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The Inland Penetration of Atmospheric Rivers

CW3E Publication Notice

The Inland Penetration of Atmospheric Rivers over Western North America: A Lagrangian Analysis

June 15, 2015

Jonathan J. Rutz, W. James Steenburgh, and F. Martin Ralph, 2015: The Inland Penetration of Atmospheric Rivers over Western North America: A Lagrangian Analysis. Mon. Wea. Rev., 143, 1924–1944. (Click here for personal-use pdf file of the article)

Schematic from Rutz et al. (2015) showing the primary pathways for the penetration of 950-hPa AR-related trajectories into the interior of western North America. Pathways associated with regimes 1–3 are represented by green, orange, and purple arrows, respectively. Regions associated with frequent AR decay are shaded in red. Topography is shaded in grayscale. Note that while this schematic highlights common regimes and pathways, individual trajectories follow many different paths.

Although atmospheric rivers (ARs) are typically regarded as coastal events, their impacts can be felt further inland as well. Recent work by Rutz et al. (2015) uses a forward trajectory analysis and AR thresholding criteria to examine the inland penetration of ARs over western North America, and identifies geographic corridors where inland-penetrating ARs are most likely. This paper builds on the earlier work led by Jon Rutz (Rutz and Steenburgh 2012 – Atmos. Sci. Lett., and Rutz et al. 2014 – Mon. Wea. Rev.) as part of his PhD dissertation with Jim Steenburgh at Univ. of Utah. Combined with recent results from Alexander et al. (2015, J. Hydrometeor) that used backward trajectories to examine inland penetration of ARs, as well as earlier studies on Arizona AR events (Neiman et al. 2013, Hughes et al. 2014; both in J. Hydrometeor.) and across the west (Ralph et al. 2014; J. Contemporary Water Resources Research and Education), it is now clear that ARs play a critical role in Western U.S. extreme precipitation, even well inland from the coastal areas where they were first studied. These results improve our understanding of water vapor transport and precipitation over the interior western U.S., and hence contribute to ongoing research interests and efforts at CW3E regarding the causes and prediction of extreme weather and water events across the Western U.S.

Abstract

Although atmospheric rivers (ARs) typically weaken following landfall, those that penetrate inland can contribute to heavy precipitation and high-impact weather within the interior of western North America. In this paper, the authors examine the evolution of ARs over western North America using trajectories released at 950 and 700 hPa within cool-season ARs along the Pacific coast. These trajectories are classified as coastal decaying, inland penetrating, or interior penetrating based on whether they remain within an AR upon reaching selected transects over western North America. Interior-penetrating AR trajectories most frequently make landfall along the Oregon coast, but the greatest fraction of landfalling AR trajectories that eventually penetrate into the interior within an AR is found along the Baja Peninsula. In contrast, interior-penetrating AR trajectories rarely traverse the southern “high” Sierra. At landfall, interior-penetrating AR trajectories are associated with a more amplified flow pattern, more southwesterly (vs westerly) flow along the Pacific coast, and larger water vapor transport (qυ). The larger initial qυ of interior-penetrating AR trajectories is due primarily to larger initial water vapor q and wind speed υ for those initiated at 950 and 700 hPa, respectively.

Inland- and interior-penetrating AR trajectories maintain large qυ over the interior partially due to increases in υ that offset decreases in q, particularly in the vicinity of topographical barriers. Therefore, synoptic conditions and trajectory pathways favoring larger initial qυ at the coast, limited water vapor depletion by orographic precipitation, and increases in υ over the interior are keys to differentiating interior-penetrating from coastal-decaying ARs.

Atmospheric Rivers Workshop: June 15-17, 2015

June 19, 2015

An atmospheric rivers (AR) workshop was held 15-17 June at the Seaside Forum at UC San Diego/Scripps Institution of Oceanography (SIO). This workshop was sponsored by the Center for Western Weather and Water Extremes (CW3E) at SIO. Mike Dettinger, David Lavers and Marty Ralph were co-chairs of this workshop. The photo below shows the workshop participants.

Left to right: Jay Jasperse (Sonoma County Water Agency), Jennifer Haase (Scripps), Lauren Muscatine (UC Davis), Brian Kawzenuk (Scripps/CW3E), Tamara Shulgina (Fulbright Scholar at Scripps/CW3E), Sasha Gershunov (Scripps/CW3E), Joel Norris (Scripps and CW3E), Roger Pierce (NOAA/NWS), Harald Sodemann (Univ. of Bergen), Marty Ralph (Scripps/CW3E; Workshop Co-Chair), Nina Oakley (Univ. of Nevada Reno), Mike Dettinger (USGS & Scripps/CW3E; Workshop Co-Chair), Dale Cox (USGS), David Lavers (Scripps/CW3E; Workshop Co-Chair), Jon Rutz (NWS), Jason Cordeira (Plymouth State Univ.), Andrew Martin (Scripps/CW3E), Allen White (NOAA/ESRL), Bin Guan (UCLA), Heini Wernli (ETH Zurich), Larry Schick (US Army Corps of Engineers), Dan Cayan (Scripps/CW3E and USGS), Julie Kalansky (Scripps/CW3E), Ryan Spackman (Science and Technology Corp. and NOAA/ESRL), Maximiliano Viale (Univ. of Chile). Attendees not in picture: Mike Anderson (California Dept. of Water Resources), Bruce Cornuelle (Scripps & CW3E), Duane Waliser (NASA/JPL)

This workshop brought together experts from around the world to survey the current state of atmospheric-river (AR) science and plan the First International Atmospheric Rivers Conference to be held in summer 2016 at Scripps’ Seaside Forum. The group also planned the development of a Monograph on atmospheric rivers that is intended to become the standard reference on the roughly 20 years of AR research. The meeting addressed an outstanding debate in the science community about the physical relationship between ARs, the warm conveyor belt (WCB) in extratropical cyclones and tropical moisture exports (TME) to the extratropics.
The workshop concluded with a plan for the conference in 2016, a strategy for the book, and development of a schematic summary of the relationships between ARs, WCBs and TMEs, each of which plays a critical and complementary role in transporting water vapor through the atmosphere, in terms of horizontal transport and sloped ascent in extratropical cyclones.
The term “atmospheric river” was first coined in 1994 to describe atmospheric water vapor transport across the mid-latitudes. Subsequent research has shown them to be responsible for the majority of extreme hydrologic events in the western United States, Europe, and South America, as well as being critical to water resources in these regions.

For real-time observations and forecasts of atmospheric rivers, please visit the “AR Portal”

Closed Low Event May 6-10: A Preliminary Synopsis

May 15, 2015

CW3E researcher Nina Oakley provides a preliminary synopsis for a closed low that developed in the Pacific Northwest and moved south along the California/Nevada border over the 6-10 May period. The closed low turned east over southern California with a very cold core moving over a warm surface. The easterly flow produced orographic precipitation on the eastern side of the Sierra with minimal precipitation on the western side of the Sierra. Large areas of the Great Basin experienced precipitation amounts that were not extreme but significant for May.

Climatology of extreme daily precipitation in Colorado

CW3E Publication Notice

Climatology of extreme daily precipitation in Colorado and its diverse spatial and seasonal variability

May 5, 2015

Seasonality of the top 10 daily precipitation events measured at Colorado COOP stations that have at least 30 years of data since 1950. Circles represent totals of 10 events. Seasons shaded as winter (DJF; blue), spring (MAM; yellow), summer (JJA; red), and fall (SON; green). Terrain elevation (m; gray shading) as in legend at left; Continental Divide shown by dashed black line.

The origins of extreme precipitation events in the Western U.S. range from landfalling atmospheric rivers, to the summer monsoon, upslope storms on the Rocky Mountain Front Range, and deep convection of the Great Plains variety. This was shown by an analysis across the west of the seasonality of the top 10 wettest days for each of thousands of COOP observer sites (Ralph et al. 2014**). Each of these sites had at least ~10,000 data points, so these top 10 days represent roughly the top 0.1% of days. Some areas were universally dominated by events in one season, or two. A couple of areas stood out in the diversity of their seasonality of extreme daily precipitation, including Colorado.

The study presented in Mahoney et al. 2015* explores this local variability more deeply, explores how the devastating flood of September 2013 in Colorado’s northern Front Range is related, and describes some of the implications of the findings for flood control and other sensitive sectors. The co-authors represent a diverse group themselves, including climate, weather, hydrology, hydrometeorology expertise from several organizations, (CIRES, NOAA/PSD, Scripps/CW3E and CSU). The paper is highlighted here as it represents an example of work on extreme events in the Western U.S. that the Center for Western Weather and Water Extremes is contributing to.

Abstract of Mahoney et al. 2015*: The climatology of Colorado’s historical extreme precipitation events shows a remarkable degree of seasonal and regional variability. Analysis of the largest historical daily-precipitation totals at COOP stations across Colorado by season indicates that the largest recorded daily precipitation totals have ranged from less than 60 mm/day in some areas to greater than 250 mm/day in others. East of the Continental Divide winter events are rarely among the top 10 events at a given site, but spring events dominate in and near the foothills; summer events are most common across the lower-elevation eastern plains, while fall events are most typical for the lower elevations west of the Divide. The seasonal signal in Colorado’s central mountains is complex; high-elevation intense precipitation events have occurred in all months of the year, including summer when precipitation is more likely to be liquid (as opposed to snow) which poses more of an instantaneous flood risk.

*Mahoney, K., F.M. Ralph, K. Wolter, N. Doesken, M. Dettinger, D. Gottas, T. Coleman, and A. White, 2015: Climatology of extreme daily precipitation in Colorado and its diverse spatial and seasonal variability. J. Hydrometeor. 16, 781-792.

** Ralph, F. M., M. Dettinger, A. White, D. Reynolds, D. Cayan, T. Schneider, R. Cifelli, K. Redmond, M. Anderson, F. Gherke, J. Jones, K. Mahoney, L. Johnson, S. Gutman, V. Chandrasekar, J. Lundquist, N.P. Molotch, L. Brekke, R. Pulwarty, J. Horel, L. Schick, A. Edman, P. Mote, J. Abatzoglou, R. Pierce and G. Wick, 2014: A vision for future observations for Western U.S. extreme precipitation and flooding– Special Issue of J. Contemporary Water Resources Research and Education, Universities Council for Water Resources, Issue 153, pp. 16-32.

Sonoma County Water Agency and CW3E — Monitoring sites expected to improve forecast capability

Sonoma County Water Agency and CW3E — Monitoring sites expected to improve forecast capability<

September 2013

South end of Lake Mendocino; September 2013 (photo by Kent Porter / Press Democrat)

The Santa Rosa Press Democrat (Sean Scully) published an article: “Weather forecasting a key concern for Sonoma County Water Agency”. CW3E is working with the Sonoma County Water Agency and the Hydrometeorology Testbed (HMT) data to improve forecasts in the region. The data being gathered from HMT will lead to improved forecasting of Atmospheric River (AR) events. These events are significant rain producers in the region and strongly impact how water is managed. Better management would lead to storage that could help prevent extremely low lake levels (as shown at Lake Mendocino above). Please find the full Press Democrat article here.

Test Beds Linking Research and Forecasting

September 10, 2013

A new article written by Marty Ralph and colleagues was recently published in the Bulletin of the American Meteorological Society focusing on the emergence of weather-related test beds. The paper provides a brief background on how these test beds successfully bridged the gap between research and forecasting operations; summarizes test bed origins, methods and selected accomplishments; and provides a perspective on the future of test beds. A personal use copy of the paper can be obtained here.

“Atmospheric Rivers”: Rising Interest in Science and the Media

April 25, 2015

The term “atmospheric river” was first coined in 1994 to describe atmospheric water vapor transport across the mid-latitudes. Subsequent research has shown them to be responsible for the majority of extreme hydrologic events in the western United States, Europe, and South America, as well as being critical to water resources in these regions.

A recent analysis conducted by Ann Coppin and Duane Waliser of NASA’s Jet Propulsion Laboratory and Marty Ralph of Scripps Institution Of Oceanography’s CW3E has highlighted the growing number of journal publications using the term “atmospheric rivers”, illustrating a growing use for this terminology (see figure below).

The number of publications using the term “atmospheric river” from 1994-2013

The number of references across different media outlets has also risen underscoring the increasing public interest in this phenomenon (see figure below).

The number of times the term “atmospheric river” has been used across various media outlets from 2005-2015

1st ARTMIP Workshop held at SIO

1^st ARTMIP Workshop held at SIO

May 15, 2017

The 1^st Atmospheric River Tracking Method Intercomparison Project (ARTMIP) Workshop was recently held at the Scripps Institute of Oceanography in La Jolla, CA. The participants consisted of an ad-hoc group of researchers working to identify and track atmospheric rivers (ARs), elongated regions of atmospheric water vapor transport that contribute significantly to high-impact weather events and hydroclimate. The American Meteorological Society has recently accepted a qualitative definition of what constitutes an AR, but many differences in AR identification and tracking algorithms exist. One goal of the meeting was to discuss key science questions that arise from these methods – most of these are questions related to uncertainty regarding AR climatology, the relationship between ARs and precipitation, and how these may be altered under future climate change scenarios. Another goal of the meeting was to reach consensus on common data sets and metrics to facilitate a quantitative comparison of these AR identification and tracking methods.

The 1^st ARTMIP Workshop was a resounding success, featuring ~20 participants from a wide variety of institutions. ARTMIP participants agreed upon a two-tiered structure for the project. Tier 1 describes the core of the ARTMIP project – common metrics that all participants have agreed to pursue. This analysis will be based on participants applying their AR identification and tracking methods to a common reanalysis data set (MERRA version 2) a period of record (1980-2016), with selected years and/or individual events also investigated. Data provided by each participant will be used to produce global and regional analyses of AR climatology and contributions to precipitation. Tier 2 describes a wide range or related analyses that will be conducted by subgroups within ARTMIP. Analyses will focus on addressing specific science questions such as how and why AR climatology varies in different regions, what are the relationships between AR and precipitation extremes, and how much do ARs contribute to poleward moisture transport globally.

The group will produce at least two papers based on Tier 1 metrics. The first will be submitted to Geoscientific Model Development (GMD) and will describe the Tier 1 experimental design and possible Tier 2 projects, as well as invite the wider community to participate in the ARTMIP. Christine Shields of NCAR will lead this paper, which will be written in the very near timeframe (mid 2017). The second paper will describe the comparative results across methods for AR climatology that are obtained through the Tier 1 analysis. Jon Rutz of the NWS will lead this paper, to be submitted to BAMS (or a similar high-profile journal) in about a year (mid to late 2018).

There are multiple Tier 2 analyses to be performed and these will comprise several additional papers. While Tier 1 focuses on uncertainty amongst the different tracking method algorithms, Tier 2 will explore other sources of uncertainty, such as the uncertainty that arises from different reanalysis products (ERA-interim, NCEP, CFSR and JRA55) and the uncertainty that arises in future climate change projections of ARs.

We have at least three proposed Tier 2 analyses focusing on climate model products. The first is to compare 25, 100, and 200 km CAM5 All-Hist (cesm1_0_3) output from the C20C+ Sub-project on Detection and Attribution (portal.nersc.gov/c20c) to explore uncertainty across model resolution. This is a different issue than the simple coarsening of the reanalysis data. The second climate model Tier 2 analysis is an actual climate change experiment that will analyze the historical and rcp8.5 25 km CAM5 simulations produced for the C20C+ (high-resolution is likely to be very important in replicating atmospheric river climatology and properties). A third experiment is to analyze CMIP5 ensemble results to explore structural uncertainty of projected changes in AR statistics. However, this is contingent on obtaining the large volume of data that must be downloaded from the ESGF. As has seen before, high frequency data can take months to assemble from the ESGF.

Jon Rutz (NOAA/NWS) and Christine Shields (NCAR) serve as ARTMIP co-chairs, while Michael Wehner (LBNL), Ruby Leung (PNNL), and Marty Ralph (UCSD) serve with them on the project committee. A second ARTMIP workshop will be held in mid- to late 2018 to discuss the results of the intercomparison and the implications for the science of Atmospheric Rivers.

Workshop Participants (left to right): Sasha Gershunov, Beth McClenny, Roger Pierce, Anna Wilson, Bin Guan, Ashley Payne, Juan Lora, Duane Waliser, Marty Ralph, Paul Ullrich, Ruby Leung, Michael Wehner, Christine Shields, Jon Rutz, Aneesh Subramanian, Scott Sellars.

Resilience in a Changing Climate: Sonoma County Adaptation Forum

April 15, 2015

CW3E director Marty Ralph and scientist Julie Kalansky presented at the Sonoma County Adaptation Forum on April 8th. The forum was modeled after state forums, but was the first regional adaptation forum in California. The forum focused on information and approaches to help mitigate the impacts of climate change in Sonoma County and surrounding areas. The audience of over 200 people included city and county leaders, utility managers, environmental groups and the public.

Both Marty and Julie presented in the first session of the morning entitled “Extreme Weather Science; Drought and Deluge in Sonoma County.” Jay Jasperse, Chief Engineer and Director of Groundwater Management at Sonoma County Water Agency, moderated the session. The other panelists included Tim Doherty, from NOAA’s Office for Coastal Management, who discussed the impacts of sea level rise on the region, and Dr. Lisa Micheli, Executive Director of Pepperwood Preserve, who presented on the importance of downscaling climate models to understand the regional response to climate change. Marty Ralph discussed the importance of atmospheric rivers (ARs) to the water supply as well as the potential flooding risk associated with ARs. This led into an explanation of the FIRO, forecast informed reservoir operations, project for improving the water supply resilience of Lake Mendocino. At the end of his presentation he introduced the first part of an ongoing NOAA-NIDIS and Sonoma County Water Agency funded project to examine how the frequency and intensity of ARs may change in future. The link below is to an interview with Marty Ralph about atmospheric rivers and the forum that was broadcasted on North Bay Public Radio.

http://radio.krcb.org/post/charting-local-adaptations-climate-change

After Marty’s presentation, Julie presented on the second part of the study including the development of a “mega-drought” stress test for the region and working with the community to understand the all the different dimensions of drought. During Julie’s presentation, she was able to involve the audience and received feedback on the vulnerabilities to drought and the difficult decisions that surround drought. The day was a great success in bringing together scientists, decisions makers and the public to discuss how to make the community more resilient to climate change.

CW3E Welcomes Brian Kawzenuk

CW3E welcomes Brian Kawzenuk

March 30, 2015

CW3E is pleased to welcome Brian Kawzenuk as a staff research associate. Brian joins us from completing his master’s work with Dr. Jason Cordeira at Plymouth State University in New Hampshire. At Plymouth Brian investigated the impacts of land-falling atmospheric rivers (ARs) on the west coast during February 2014. The structure and dynamics of AR events as well as their influence on extreme precipitation over the west coast were explored. Brian looks forward to continuing his study of AR events and using his extensive analytic and programming skills to help the CW3E team develop stakeholder tools. Brian grew up in central New York State and has always had a passion for meteorology. We’re delighted to welcome him to the team and hope he enjoys the milder climate of the La Jolla region.