Short diagnosis of development of a tropic surge, cut-off low and AR features

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).”

(Please click here for powerpoint slides)

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.

Publication Notice: Climatological Characteristics of Atmospheric Rivers and Their Inland Penetration over the Western United States

CW3E Publication Notice

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.

Heavy rain in the Pacific northwest

Heavy Rain in the Pacific Northwest

October 27, 2014

Heavy rain struck the Pacific Northwest (Figs. 1, 2) from a series of modest intensity storms, including landfall of embedded atmospheric river conditions (Fig. 3). Precipitation totals during the week of 17-24 October approached or exceeded 10 inches in many of the normally wet mountain locations from northwest Washington to northwest California. One of the largest totals, about 15 inches was in the Olympic Mountains. Although some precipitation fell in the headwaters to reservoirs in northern-most California, there was a sharp southern edge to the heavy precipitation that occurred in northern California.


Fig. 1. Examples of some of the heaviest precipitation in the region from 17 to 24 October 2014.


Fig. 2. 14-day precipitation ending 1200 UTC 25 Oct 2014.


Fig. 3. A well-defined atmospheric river on 23 October contributed to some of the heaviest precipitation, including sites that received over 5 inches of rain in 24 hours in southwest Oregon and Northwest California around the time of this SSM/I satellite image (courtesy of CIMSS).

The southern end of the AR passed over the Russian River on 25 Oct (Fig. 4) and produced the first rainfall event of the season of over 1 inch accumulation. It lasted about 15 h, roughly half the duration of an average AR in the region. Also, snow was observed on Donner Pass (I-80).


Fig. 4. The atmospheric river observatory at Bodega Bay, CA (part of the new state-wide extreme precipitation network sponsored by CA DWR and developed by NOAA and Scripps) documented the passage of the southern end of the first AR of the season in northern California.

The “Northern Sierra 8-Station Index” received roughly 1.2 inches, bringing the season total to 2.7 inches, which is about normal to date for the first 4 weeks of the new water year. More significant precipitation is predicted for the Pacific Northwest in the next week, from 24-31 October 2014 (Fig. 5). Note once again the sharp southern edge to the heavy precipitation, as occurred in the previous week.


Fig. 5. Precipitation forecast for 24-31 October 2014 courtesy of the National Weather Service.

California Drought: 2013/14 4th driest water year on record

California Drought: 4th driest water year depletes reservoirs

October 1, 2014

CW3E partners at the Sonoma County Water Agency (SCWA) are quoted in a recent article in the Santa Rosa Press Democrat marking water year 2013/14 as the 4th driest on record.


The baked lakebed of Lake Mendocino shows a carp head (photo taken 30 September 2014 by Press Democrat photographer Kent Porter)

Dwindling reservoir levels are one of the main concerns due to the drought – Lake Mendocino is only 27% full. The article mentions the research partnership between SCWA and CW3E. Understanding the key role of atmospheric rivers in the area’s water supply is a focus of the research agreement. Please find the full article (including additional photos and a video) at:

NASA NEWS program selects joint JPL+CW3E AR proposal

NASA NEWS program selects joint JPL+CW3E AR proposal

August 28, 2014


NASA recently announced it selections for its 2013 NASA Energy and Water Cycle (NEWS) solicitation. Included in the selections was a proposal led by Duane Waliser of JPL as PI, and Marty Ralph of CW3E and Bin Guan of UCLA as co-PIs. The proposal aims to study “Atmospheric Rivers: Water Extremes that Impact Global Climate, Regional Weather and Water Resources”, with objectives on observation characterization, including emphasis on atmospheric and terrestrial water budgets, and on the development of methodologies and metrics for model evaluation. The proposal is for a three year study, and funding is expected to start early in FY 2015.

Publication Notice: An Airborne Study of an Atmospheric River over the Subtropical Pacific during WISPAR: Dropsonde Budget-Box Diagnostics and Precipitation Impacts in Hawaii

CW3E Publication Notice

An Airborne Study of an Atmospheric River over the Subtropical Pacific during WISPAR: Dropsonde Budget-Box Diagnostics and Precipitation Impacts in Hawaii

Satellite image swath at 1921 UTC 3 Mar 2011 of IWV (cm; color scale at bottom) from the SSMIS. The unadjusted G-IV flight track is superimposed, along with dots marking the positions and times of the dropsonde releases late on 3 Mar (2007–2343 UTC) and early on 4 Mar (0003–0302 UTC) 2011. The operational rawinsonde locations at Lihue (LIH) and Hilo (ITO) on Hawaii are shown. Manually smoothed satellite-derived sea surface temperatures (8C; contours) from the Reynolds et al. (2007) daily 0.25830.258 resolution optimally interpolated infrared product are also shown. (From Neiman et al. 2014.).

The upcoming CalWater-2 experiment, which is organized by CW3E scientists and others from NOAA, NASA, DOE, USGS and elsewhere, will be using research aircraft to observe atmospheric rivers over the Northeastern Pacific Ocean and U.S. West Coast. This presents an opportunity to measure atmospheric river (AR) structure and embedded physical processes that control the water vapor transport budget. This paper develops some of the key diagnostic tools needed and demonstrates them using a flight pattern and dropsondes designed to calculate the vertical profile of water vapor flux divergence (and other key parameters) in an AR. These tools and associated flight strategies will be critical to future airborne field campaigns that will enable CW3E and colleagues to diagnose key AR-relevant physical processes and to then assess in detail their representation in weather and climate models.

In 2011 a short airborne campaign was conducted to demonstrate the ability of an unmanned aircraft (Global Hawk) to fly over ARs and sample them using dropsondes. As part of this successful demonstration of UAS technology – WISPAR – a special flight of the NOAA G-IV aircraft was also conducted. The paper presents a summary of this flight, including use of drospondes from a box pattern over an AR to calculate vertical profiles of divergence, water vapor flux divergence, sensible heat flux divergence and vertical air motions. The same AR studied here also produced over 10 inches of rain on the normally dry side of the Hawaiian Islands due to the anomalous water vapor transport conditions associated with the AR hitting the region.


The Winter Storms and Pacific Atmospheric Rivers (WISPAR) experiment was carried out in January–March 2011 from the National Aeronautics and Space Administration (NASA) Dryden Flight Research Center as a demonstration for utilizing unmanned aerial systems in meteorological research and operations over data-sparse oceans. One of the campaign’s three missions was coordinated with a manned National Oceanic and Atmospheric Administration Gulfstream-IV (G-IV) flight out of Honolulu, Hawaii, on 3–4 March 2011. The G-IV, which flew through a developing atmospheric river (AR) west of Hawaii, represents the cornerstone observing platform for this study and provided the southernmost dropsonde observations of an AR published to date in the subtropical Northern Hemisphere. The AR exhibited characteristics comparable to those observed in previous studies farther north in the subtropics and midlatitudes, save for larger integrated water vapor and weaker winds in the AR core and stronger equatorward vapor fluxes in the shallow post-cold-frontal northeasterly flow. Eight dropsondes released in a ~200-km-wide box formation provided a novel kinematic assessment of tropospheric vorticity, divergence (mass, water vapor, sensible heat), and vertical velocity in the AR region, as well as sea surface fluxes. The budget-box diagnostics were physically consistent with global-gridded reanalysis datasets, while also providing useful additional kinematic and thermodynamic information on the mesoscale. Meteorological impacts of the AR were assessed on Hawaii’s island of Kauai, where the state’s heaviest rainfall was observed for this case. Rainfall on Kauai was modulated significantly by its steep orography, including on the normally dry side of the island where heavy rains fell.

A personal use copy of the article is available here.

CalWater-ACAPEX 2015 Planning Workshop

CalWater-ACAPEX 2015 Planning Workshop

Scripps Institution of Oceanography

La Jolla, California

CalWater 2 Co-Leads: Marty Ralph, Kim Prather, Dan Cayan (Scripps)

Organizing Committee: Chris Fairall (NOAA), Ruby Leung (PNNL), Andrew Martin (Scripps), Ryan Spackman (NOAA/STC)

CalWater2 – ACAPEX Observational Strategy Winter 2014-15

CalWater-2 took major steps from vision to reality on 22-24 April 2014 at Scripps Institution of Oceanography when roughly 40 key individuals (scientists, engineers, aircraft and ship managers, and students) met to plan for major field deployments in 2015. The following facilities are committed (or nearly so) to a field campaign between roughly 10 January and 10 March 2015:

  • DOE – G-1 aircraft
  • DOE AMF-2 ocean-atmosphere facility on the NOAA Research Vessel (ship) Ron Brown
  • NOAA G-IV aircraft
  • NOAA P-3 aircraft
  • ATOFMS mobile, land-based aerosol-sensor suite
  • EFREP hydrometeorological Mesonetwork in California

The DOE facilities are part of the ARM Cloud Aerosol Precipitation Experiment (ACAPEX) experiment addressing (1) aerosol impacts on clouds and precipitation and (2) atmospheric rivers. The NOAA facilities were requested also based on the CalWater vision, with an emphasis on atmospheric-river science questions.

The workshop concluded with a plan for specific start and end dates for each facility, narrowed options for where to operate them, a plan for a field operations center (and a specific possible location), strategies for developing coordinated ship and aircraft operations, and plans for the forecasting capabilities needed to guide missions. In addition, the 12-member CalWater Core Scientific Steering Group met afterword and reviewed plans for 2016-2018 and strategies to advance the longer term Calwater Vision. The Steering Group committed to organizing two special sessions and a side meeting (for last minute coordinations of the 2015 CalWater and ACAPEX activities) at the Fall Meeting of AGU in December 2014, and a journal article describing the program. The proposed AGU sessions are:

  1. CalWater Theme 1: Cloud-Aerosol-Precipitation Interactions in California (Conveners: Daniel Rosenfeld, Kimberly Prather),
  2. Atmospheric Rivers: Observations, Dynamics, Modeling, Impacts and Applications (Conveners: Marty Ralph, Duane Waliser, Jason Cordeira).

The presentations from the Workshop are available here.

Workshop Sponsored by:

  • Scripps, Center for Western Weather and Water Extremes (CW3E)
  • Scripps Center for Aerosol Impacts on Climate and the Environment (CAICE)
  • Science and Technology Corporation (STC)

Workshop Sponsored by:Workshop Participants

Photo of most workshop participants at the CalWater 2015 – ACAPEX workshop at Scripps, April 2014.

CalWater2 Workshop Participants

ARs Play Role in Greenland Melt Episodes

Atmospheric Rivers Play Key Role in Rare Greenland Melt Episodes

Integrated Water Vapor (IWV) impacting Greenland on July 9, 2012 based on data from the 20th Century Reanalysis.

Researchers at NOAA’s Earth System Research Laboratory and the Center for Western Weather and Water Extremes here at Scripps have published a new article examining the processes responsible for the unusual melting episode in Greenland during the summer of 2012 when temperatures at the summit of Greenland rose above freezing for the first time since 1889. They found a number of climate factors were present in both 1889 and 2012 including strong atmospheric rivers transporting warm, moist air towards Greenland’s west coast. The research article was published in the Journal of Geophysical Research – Atmospheres.

A more in depth news story on this research can be found on the Scripps website.

A personal use copy of the article is available here.

Vision for future observations of extreme events in Western US

Vision for Future Observations of Extreme Precipitation and Flooding in the Western U.S.

A journal article entitled: A Vision for Future Observations for Western U.S. Extreme Precipitation and Flooding, by CW3E Director F. Martin Ralph and colleagues was recently published in the April 2014 issue of the Journal of Contemporary Water Research and Education

The paper describes how new technologies and paradigms using the most recent technological and scientific advances can be used to better monitor and predict extreme storms that lead to flooding in the Western U.S. The strategy is intended to add new technology to existing observational networks rather than replacement. The full journal article can be accessed here.

Schematic network of new sensors (land-based) to improve monitoring, prediction, and climate trend detection for hydrometeorological conditions that create extreme precipitation and flooding.