Climate change intensification of horizontal water vapor transport in CMIP5

CW3E Publication Notice

Climate change intensification of horizontal water vapor transport in CMIP5

June 25, 2015

Lavers, D.A., F.M. Ralph, D.E. Waliser, A. Gershunov, and M.D. Dettinger, 2015: Climate change intensification of horizontal water vapor transport in CMIP5. Geophys. Res. Lett., 42, doi:10.1002/2015GL064672.

Projected multimodel mean changes in the mean, standard deviation, and 95th percentile of winter water vapor transport (over 2073-2099) in the RCP4.5 and RCP8.5 scenarios. The multimodel mean percentage changes are shown in the right column. From Lavers et al. (2015).

Research over the last decade has shown that the majority of heavy precipitation and flood events on the western edges of mid-latitude land masses are connected to intense water vapor transport. This vapor transport is found within the atmospheric river region of extratropical cyclones. As climate change is expected to create a warmer atmosphere capable of supporting more water vapor, it is also thought that the global water cycle will intensify leading to more vapor flux and hydrological extremes, such as floods and droughts. Any changes to the water vapor transport by the atmosphere are likely to have hydrological ramifications of great significance to hydrometeorological applications.

The research presented in Lavers et al. (2015) investigates the historical and projected changes to water vapor transport in the latest Climate Model Intercomparison Project Phase 5 archive. Using output from 22 models, robust increases in vapor flux by the end of the 21st Century are found, which suggests the likelihood for larger precipitation and floods in the future. This research was a collaborative effort led by CW3E, with the aim of ascertaining the projected global water cycle changes that may need to be considered in the future.


Global warming of the Earth’s atmosphere is hypothesized to lead to an intensification of the global water cycle. To determine associated hydrological changes, most previous research has used precipitation. This study, however, investigates projected changes to global atmospheric water vapor transport (integrated vapor transport (IVT)), the key link between water source and sink regions. Using 22 global circulation models from the Climate Model Intercomparison Project Phase 5, we evaluate, globally, the mean, standard deviation, and the 95th percentiles of IVT from the historical simulations (1979–2005) and two emissions scenarios (2073–2099). Considering the more extreme emissions, multimodel mean IVT increases by 30–40% in the North Pacific and North Atlantic storm tracks and in the equatorial Pacific Ocean trade winds. An acceleration of the high-latitude IVT is also shown. Analysis of low-altitude moisture and winds suggests that these changes are mainly due to higher atmospheric water vapor content.

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.


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

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”

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

“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

Sonoma County Water Agency video posted about Atmospheric Rivers

Sonoma County Water Agency (SWCA) Video posted about Atmospheric Rivers (ARs)

March 4, 2015

CW3E is pleased to be part of a recent video produced by our partners at the Sonoma County Water Agency (SCWA) and hosted by SCWA Director Shirlee Zane. This video focuses on the importance of Atmospheric Rivers (ARs) to California’s precipitation. Extremes of both drought and flood are examined for their link to ARs and impact on the Sonoma region. Emphasis is placed on the importance of understanding ARs and applying that knowledge to create better forecast information to help SCWA prepare for drought and potential flood conditions. Shirlee points out a key goal of our collaboration: “retain water without increasing flood risk”.

DWR Video posted about CalWater and ARs

DWR Video posted about CalWater and ARs

February 27, 2015

CW3E is pleased to be part of a recent video produced by Elissa Lynn, program manager at California’s Department of Water Resources (a CW3E partner). This video focuses on the recent CalWater 2015 – ACAPEX Field Campaign and the importance of Atmospheric Rivers (ARs) to California’s precipitation. This video provides excellent background information about ARs and how unique CalWater 2015 was with the availability of 4 different aircrafts and a NOAA research vessel examining ARs simultaneously. The importance of atmospheric aerosols from humans and their potential link to precipitation quantity is also described in this video.

California Precipitation: summary handout

California Precipitation: summary handout

February 8, 2015

Coefficient of variation (the standard deviation divided by the average) of total precipitation based on water year data from 1951-2008.

Please click here for the summary handout

CW3E and partners from the California Department of Water Resources, CNAP and the Southwest Climate Science Center have released a summary handout describing California precipitation. The seasonality and variability of precipitation for the state are examined in this summary. Special emphasis is on the link between large storms (AR storms) and the total precipitation for a season. The figure above (Dettinger et al., 2011) illustrates that how much variability there is from year to year in precipitation. The green and blue circles over California indicate the largest year-to-year variability is over this state at an order of about half the annual average precipitation.

LA Times: Focus on ARs and CW2-ACAPEX

LA Times: Focus on ARs and CW2-ACAPEX

January 19, 2015

Photo by Allen J. Schaben / Los Angeles Times: Sunset through clouds over Los Angeles

A recent LA Times article “California drought could end with storms known as atmospheric rivers” highlighted the importance of ARs to California’s water status and the start of the CalWater2 – ACAPEX field campaign (article by Tony Barboza). This article provides an excellent summary of the role of ARs in California’s water supply – from drought to flood. It emphasizes that ARs are known to have a strong link to ending droughts (article by CW3E researcher Mike Dettinger, Journal of Hydrometeorology). As well as highlighting the importance of ARs the article mentions the effort to better understand ARs with the massive data collection effort undergoing now by university and government scientists in CalWater2 – ACAPEX. Find more information about CW2-ACAPEX here.

First International Atmospheric Rivers Conference

First International Atmospheric Rivers Conference

Note: the full conference has been postponed to 2016

Atmospheric rivers (ARs) play a key role in the water cycle as the primary mechanism conveying water vapor through mid-latitude regions. The precipitation that ARs deliver in many parts of the world, especially through orographic precipitation proceses, is important for water resources; but it also regularly is a hazard, with floods resulting. The aims of the First International Atmospheric Rivers Conference are

  • to discuss and identify differing regional perspectives and conditions from around the world,
  • to evaluate the current state and applications of the science of the mid-latitude atmospheric water cycle, with particular emphasis on atmospheric rivers and associated or parallel processes (e.g., tropical moisture exports),
  • to assess current forecasting capabilities and developing applications, and
  • to plan for future scientific and practical challenges.

The conference aims to bring together experts from academia and applications to form a real community of interests. Questions on the table include: What meteorological conditions constitute ARs and what do not? How can ARs (and related processes) best be identified and categorized? What are the most promising new research directions for putting AR science into its proper meteorological/climatological context and improving its applicability?

Additional contributions are now invited from the scientific community

If you have an interest in ARs (or related topics) and an interest in participating please contact the chairs Marty Ralph or Mike Dettinger.

Please click here for more details.

Antarctic Atmospheric Rivers

CW3E Publication Notice

The role of atmospheric rivers in anomalous snow accumulation in East Antarctica

December 4, 2014

Gorodetskaya, I.V., M. Tsukernik, K. Claes, F.M. Ralph, W.D. Neff and N.P.M. Van Lipzig, 2014: The role of atmospheric rivers in anomalous snow accumulation in East Antarctica. Geophysical Research Letters, 41, 6199-6206.

(Please click here for a personal use copy of the article)

Integrated water vapor (colors) at 00Z on 15 February 2011. Red arrows indicated vertically integrated total moisture transport within the atmospheric river as identified using the definition adapted for Antarctica. Black contours are 500 hPa geopotential heights, where L shows a closed trough at 500 hPa influencing Dronning Maud Land and H shows the blocking high-pressure ridge downstream of the low. White square shows Princess Elisabeth station location. Based on the ERA-Interimm reanalysis.

Understanding changes in the Antarctic ice sheet mass are important for predicting global sea level rise. Recently, East Antarctica gained substantial mass, counterbalancing the increasing ice discharge from West Antarctica in these years. Occasional large snowfall events explained this increased mass load, which has been especially high in 2009 and 2011. Ground-based measurements at the Belgian Antarctic station Princess Elisabeth, established at the ascent to the East Antarctic plateau, have well captured these occasional intense snowfalls and associated snow accumulation responsible for 2009 and 2011 mass anomalies. The question is what has been causing this high accumulation?

Most of the water vapor transforming into the Antarctic snowfall arrives from lower latitudes. We have established that atmospheric rivers explain all extremely high snow accumulation events leading to the mass anomaly at Princess Elisabeth station in 2009 and 2011. These narrow bands of high moisture content have been more known in mid latitudes for their, sometimes catastrophic, impacts, such as heavy precipitation resulting in floods. The atmospheric rivers reaching the Antarctic ice sheet bring a lot of moisture from as far as subtropics and result in intense snowfall when reaching the steep ascent to the Antarctic plateau.
In addition, the work represents a significant advance in the understanding of how the global water cycle is affected by atmospheric rivers by

  • diagnosing their role in recent Antarctica extreme snowfall events,
  • developing an AR-detection methodology to track ARs into Polar Regions and
  • exploring their role in cryospheric processes of importance to global sea level in a changing climate.


Recent, heavy snow accumulation events over Dronning Maud Land (DML), East Antarctica, contributed significantly to the Antarctic ice sheet surface mass balance (SMB). Here we combine in situ accumulation measurements and radar-derived snowfall rates from Princess Elisabeth station (PE), located in the DML escarpment zone, along with the European Centre for Medium-range Weather Forecasts Interim reanalysis to investigate moisture transport patterns responsible for these events. In particular, two high-accumulation events in May 2009 and February 2011 showed an atmospheric river (AR) signature with enhanced integrated water vapor (IWV), concentrated in narrow long bands stretching from subtropical latitudes to the East Antarctic coast. Adapting IWV-based AR threshold criteria for Antarctica (by accounting for the much colder and drier environment), we find that it was four and five ARs reaching the coastal DML that contributed 74–80% of the outstanding SMB during 2009 and 2011 at PE. Therefore, accounting for ARs is crucial for understanding East Antarctic SMB.