Publication Notice: CalWater Field Studies Designed to Quantify the Roles of Atmospheric Rivers and Aerosols in Modulating U.S. West Coast Precipitation in a Changing Climate

CW3E Publication Notice

CalWater Field Studies Designed to Quantify the Roles of Atmospheric Rivers and Aerosols in Modulating U.S. West Coast Precipitation in a Changing Climate

November 28, 2016

Ralph F.M., K. A. Prather, D. Cayan, J.R. Spackman, P. DeMott, M. Dettinger, C. Fairall, R. Leung, D. Rosenfeld, S. Rutledge, D. Waliser, A. B. White, J. Cordeira, A. Martin, J. Helly, and J. Intrieri, 2016: CalWater Field Studies Designed to Quantify the Roles of Atmospheric Rivers and Aerosols in Modulating U.S. West Coast Precipitation in a Changing Climate. Bull. Amer. Meteor. Soc. 97, yyy-zzz. doi: 10.1175/BAMS-D-14-00043.1.

This paper summarizes the 8-year-long CalWater program of field studies, from planning to field operations and analysis efforts. It also summarizes the major motivations for the program as well as science gaps addressed, and serves as the standard reference for future CalWater analysis papers.

Contact: F. Martin Ralph (


Quantifying the roles of atmospheric rivers and aerosols in modulating U.S. West Coast precipitation, water supply, flood risks and drought in a changing climate.

The variability of precipitation and water supply along the U.S. West Coast creates major challenges to the region’s economy and environment, as evidenced by the recent California drought. This variability is strongly influenced by atmospheric rivers (AR), which deliver much of the precipitation along the U.S. West Coast and can cause flooding, and by aerosols (from local sources and transported from remote continents and oceans) that modulate clouds and precipitation. A better understanding of these processes is needed to reduce uncertainties in weather predictions and climate projections of droughts and floods, both now and under changing climate conditions.

To address these gaps a group of meteorologists, hydrologists, climate scientists, atmospheric chemists, and oceanographers have created an interdisciplinary research effort, with support from multiple agencies. From 2009-2011 a series of field campaigns (CalWater 1) collected atmospheric chemistry, cloud microphysics and meteorological measurements in California and associated modeling and diagnostic studies were carried out. Based on remaining gaps, a vision was developed to extend these studies offshore over the Eastern North Pacific and to enhance land-based measurements from 2014-2018 (CalWater 2). The data set and selected results from CalWater 1 are summarized here. The goals of CalWater-2, and measurements to date, are then described.

CalWater is producing new findings and exploring new technologies to evaluate and improve global climate models and their regional performance and to develop tools supporting water and hydropower management. These advances also have potential to enhance hazard mitigation by improving near-term weather prediction and subseasonal and seasonal outlooks.

Publication Notice: Forecasting Atmospheric Rivers during CalWater 2015

CW3E Publication Notice

Forecasting Atmospheric Rivers during CalWater 2015

November 22, 2016

Cordeira, J., F. Ralph, A. Martin, N. Gaggini, R. Spackman, P. Neiman, J. Rutz, and R. Pierce, 0: Forecasting Atmospheric Rivers during CalWater 2015. Bull. Amer. Meteor. Soc., 0, doi: 10.1175/BAMS-D-15-00245.1.

As part of CW3E’s mission and goals a new set of atmospheric river (AR)-focused diagnostic and prediction tools have been created, in close partnership with Plymouth State University’s Prof. Jason Cordeira, and building upon work done earlier at NOAA under the HMT Program (see Ralph et al. 2013 BAMS, Wick et al. 2013 Wea. Forecasting). These developments were accelerated and focused by the needs for specialized AR forecast displays to support the CalWater field campaigns in 2014 and 2015 (see Ralph et al. 2016, BAMS). CalWater used research aircraft to observe atmospheric rivers and carried out aerosol science. These developments are summarized in a paper on the forecasting tools that were used in the CalWater field campaign by CW3E researchers and collaborators (Cordeira et al.) that was recently published in Bulletin of the American Meteorological Society (BAMS). The paper details some of the new AR forecasting tools developed using NCEP Global Forecast System and Global Ensemble Forecast System. A novel AR landfall detection forecast tool illustrates the probability of AR conditions at different locations along the western coast of the US. Another new forecast tool that used the various ensemble members illustrates the possible range of integrated water vapor transport (IVT) at a specific location using each of the ensemble members. In addition, the high quality plots of forecasted IVT and observed integrated water vapor supported the CalWater field campaign. Beyond supporting the CalWater Field Campaign, these new forecasting tools will likely improve AR forecasting throughout the West Coast. All these and more of the new forecasting tools can be found on the CW3E website under “Atmospheric River Resources.”

84-h NCEP GFS gridded forecast of IVT magnitude (kg m-1s-1 and direction; initialized at 1200 UTC on 3 February 2015; (b) as in (a), except for the verifying analysis of IVT magnitude and direction at 0000 UTC 7 February 2015 with overlaid draft flight track of the NOAA G-IV aircraft (c) GPS-derived IWV (mm) at 0015 UTC 7 February 2015.


Atmospheric Rivers (ARs) are long and narrow corridors of enhanced vertically integrated water vapor (IWV) and IWV transport (IVT) within the warm sector of extratropical cyclones that can produce heavy precipitation and flooding in regions of complex terrain, especially along the U.S. West Coast. Several field campaigns have investigated ARs under the “CalWater” program of field studies. The first field phase of CalWater during 2009–2011 increased the number of observations of precipitation and aerosols, among other parameters, across California and sampled ARs in the coastal and near-coastal environment, whereas the second field phase of CalWater during 2014–2015 observed the structure and intensity of ARs and aerosols in the coastal and offshore environment over the Northeast Pacific. This manuscript highlights the forecasts that were prepared for the CalWater field campaign in 2015 and the development and use of an “AR portal” that was used to inform these forecasts. The AR portal contains archived and real-time deterministic and probabilistic gridded forecast tools related to ARs that emphasize water vapor concentrations and water vapor flux distributions over the eastern North Pacific, among other parameters, in a variety of formats derived from the NCEP Global Forecast System and Global Ensemble Forecast System. The tools created for the CalWater 2015 field campaign provided valuable guidance for flight planning and field activity purposes, and may prove useful in forecasting ARs and better anticipating hydrometeorological extremes along the U.S. West Coast.

Click here for personal use PDF file

Points of contact: Jason Cordeira, F. Martin Ralph, Brian Kawzenuk

Mesoscale Frontal Wave AR during CalWater-2014

CW3E Publication Notice

An Airborne and Ground-Based Study of a Long-Lived and Intense Atmospheric River with Mesoscale Frontal Waves Impacting California during CalWater-2014

May 10, 2016

Neiman, P.J., B.J. Moore, A.B. White, G.A. Wick, J. Aikins, D.L. Jackson, J.R Spackman, and F.M. Ralph, 2016: An Airborne and Ground-Based Study of a Long-Lived and Intense Atmospheric River with Mesoscale Frontal Waves Impacting California during CalWater-2014. Mon. Wea. Rev., 144, 1115-1144.

This study provides the most comprehensive observations to date of a mesoscale frontal wave associated with an atmospheric river, including its structure offshore, landfall characteristics and impacts on precipitation. It utilizes research aircraft, a unique array of coastal hydrometeorological measurements and inland data. This paper reflects the broader scientific collaboration between CW3E and NOAA/PSD, and adds to the knowledge of phenomena that are critical to creating extreme precipitation on the U.S. West Coast – a major thrust of CW3E. Dr. Ralph contributed to this paper by proposing the experiment (Ralph et al. 2016 BAMS), identifying the science objective for the flights (i.e., mapping out the structure of a mesoscale frontal wave with dropsondes and airborne radar), laying out the flight tracks, guiding the mission onboard, having been the PI of the major projects that created the unique land-based observing network (NOAA HMT- Ralph et al. 2013 BAMS, and the DWR-sponsored EFREP mesonet – White et al. 2013 JTech) used in the study and contributing to the analysis and interpretation of the measurements in this paper.

Contacts: Paul Neiman ( and F. Martin Ralph (


The wettest period during the CalWater-2014 winter field campaign occurred with a long-lived, intense atmospheric river (AR) that impacted California on 7–10 February. The AR was maintained in conjunction with the development and propagation of three successive mesoscale frontal waves. Based on Lagrangian trajectory analysis, moist air of tropical origin was tapped by the AR and was subsequently transported into California. Widespread heavy precipitation (200–400 mm) fell across the coastal mountain ranges northwest of San Francisco and across the northern Sierra Nevada, although only modest flooding ensued due to anomalously dry antecedent conditions. A NOAA G-IV aircraft flew through two of the frontal waves in the AR environment offshore during a ;24-h period. Parallel dropsonde curtains documented key three dimensional thermodynamic and kinematic characteristics across the AR and the frontal waves prior to landfall. The AR characteristics varied, depending on the location of the cross section through the frontal waves. A newly implemented tail-mounted Doppler radar on the G-IV simultaneously captured coherent precipitation features. Along the coast, a 449-MHz wind profiler and collocated global positioning system (GPS) receiver documented prolonged AR conditions linked to the propagation of the three frontal waves and highlighted the orographic character of the coastal-mountain rainfall with the waves’ landfall. Avertically pointing S-PROF radar in the coastal mountains provided detailed information on the bulk microphysical characteristics of the rainfall. Farther inland, a pair of 915-MHz wind profilers and GPS receivers quantified the orographic precipitation forcing as the AR ascended the Sierra Nevada, and as the terrain-induced Sierra barrier jet ascended the northern terminus of California’s Central Valley.

California Storm of 10-12 December 2014: A Synopsis Including Landfalling Atmospheric River Conditions

California Storm of 10-12 December 2014: A Synopsis Including Landfalling Atmospheric River Conditions

July 10, 2015

CW3E researcher Brian Kawzenuk provides an analysis and synopsis of an Atmospheric River that made landfall along the U.S. West Coast over the 10-12 December 2014 period. The AR made initial landfall along the Oregon coast and propagated south before dissipating over southern California. Up to 350 mm of 72-hour precipitation was produced in northern California representing up to 45% of total water year to date precipitation. The precipitation from this event provided many drought-stricken California reservoirs with significant amounts of water supply and improved drought conditions throughout northern California.

Above is a sequence of 30-minute NEXRAD radar composite imagery from 10-13 December 2014 which shows the penetration of the heaviest precipitation.









The above loop shows the strong atmospheric river making landfall and the associated integrated water vapor (color bar in cm).

















Please click here for pdf file of this information.

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.

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.

CalWater-2/ARM Cloud Aerosol Precipitation Experiment (ACAPEX)

CalWater-2/ARM Cloud Aerosol Precipitation Experiment (ACAPEX): AGU session

January 2, 2015

The influence of Atmospheric Rivers (ARs) on wet extremes since 1950 are shown by the fraction of AR landfall days (green portion of pie chart). Note, for example, 87% of flood days for the Russian River are AR landfall days.

The Fall 2014 AGU meeting in San Francisco hosted a workshop/press conference describing the upcoming 2015 field campaign: CalWater2 / ARM Cloud Aerosol Precipitation Experiment (ACAPEX). Scientists from Scripps Institution of Oceanography at UC San Diego, the Department of Energy’s Pacific Northwest National Laboratory (PNNL), and NOAA discussed the impetus behind the field campaign to begin in early 2015. The panel described how ground-based, multiple-aircraft, and ship-based measurements will help provide a better understanding of how California gets its rain and snow, how human activities are influencing precipitation, and how the new science provides potential to inform water management decisions relating to drought and flood. One of the related presentations was given by CW3E PI Mike Dettinger. Dr. Dettinger, AGU 2014 Fellow, described historical and future impacts of land-falling ARs. The image above, from his presentation, depicts the influence of AR land-falling days on extreme wet events in California (Russian River floods, flood plain inundations and levee breaks).

Click here for the UCSD/SIO press release about the workshop / press conference.

Click here for the related UCSD/SIO news story about “Refilling California’s Reservoirs—The Roles of Aerosols and Atmospheric Rivers”.

Click here to follow the CalWater-2 / ACAPEX field project forecasts.

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 2015 Advanced Planning Workshop

CalWater 2015 Advanced Planning Workshop

June 10, 2014

CW3E is pleased to host a CalWater 2015 Advanced Planning Meeting for Forecasting at Scripps Institution of Oceanography from 24-25 June 2014. The purpose of this meeting was to begin preparing for a major deployment of 3 aircraft, a ship and ground-based observation sites this winter. Specifically the aim was to focus on the development of forecast requirements specific to the field experiment and implementation of forecasts for meteorological and chemical/aerosol dimentions of CalWater-2015. Coordinators of this meeting were Jason Cordeira (Plymouth State University), Andrew Martin (Scripps Institution of Oceanography) and Jonathan Rutz (NOAA/National Weather Service). Other participants included Ryan Spackman and Natalie Gaggani (NOAA STC/PSD) and Roger Pierce (NOAA/NWS).