CW3E Publication Notice: Flood runoff in relation to water vapor transport by atmospheric rivers over the western United States

CW3E Publication Notice

Flood runoff in relation to water vapor transport by atmospheric rivers over the western United States

December 1, 2017

CW3E long-time collaborator, Mike Dettinger, and USGS colleague, recently published a paper in Geophysical Research Letter titled: Flood runoff in relation to water vapor transport by atmospheric rivers over the western United States.

In the study they analyzed historical flood flows at over 5000 streamgages across the western US in relation to landfalling atmospheric-river storms. Specifically, they focused on the probabilities of floods flows occurring as conditioned by the presence of an atmospheric river and by the water vapor-transport rates in the atmospheric river. Through this analysis they were able to show that stronger the atmospheric river, the more likely are flood flows to develop.

Along the west coast, these peak flows coincide with atmospheric rivers about 80+% of the time, falling off to about 40-50% of the time in southern California, and falling off the farther inland the river basin (with notable regional anomalies, e.g., around Phoenix and in northern Idaho).

CW3E Hosts California DWR Winter Outlook Workshop

CW3E Hosts California DWR Winter Outlook Workshop

ovember 6, 2017

CW3E hosted the annual California Department of Water Resources Winter Outlook Workshop (WOW) from Nov. 1-3, at Scripps Institution of Oceanography. The purpose of the workshop is to highlight the latest science in seasonal to subseasonal (S2S), 1-month to 3-month, atmospheric forecasting. This timescale bridges the gap between weather and climate prediction. The meeting was organized by Jeanine Jones, ‎Interstate Resources Manager, and covered a variety of topics including paleoclimate, week three predictions, atmospheric rivers (ARs), summer North American monsoon, new forecasting tools, and drought.

During the first day, Dave Meko, from University of Arizona, discussed paleodrought in Southern California and how this compared to paleodrought in Northern California and in the Colorado River Basin. The second day of the workshop highlighted recent accomplishments in subseasonal to seasonal forecasting. Dr. David DeWitt, NCEP CPC Director, presented on the S2S activities on-going at the National Weather Service (NWS) Climate Prediction Center (CPC). Dr. Marty Ralph, Director of CW3E, gave an overview of the activities at CW3E related to observations, modeling and S2S prediction ARs. This was followed by a session chaired by Dr. Duane Waliser and Dr. Aneesh Subramanian on current S2S activities at CW3E and an experimental CW3E week-3 AR outlook product. The session had three talks presented by Dr. Alexander Gershunov, Dr. Michael DeFlorio, and Dr. Aneesh Subramanian on the experimental CW3E week-3 AR outlooks and the multi-pronged effort to design and evaluate the product. The day ended with presentation by Yolande Serra, from the University of Washington, on the dynamics and predictability of the summer monsoon. The third and final day began with a presentation on the influence of ARs in the Colorado River Basin as well an historical perspective on atmospheric river maps by Jon Rutz, National Weather Service. The last presentation of the day was by Dan Cayan on how various atmospheric patterns can lead to drought in the western U.S. WOW provided an opportunity for CW3E researchers and collaborators to share their latest advancements in subseasonal to seasonal forecasting and discuss future research collaboration and needs.

CW3E Director, Marty Ralph, discusses the research and activities at CW3E during the DWR WOW.

During the workshop Jeanine Jones also presented Department of Water Resources Climate Service Awards to Dr. Jason Cordeira, Plymouth State University, Dr. Duane Waliser, NASA JPL and Dr. Dave Meko, University of Arizona. The awards highlight the three individuals’ contribution to climate science as it applies to DWR operations. Jason Cordiera, a close collaborator of CW3E spoke of the honor, “My collaboration with CW3E has led to the synergistic development of many weather forecast tools that have benefited and informed water resource management and related impact-based decision support. Receipt of the CA DWR Climate Science Service Award reflects the dedication of many individuals at CW3E and Plymouth State who support and provide invaluable resources to maintain a productive research and application environment. Thank you to the CA DWR for the honor and I look forward to continued collaboration in pursuit of improving our ability to understand and forecast hydrological extremes”.

Recipients of the Department of Water Resources Climate Service Awards, presented at the WOW. From left, Dr. Duane Waliser, NASA JPL, Dr. Dave Meko, University of Arizona, and Dr. Jason Cordeira, Plymouth State University.

Atmospheric Rivers Highlighted in the U.S. Fourth National Climate Assessment

Atmospheric Rivers Highlighted in the U.S. Fourth National Climate Assessment

November 6, 2017

Click here for a pdf of this information.

The Fourth National Climate Assessment, released last week, highlights atmospheric rivers as a key topic of its chapter on “Extreme Storms.” The other storm types addressed in this section are “tropical storms (hurricanes and typhoons),” “severe convective storms (thunderstorms)” and “winter storms.

The “Key findings” on atmospheric rivers are: “The frequency and severity of landfalling “atmospheric rivers” on the U.S. West Coast (narrow streams of moisture that account for 30%–40% of the typical snowpack and annual precipitation in the region and are associated with severe flooding events) will increase as a result of increasing evaporation and resulting higher atmospheric water vapor that occurs with increasing temperature. (Medium confidence).”

This major report further highlighted the atmospheric river topic by using a satellite image of an atmospheric river hitting the U.S. West Coast in February 2017 for the cover page of the entire report.



Contacts: Duane Waliser, F. Martin Ralph

CW3E Graduate Student to Participate in United Nations Convention Next Month

CW3E Graduate Student to Participate in United Nations Convention Next Month

October 23, 2017

Tashiana Osborne, a graduate student within CW3E, will be attending the 23rd United Nations Framework Convention on Climate Change in Bonn, Germany next month. During the convention, Tashiana will lead a press conference centered on oceanic and atmospheric phenomena with another Scripps student. Her attendance at the Convention along with two other Scripps graduate students was highlighted in a recent San Diego Union Tribune article. Tashiana was interviewed about atmospheric rivers and their importance to California’s water supply as well as their potential to lead to flooding. Read more here about Tashiana and other graduate students heading to the Convention on Climate Change.

CW3E, UCAR, and NCAR Meet to Discuss West-WRF Regional Model Development

CW3E, UCAR, and NCAR Meet to Discuss West-WRF Regional Model Development

October 9, 2017

On 4-5 October 2017, CW3E had the privilege of hosting visitors from NCAR and UCAR to discuss the development and implementation of West-WRF, the regional forecast model that CW3E is developing focusing on extreme precipitation. The team from UCAR and NCAR included Bill Kuo, the director of UCAR Community Programs, who also helped lead the development of WRF. Chris Davis, NCAR associate director and leader of the Mesoscale and Microscale Meteorology (MMM) Laboratory also attended, along with, David Gill, Jake Liu, and Wei Wang, WRF experts in computation, data assimilation, and modeling, respectively.

The first day of the two day visit began with CW3E director, Marty Ralph, briefing the NCAR/UCAR visitors on CW3E, and how West-WRF supports the mission and goals of the center. After this introduction, CW3E researchers and staff had the opportunity to learn about best practices with respect to WRF computation, modeling, and data assimilation, as well as the new MPAS modeling system. The entire CW3E group had lunch with the NCAR/UCAR visitors and had a chance to hear all the CW3E updates including on AR reconnaissance, publications, and instrument deployments.

After lunch, the CW3E West-WRF team shared the current applications and status of the West-WRF development with the UCAR/NCAR team. The afternoon ended with Bill Kuo giving the CASPO seminar on assessment of the impacts of assimilation of COSMIC radio occultation measurements in typhoon forecasts. The second day of the visit, allowed for detailed discussions on many of the technical aspects of West-WRF development and applications. The UCAR/NCAR team provided recommendations to the CW3E researches and staff on ways to improve the implantation of West-WRF as well as design experiments. In addition the groups discussed ways for the CW3E team to provide feedback in the WRF development at NCAR/UCAR through sharing new code for verification metrics and scientific and technical advancements made through recent experiments. The meeting was a very productive initial collaboration between CW3E and UCAR/NCAR and we are looking forward to many more. The engagement of UCAR and NCAR in supporting one of its member institutions technical development efforts is greatly appreciated.

CW3E Publication Notice: Dropsonde Observations of Total Integrated Water Vapor Transport within North Pacific Atmospheric Rivers

CW3E Publication Notice

Dropsonde Observations of Total Integrated Water Vapor Transport within North Pacific Atmospheric Rivers

Spetember 14, 2017

F. Martin Ralph, director of CW3E, along with collaborators, recently published a paper in the American Meteorological Society’s Journal of Hydrometeorology: Ralph, F. M., S. Iacobellis, P. Neiman, J. Cordeira, J. Spackman, D. Waliser, G. Wick, A. White, and C. Fairall, 2017: Dropsonde Observations of Total Integrated Water Vapor Transport within North Pacific Atmospheric Rivers. J. Hydrometeor., 18, 2577-2596,

This study uses vertical profiles of water vapor, wind, and pressure obtained from 304 aircraft dropsondes across 21 ARs, in the midlatitudes as well as the subtropics, which were deployed during various experiments since the winter of 1998, including CALJET (Ralph et al. 2004), Ghostnets (Ralph et al. 2011), WISPAR (Neiman et al. 2014), CalWater-2014, CalWater-2015 (Ralph et al. 2016), and AR Recon-2016. Dropsondes provide the best measurements to date of horizontal water vapor transport in atmospheric rivers (ARs) and can document AR structure. Different methods of defining AR edges, using either integrated vapor transport (IVT) or integrated water vapor (IWV), were compared.

The study found that total water vapor transport (TIVT) in an AR averaged nearly 5×108 kg s-1, which is 2.6 times larger than the average discharge of liquid water from the Amazon River. The mean AR width was 890 ± 270 km. Subtropical ARs contained larger IWV but weaker winds than midlatitude ARs, although average TIVTs were nearly the same. Mean TIVTs calculated with an IVT-threshold versus an IWV- threshold produced results that differed by only 4% on average, although they did vary more between midlatitudes and subtropical regions. In general, important AR characteristics such as width and TIVT are less dependent on latitude when the IVT-threshold is used, and the IWV threshold often was not crossed on the warm side of subtropical ARs, so IVT represents a more robust threshold across a wider range of conditions than IWV.

Results were summarized in a schematic to illustrate the AR structure in 3 dimensions (see below). This schematic was used in the AR definition that was recently published in the American Meteorological Society’s Glossary of Meteorology.

Figure 1: Schematic summary of the structure and strength of an atmospheric river based on dropsonde measurements analyzed in this study, and on corresponding reanalyses that provide the plan-view context. (a) Plan view including parent low pressure system, and associated cold, warm, stationary and warm-occluded surface fronts. IVT is shown by color fill (magnitude, kg m-1 s-1) and direction in the core (white arrow). Vertically integrated water vapor (IWV, cm) is contoured. A representative length scale is shown. The position of the cross-section shown in panel (b) is denoted by the dashed line A-A’. (b) Vertical cross-section perspective, including the core of the water vapor transport in the atmospheric river (orange contours and color fill) and the pre-cold-frontal low-level jet (LLJ), in the context of the jet-front system and tropopause. Water vapor mixing ratio (green dotted lines, g kg-1) and cross-section-normal isotachs (blue contours, m s-1) are shown. Magnitudes of variables represent an average mid-latitude atmospheric river with lateral boundaries defined using the IVT threshold of 250 kg m-1 s-1. Depth corresponds to the altitude below which 75% of IVT occurs. Adapted primarily from Ralph et al. 2004 and Cordeira et al. 2013.

CW3E Publication Notice: The Chiricahua Gap and the Role of Easterly Water Vapor Transport in Southeastern Arizona Monsoon Precipitation

CW3E Publication Notice

The Chiricahua Gap and the Role of Easterly Water Vapor Transport in Southeastern Arizona Monsoon Precipitation

Spetember 13, 2017

Click here for personal use pdf file

This study is a collaborative effort between CW3E and University of Arizona that identifies a terrain feature along the Arizona-New Mexico border just north of Mexico that is potentially important to the weather and climate of the southeast Arizona summer monsoon. The terrain feature is a “gap” that is approximately 250 km across and 1 km deep and represents the lowest terrain elevation along the 3000-km length the Continental Divide from 16-45°N. The name “Chiricahua Gap” is introduced to identify this key terrain feature, which reflects the name of a nearby mountain range in southeast Arizona and the region’s Native American history. The importance of the Chiricahua Gap is that it represents the primary pathway in which low altitude atmospheric water vapor is transported across the Continental Divide.

Motivated by identification of the Chiricahua Gap, upper-air observations from a wind profiling radar in Tucson, model reanalyses (Climate Forecast System Reanalysis), and gridded daily precipitation data (NCEP Stage-IV) are used to construct a case study and 15-year climatology to link summer monsoon rainfall events in southeast Arizona to low-altitude water vapor transport within the Chiricahua Gap. The results show that 76% of the wettest summer monsoon days in southeast Arizona during 2002-2016 occurred in conditions of low-altitude easterly water vapor transport in the Chiricahua Gap on the previous day. This result highlights how low-altitude water vapor associated with the wettest summer monsoon days in southeast Arizona originates from the east side of the Continental Divide, which differs from previous studies published since the 1970s. Much of the recent scientific literature points to southwesterly surges of low-altitude water vapor from over the Gulf of California as the primary driver of rainfall over southern Arizona during the summer monsoon. The current study by F. M. Ralph and T. J. Galarneau shows that the source region of low-altitude water vapor in southeast Arizona during the summer monsoon is potentially more complex, and is significantly influenced by source regions east of the Divide.

The paper is an example of CW3E expanding its research to examine the dynamics of the North American monsoon. Because monsoon is an important source or water for the US southwest and can cause flooding events, particularly flash floods, better understanding and improving forecasts of the North American monsoon is and important component of CW3E achieving its goal of revolutionizing the physical understanding, observations, weather predictions, of extreme events in Western North America and their impacts on floods, droughts, hydropower, ecosystems and the economy.

Figure 1: Terrain height (shaded in m) over Arizona, New Mexico, western Texas, and northern Mexico. Key terrain features are labeled in black. The location of Tucson, Arizona, is labeled by the black-filled circle. Low-altitude easterly water vapor transport through the Chiricahua Gap is shown by the blue arrows. This figure is modified from Fig. 1b in Ralph and Galarneau (2017).

CW3E Publication Notice: Characterizing the Influence of Atmospheric River Orientation and Intensity on Precipitation Distribution over North-Coastal California

CW3E Publication Notice

Characterizing the Influence of Atmospheric River Orientation and Intensity on Precipitation Distributions over North-Coastal California

Spetember 12, 2017

Chad Hecht, a CW3E staff researcher, and Jason Cordeira, a CW3E affiliate and professor at Plymouth State University, recently published an article in AGU Geophysical Research Letters: Hecht, C. W., and J. M. Cordeira, 2017: Characterizing the Influence of Atmospheric River Orientation and Intensity on Precipitation Distribution over North-Coastal California. Geophys. Res. Lett., 44, doi:10.1002/2017GL074179. click here for personal use pdf file.

The key result of the study found that south-southwesterly oriented atmospheric rivers (ARs) produce significantly more Russian River watershed areal-average precipitation compared to westerly ARs (median areal-precipitation of 13 mm vs. .5 mm). This difference in precipitation accumulations is attributed to both the orientation of water vapor flux relative to the watershed topography and large-scale forcing that results in ascent.

The study uses clustering to objectively identify different orientations and intensities of ARs that make landfall over the California Russian River watershed (Fig. 1). Daily averaged IVT was calculated using 11-years of National Centers for Environmental Prediction (NCEP)–Climate Forecast System Reanalysis (CFSR) data spanning from 1 January 2004 to 31 December 2014. The paper analyzed the synoptic-scale flow configurations and resulting precipitation accumulations and distributions of westerly and south-southwesterly oriented ARs (Orange and Blue clusters in Fig. 1b).

Figure 1. (a) Domain averaged daily IVT direction (angular coordinate) and magnitude (kg m/s ; radial coordinate) for all days from 1 January 2004 to 31 December 2014 that data were available. Markers are color-coded based on 24-h accumulated precipitation (mm). The colored lines illustrate the average IVT for days with precipitation >10 (black), >25 (blue) and >50 mm (red). The 200 kg/m/s threshold that was applied in this study is shown by the black circle. (b) As in (a) except for days with daily average IVT ≥200 kg/m/s and color-coded based on K-means cluster.

Composite analyses illustrate the vastly different synoptic-scale characteristics associated with westerly and south/southwesterly ARs (Cluster 2 and Cluster 3). These different synoptic-scale flow configurations result in differences in synoptic scale forcing co-located over the composite AR and the Russian River watershed (Fig. 2).

Figure 2. (a, b) Composite mean IVT (kg/m/s ; plotted according to the reference vector in the upper right), SLP (hPa; contoured), and IWV (mm; color-coded according to scale), (c, d) composite mean 250-hPa geopotential height (dam; contoured), wind speed (m s–1 ; colorcoded according to scale), and IWV (mm; dashed blue contour), and (e, f) composite mean 700-hPa geopotential height (dam; solid contours), Q-vectors (1011 K/m/s ; plotted according to the reference vector in the bottom right), Q-vector divergence (1016 K/m/s ; color-coded according to scale) and potential temperature (K; dashed red contours) at t–12 h during (a,c,e) westerly and (b,d,f) south–southwesterly ARs.

The large difference in Russian River watershed area-averaged precipitation between westerly and south-southwesterly ARs (Fig. 3a) is not likely explained by statistically similar cluster IVT magnitudes (i.e., AR intensity; Fig. 3b) and IWV values (Fig. 3e) but likely a combination of a more favorable southwesterly IVT direction (i.e., AR orientation) relative to the orientation of the local topography and favorable synoptic-scale forcing for ascent (Fig. 2) illustrated by Q-vector convergence (Fig. 3d). While both AR types exhibit significantly statistically similar mean IVT, south-southwesterly ARs are associated with statistically significantly higher mean low-level IVT (1000–850 hPa; Fig. 3c).

Results from this study suggest that extreme precipitation produced by ARs is the result of both upslope moisture flux and quasi-geostrophic forcing for ascent.

Figure 3. Box and whisker plots of Russian River Watershed (a) area-average 24-h precipitation (mm), (b) domain average IVT (kg/m/s ), (c) domain average lower tropospheric (1000–850 hPa) IVT (kg/m/s ), (d) domain average Q-vector divergence (1016 K/m/s ), and (e) domain average IWV (mm) for westerly (orange) and south–southwesterly (blue) ARs. The boxes represent the interquartile range of the data and the whiskers represent upper and lower quartile of the data. The horizontal line within the boxes is the median value. The colored dots represent outliers and the asterisks represent extreme outliers. The box in the upper-left corner of each panel indicates the result of the independent samples t-test with 95% confidence (white indicates significantly statistically similar means and black indicates significantly statistically different).

Support for this project was provided by the State of California-Department of Water Resources and the U.S. Army Corps of Engineers, both as part of broader projects led by CW3E. A majority of this work was conducted while Chad was a graduate student at Plymouth State University. Dr. Cordeira and his graduate students at Plymouth State University actively collaborate with CW3E on topics related to atmospheric rivers, such as analyzing, understanding, and forecasting their impacts along the U.S. West Coast.

CW3E Publication Notice: Hourly Storm Characteristics along the U.S. West Coast: Role of Atmospheric Rivers in Extreme Precipitation

CW3E Publication Notice

Hourly Storm Characteristics along the U.S. West Coast: Role of Atmospheric Rivers in Extreme Precipitation

July 10, 2017

Fifty-five years of gridded hourly precipitation observations (CPC Hourly U.S. Precipitation) are used in this study to identify storm characteristics which most strongly modulate extreme storms along the U.S. West Coast. By investigating storms at fine (hourly) time scales, we showed that U.S. West Coast storm total precipitation is more strongly modulated by storm durations than by storm intensities, whereas in the Southeast U.S., storm intensities more strongly dictate the storm total precipitation (Figure 1, presented as Figure 2 in Lamjiri et al. [2017]). This study also showed that the most extreme precipitation events along the U.S. West Coast are associated with the most persistent atmospheric rivers, rather than the high intensity ARs. Therefore, it is of high importance to improve forecast skill of the duration of storms over the U.S. West Coast, which provides valuable information that could be used to mitigate flood risks and enhance water reservoir management. More details are provided in the full manuscript, which was published in the AGU journal Geophysical Research Letters: Lamjiri, M. A., M. D. Dettinger, F. M. Ralph, and B. Guan, 2017: Hourly storm characteristics along the U.S. West Coast: Role of atmospheric rivers in extreme precipitation, Geophys. Res. Lett., 44, doi:10.1002/2017GL074193. click here for personal use pdf file

Figure 1 Correlation coefficient of storm-precipitation totals with storm durations (a), maximum intensities (b), and average intensities(c) based on hourly precipitation observations from 1948-2002.


Gridded hourly precipitation observations over the conterminous US, from 1948 to 2002, are analyzed to determine climatological characteristics of storm precipitation totals. Despite generally lower hourly intensities, precipitation totals along the U.S. West Coast (USWC) are comparable to those in Southeast U.S. (SEUS). Storm durations, more so than hourly intensities, strongly modulate precipitation-total variability over the USWC, where the correlation coefficients between storm durations and storm totals range from 0.7 to 0.9. Atmospheric rivers (ARs) contribute 30-50% of annual precipitation on the USWC, and make such large contributions to extreme storms that 60-100% of the most extreme storms, i.e. storms with precipitation-total return intervals longer than two years, are associated with ARs. These extreme storm totals are more strongly tied to storm durations than to storm hourly or average intensities, emphasizing the importance of AR persistence to extreme storms on the USWC.

CW3E Graduate Students Complete Advanced Study Program in Colorado

CW3E Graduate Students Complete Advanced Study Program in Colorado

June 29, 2017

CW3E graduate students Meredith Fish and Tashiana Osborne were selected to participate in the competitive Advanced Study Program on the Interaction of Precipitation with Orography. The program is a two-week colloquium held at the National Center for Atmospheric Research (NCAR) Mesa Lab in Boulder, Colorado.

Osborne and Fish, along with 24 other students from around the globe, heard dynamic talks from professors, researchers, academics, and professionals from federal agencies such as NCAR and NOAA and universities such as the University of Washington, the University of Colorado Boulder and the University of Miami, as well as many others. The lectures focused on precipitation in the world’s mountainous regions. One of the speakers was CW3E post-doc Nick Siler, who spoke about his research on orographic rain shadows. Talks addressed a variety of topics including the dynamical flow and physical science, challenges in weather and climate modeling around these regions and interactions of between the atmosphere, land and ocean, as well as professional development.

Students visit the Mountain Research Station where researchers focus on advancing the study of mountain ecosystems; Elevation: ~9500 feet. Photo credit: Richard Neale, Project Scientist at NCAR

Students also grew through hands-on practical sessions analyzing observed precipitation datasets, such as PRISM and TRMM, and running the NCAR Weather Research and Forecasting Model (WRF) and Community Earth System Model (CESM). CW3E has designed a version of WRF that is tailored for West Coast atmospheric rivers, with an aim to enhance understanding of precipitation processes. Following this colloquium, students are able to bring the practical expertise gained at NCAR back to Scripps and CW3E to further enhance scientific understanding of precipitation over orography. During the last week of the colloquium, students applied new skills and knowledge to design and complete their own research project incorporating WRF and CESM modeling techniques to present on the last day.

Fun group photo of the students participating in the colloquium; featuring NCAR Project Scientist, Richard Neale. Photo Credit: Valerie Sloan, Director of GEO REU Network and Internship Specialist at the University Corporation for Atmospheric Research (UCAR)