interning at Goddard, Tashiana had a unique opportunity to speak with an astronaut currently aboard the International Space Station (ISS).

NASA Astronaut Ricky Arnold formerly worked in the marine sciences and also as a science teacher in various countries. He became an astronaut in 2004 with his first space mission in March 2009, lasting almost 13 days. In March 2018, Arnold launched to the ISS as part of Expedition 55. This time, he won’t be back on Earth until October.

Speaking with NASA astronaut, Ricky Arnold, currently aboard the ISS (Photo Credit: NASA Goddard)

Arnold, while showcasing the fun of microgravity, answered questions from visitors, employees, and interns at Goddard Space Flight Center during the July ISS Downlink event. He also shared his goal to “…shine a light on one of the most important jobs there is; being a classroom teacher” through his A Year Of Education on Station efforts (providing K-16 students and educators with NASA STEM activities).

Especially considering that the audience consisted of younger students, teachers, and others in various stages of life, Tashiana asked an Earth Science-related question she thought might interest others, and would allow Ricky to share his unique perspective as an astronaut

Tashiana’s Questions:

Hi Ricky. Thanks for answering our questions.
What can we learn from space missions and exploring other planets that might help us in adapting to a changing climate here on Earth?

Ricky’s Answer:

Tashiana – Fantastic question, particularly [while] at Goddard Space Flight Center which invests so much energy in studying our home planet. Studying other planets helps us understand the mechanics of things that happen here on Earth.

Right beside us in our solar system we have a planet, Venus, which suffered from runaway greenhouse gas and climate change. And Mars, which probably had a thick atmosphere at one point, and now really has a very thin one.

[On] Mars, much like on Earth, when you want to understand what happened in past climates, we go to the poles and we drill into the ice and look at what the atmosphere was like. I think getting an understanding of the mechanisms and what happened over time to the atmosphere and other parts of the planet will give us a better understanding of how climate change is [affecting] and will continue to affect Earth.

Ricky answered interesting and fun questions asked by participants; from preschoolers to grandparents and engineers:

  • What do you see in outer space?
  • How does it feel when you launch?
  • What are some long-term health issues from being in space?
  • How do you shower in space?
  • How has your perspective of Earth changed after viewing it from space?
  • And more….

Watch the astronaut Q&A video here.

CW3E Welcomes Dr. Luca Delle Monache

CW3E Welcomes Dr. Luca Delle Monache

August 31, 2018

Luca Delle Monache started on 13 August 2018 as the Deputy Director of the Center for Western Weather and Water Extremes (CW3E), Scripps Institute of Oceanography, University of California San Diego. This new senior leadership position within CW3E was created in response to the growth in the Center’s scope and complexity of activities. Its goal is to provide support for the Director in managing activities within the Center, and in developing new science and applications directions to support CW3E’s Vision and Mission. Specifically, Dr. Delle Monache will oversee the development of the Center’s modeling, data assimilation, postprocessing, and artificial intelligence capabilities, with the goal of maintaining state-of-the-art models and tools while actively exploring innovative algorithms and approaches. In close coordination with the Center Director and the management team, he will also develop new scientific and programmatic strategies to maintain and further expand CW3E leadership on understanding, observing, and predicting extreme events in Western North America.

He earned a Laurea (~M.S.) in Mathematics from the University of Rome, Italy (1997), an M.S. in Meteorology from the San Jose State University, U.S. (2002), and a Ph.D. in Atmospheric Sciences from the University of British Columbia, Canada (2005). His interests include the design of ensemble methods, probabilistic prediction and uncertainty quantification, numerical weather prediction, data assimilation, inverse modeling, postprocessing methods including artificial intelligence algorithms, renewable energy, air quality and transport and dispersion modeling, and accelerated simulations on graphics processing units (GPUs). Among his main scientific accomplishments, there is the development during his Ph.D. of the first ensemble for air quality prediction, and later in his career the design of the analog ensemble which has been applied successfully in several of the fields listed above, and is based on a significant shift in the paradigm of ensemble design. Dr. Delle Monache has been the principal investigator of several multi-institution projects funded by the National Science Foundation, the National Oceanic and Atmospheric Administration, the National Aeronautics and Space Administration, the Department of Energy, the Department of Defense, and the private sector. Before joining CW3E, he was a postdoc and then a staff scientist at the Lawrence Livermore National Laboratory, Livermore, California (2006-2009), and a project scientist and then the Science Deputy Director of the National Security Applications Program at the National Center for Atmospheric Research, Boulder, Colorado (2009-2018).

CW3E Welcomes Dr. Edwin Sumargo

CW3E Welcomes Dr. Edwin Sumargo

August 30, 2018

Dr. Edwin Sumargo joined the CW3E as a Postdoctoral Researcher in August 2018. Edwin obtained his Ph.D. in Earth Science from the Scripps Institution of Oceanography at UC San Diego, under the supervision of Dan Cayan. During his graduate study, Edwin studied the space-time variability of mountain cloud cover over the western U.S. and its influence on springtime snowmelt and streamflow variations. His last project involved watershed-scale hydrologic modeling, in which he demonstrated the use of satellite remote-sensed solar radiation to improve Precipitation-Runoff Modeling System’s simulation. At CW3E, Edwin investigates the Russian River Basin’s hydrometeorological variability and works on the hydrologic modeling verification and evaluation in the Russian River Basin as a part of the FIRO project. His work will involve the incorporation of soil moisture observations and high-resolution gridded precipitation data in hydrologic models.

CW3E Publication Notice: Assessment of Numerical Weather Prediction Model Reforecasts of the Occurrence, Intensity, and Location of Atmospheric Rivers along the West Coast of North America

CW3E Publication Notice

Assessment of Numerical Weather Prediction Model Reforecasts of the Occurrence, Intensity, and Location of Atmospheric Rivers along the West Coast of North America

August 24, 2018

Colorado State University (CSU) graduate student Kyle Nardi and co-authors Dr. Elizabeth A. Barnes of CSU and CW3E director Dr. F. Martin Ralph have published a study assessing how well numerical weather prediction (NWP) models predict landfalling atmospheric rivers (ARs) along the West Coast of North America at short and medium-range lead times (1-14 days). The article, titled “Assessment of Numerical Weather Prediction Model Reforecasts of the Occurrence, Intensity, and Location of Atmospheric Rivers along the West Coast of North America”, is now available as an Early Online Release in Monthly Weather Review (Nardi et al., 2018).

The study, funded by CW3E, examined AR landfall re-forecasts from the control runs of nine operational weather models, including those from ECMWF and NCEP. Re-forecasts were assessed in terms of three components of an effective AR forecast: AR occurrence, intensity, and landfall location. The study found that, in terms of AR occurrence, models provide little additional skill compared to a random forecast at leads approaching 14 days. In addition, at leads of 1-7 days, occurrence-based skill is significantly lower in British Columbia and Alaska compared to California, Oregon, and Washington (see below).

Figure 1. (Fig. 5 from Nardi et al. 2018) Model-averaged Peirce Skill Score (PSS) for three sub-regions along the West Coast of North America. PSS ranges from -1 to 1, with 1 implying a perfect forecast and 0 implying no additional skill compared to a random forecast.

With respect to AR intensity re-forecasts, the magnitude of landfall integrated water vapor transport (IVT) error stays fairly constant with lead time. However, at leads greater than 3 days, models tend to over-predict landfall IVT. The study also found that the magnitude of AR landfall location error increases with lead time, with northward errors favored in California, Oregon, and Washington. However, these northward errors are not as frequent as expected from climatology. Overall, these results provide an updated overview of how well our current weather models perform when predicting landfalling ARs. The findings also highlight the need for further examination related to the causes of model errors at lead times of 1-14 days.

This work was funded under a sub-award from the Forecast Informed Reservoir Operations (FIRO) project. Data used in the study came from ECMWF through the Subseasonal to Seasonal (S2S) International Project.

Nardi, K., E. Barnes, and F. Ralph, 2018: Assessment of Numerical Weather Prediction Model Re-Forecasts of the Occurrence, Intensity, and Location of Atmospheric Rivers along the West Coast of North America. Mon. Wea. Rev., doi:10.1175/MWR-D-18-0060.1

CW3E Publication Notice: Empirical Return Periods of the Most Intense Vapor Transports during Historical Atmospheric River Landfalls on the U.S. West Coast

CW3E Publication Notice

Empirical Return Periods of the Most Intense Vapor Transports during Historical Atmospheric River Landfalls on the U.S. West Coast

August 21, 2018

Atmospheric rivers (ARs) come in all intensities from weak to extremely intense, dropping precipitation amounts –when they encounter mountain ranges on the West Coast– ranging from modest and mostly beneficial to large and mostly hazardous. Because of this wide range of intensities and outcomes, clear and simple communication of the level of storm and flood risks posed by a given storm in observations and forecasts is an important challenge.

A new paper from CW3E in the Journal of Hydrometeorology, ‘Empirical Return Periods of the Most Intense Vapor Transports during Historical Atmospheric River Landfalls on the U.S. West Coast,’ explores the estimation and use of AR return periods as a way to describe very strong ARs to the scientific community and public simply and usefully. The paper is the product of recent research by Mike Dettinger of the US Geological Survey and a long-time CW3E partner, by Marty Ralph, CW3E Director, and by Jonathan Rutz, NWS Salt Lake City and another long-time CW3E partner.

The idea explored is that, for example, a particularly strong AR might be described as being of a magnitude “only seen historically about once in 10 years,” rather than relying on descriptions like “twice as intense as the last one” (whatever that means) or “like the great storm of Feb 7 2017” (which few of us would recall in any detail). The paper documents how strong the most intense AR landfalls (as measured by the amounts of moisture they deliver) have been in the 37-yr period from 1980-2016, and how often storms of various (strong) intensities have made landfall at each latitude along the West Coast (e.g., Fig. 1) and by season of the year. In making these estimates, Dettinger et al. found that the ARs that have delivered the most intense observed rates of vapor transport have made landfall on and near the Oregon coast, with maximum storm intensities falling off to the north and even more so to the south towards southern California where the maximum intensities are less than half the intensity of the strongest Oregon landfalls. When storm-total vapor deliveries are considered, the largest storm totals have followed a broadly similar geographic pattern. Surprisingly though, these largest storm totals were as large as they were not because of their rates of transport but rather because of how long they persisted on a given coastal location.

Figure 1. Historical return periods (and exceedence probabilities) of annual-maximum integrated vapor transport rates, water years 1981–2016, along the U.S. West Coast at grid cells indicated by black dots in maps along right-hand side of the panels, based on MERRA-2.

These patterns of strong-AR landfall provide important perspectives regarding the likelihoods of recurrence of the most intense AR storms that have reached the West Coast, and because precipitation intensities and totals from AR storms depend directly (albeit not solely) on vapor-transport rates and totals, the historical return periods of AR storms offer a useful way to categorize large-AR storms and the risks they pose.

Dettinger, M.D., Ralph, F.M., and Rutz, J.J., 2018, Empirical return periods of the most intense historical atmospheric-river vapor transports on the US West Coast Journal of Hydrometeorology19, 1363–1377 doi: 10.1175/JHM-D-17.0247.1.

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 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.