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.
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.
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.
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.
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.
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.
“And it never failed that during the dry years the people forgot about the rich years and during the wet years they lost all memory of the dry years. It was always that way.” —John Steinbeck, East of Eden, 1962
Heavy rains are predicted for California this week, and after the extreme drought of the past few years, California welcomes the moisture. But can there be too much of a good thing?
While drought is a significant natural hazard Californians must contend with, the natural hazards of severe weather and flooding are equally significant in the feast or famine cycle of storms in California.
NOAA’s National Weather Service has issued several watches, warnings, and advisories for across California. Flash floods and high winds are expected in many areas.
Drought is a familiar occurrence in California. Indeed, at a year-to-year or shorter time scale, California has a remarkably variable hydroclimate, experiencing larger year-to-year variations in precipitation than anywhere else in the U.S.
In large part, this extreme variability arises from the small number of storms that provide most of the state’s precipitation each year. If a few large storms happen to bypass California in a given winter, precipitation totals are proportionally much reduced and we risk drought. But the wet, drought-busting months are typically reflections of one or two extremely large storms, with almost half of the large drought-busting storms resulting from landfalling atmospheric rivers or “pineapple expresses.”
Atmospheric Rivers (ARs) are constantly moving and evolving pathways of water vapor transport that are thousands of kilometers long but only about 500 km wide and that contain large quantities of water vapor and strong winds They are naturally occurring parts of the global water cycle, responsible for more than 90 percent of all atmospheric vapor transport at latitudes of the conterminous United States. When an AR reaches and encounters mountains in the West Coast states, the fast moving, moisture-laden air contained in ARs generally flows up and over the coastal and Sierra Nevada ranges, leading to almost ideal conditions for producing intense and sustained precipitation. Because of the intensity and persistence of their rains, ARs are the cause of many of the most extreme storms along the West Coast and a large majority of the floods in that region.
Atmospheric rivers have, in recent years, been recognized as the cause of the large majority of major floods in rivers all along the U.S. West Coast and as the source of 30 – 50 percent of all precipitation in the same region. In terms of droughts in California, about 33 – 40 percent of all persistent drought endings have been brought about by landfalling AR storms, with more localized low-pressure systems responsible for most of the remaining drought breaks.
In 2010, the USGS Science Applications for Risk Reduction (SAFRR) program created an interdisciplinary scenario about large atmospheric river storms in California called ARkStorm. The ArkStorm scenario brought together experts in climate, weather, economics, geography, and other disciplines to create a hypothetical, but scientifically plausible scenario of a future large storm that is providing emergency responders, resource managers, and the public with a realistic example that they can use to determine the possible consequences of a really large AR storm might be.
A new Center at UC San Diego’s Scripps Institution of Oceanography has established a regional effort on atmospheric rivers and other types of extreme weather and water events in the Western U.S. The Center for Western Weather and Water Extremes (CW3E) is developing an “AR Portal” with partners across the nation, including NOAA, California Department of Water Resources, Plymouth State University, and the USGS. The portal brings together advances in AR science, monitoring and prediction, and builds heavily on data from the new AR monitoring network installed across California, and takes unique advantage of existing USGS, NOAA and other monitoring and prediction systems by developing tools tailored to the AR phenomenon.
Pacific Northwest Flooding (slide 3; M. Ralph and L. Schick)
CW3E director Marty Ralph and US Army Corps of Engineers researcher Larry Schick provide a summary of recent flooding in the Pacific Northwest. The heavy precipitation (ending November 28) resulted in an R-CAT 2 event (an event which produces 12-16 inches of precipitation in a 3-day period).
During this event one can see an orographic enhancement of precipitation amounts with a rain “shadow” (low amounts of precipitation) in the Seattle region (slide 2). The heavy precipitation resulted in several streamflow sites exceeding flood stage (shown above; slide 3). The Northwest River Forecast Center had an excellent forecast of peak flow on the Skagit River (slide 4).
Short diagnosis of development of a tropic surge, cut-off low and AR features
December 2, 2014
Storm surge summary (slide 10; M. Ralph)
CW3E director Marty Ralph provides a short diagnosis of an interesting case with a variety of features coming together to generate very large IWV in this currently landfalling storm.
Dr. Ralph notes “This could be a useful event to diagnose more deeply given its relevance to many things we are working on, and the debates about AR, cut-off low, tropical moisture exports, etc. The IVT perspective needs to be explored as well, but the IWV features are quite telling. Jay Cordeira had shared a brief synopsis including a cross-section from GFS that showed the vapor transport over LA maximized at about 3.5 km, which may be more like a ‘tropical moisture export’ structure (Knippertz et al).”
The attached ppt includes an isochrone analysis of the northern edge of the tropical water vapor reservoir (using 4 cm IWV – summary shown above and in slide 10) and its landfall (as seen in the GPS-Met network – slide 11). Also, the snow levels are well-observed with the new SLR network and shows strong north-south variation (slide 12)..
Forecasts are available from the California Nevada River Forecast Center (CNRFC): click here for precipitation forecasts..
Forecasts are also available from the weather service forecast office of the National Weather Service in San Diego: please click here.
Climatological Characteristics of Atmospheric Rivers and Their Inland Penetration over the Western United States
Mean duration (h) of AR conditions based on IVT250. Histograms of IVT250 ARduration at selected coastal (left) and interior locations (right).
This paper quantifies the climatological frequency and duration of atmospheric rivers (ARs) over the western U.S., as well as the contribution of ARs to heavy precipitation events and cool-season hydroclimate over this region. ARs are objectively identified within reanalysis data based on integrated water vapor transport, which is not only shown to be well-correlated with cool-season precipitation over the West, but also useful for tracking AR penetration from the coast to the interior. Hence, this study lays the groundwork for the development of forecasting tools that will enhance the predictability of ARs and their impacts on the western U.S. This paper presents key findings from a dissertation completed by Jon Rutz at the University of Utah, and is coauthored by his advisor, Jim Steenburgh, and by the CW3E director, F. Martin Ralph. It has also become one of the 10 most-read articles in Monthly Weather Review for the year.
Narrow corridors of water vapor transport known as atmospheric rivers (ARs) contribute to extreme precipitation and flooding along the West Coast of the United States, but knowledge of their influence over the interior is limited. Here, the authors use Interim European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-Interim) data, Climate Prediction Center (CPC) precipitation analyses, and Snowpack Telemetry (SNOTEL) observations to describe the characteristics of cool-season (November–April) ARs over the western United States. It is shown that AR frequency and duration exhibit a maximum along the Oregon–Washington coast, a strong transition zone upwind (west) of and over the Cascade–Sierra ranges, and a broad minimum that extends from the “high” Sierra south of Lake Tahoe eastward across the central Great Basin and into the deep interior. East of the Cascade–Sierra ranges, AR frequency and duration are largest over the interior northwest, while AR duration is large compared to AR frequency over the interior southwest. The fractions of cool-season precipitation and top-decile 24-h precipitation events attributable to ARs are largest over and west of the Cascade–Sierra ranges. Farther east, these fractions are largest over the northwest and southwest interior, with distinctly different large-scale patterns and AR orientations enabling AR penetration into each of these regions. In contrast, AR-related precipitation over the Great Basin east of the high Sierra is rare. These results indicate that water vapor depletion over major topographic barriers is a key contributor to AR decay, with ARs playing a more prominent role in the inland precipitation climatology where lower or less continuous topography facilitates the inland penetration of ARs.
A personal use copy of the article is available here.