CW3E Publication Notice: The Chiricahua Gap and the Role of Easterly Water Vapor Transport in Southeastern Arizona Monsoon Precipitation

CW3E Publication Notice

The Chiricahua Gap and the Role of Easterly Water Vapor Transport in Southeastern Arizona Monsoon Precipitation

Spetember 13, 2017

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This study is a collaborative effort between CW3E and University of Arizona that identifies a terrain feature along the Arizona-New Mexico border just north of Mexico that is potentially important to the weather and climate of the southeast Arizona summer monsoon. The terrain feature is a “gap” that is approximately 250 km across and 1 km deep and represents the lowest terrain elevation along the 3000-km length the Continental Divide from 16-45°N. The name “Chiricahua Gap” is introduced to identify this key terrain feature, which reflects the name of a nearby mountain range in southeast Arizona and the region’s Native American history. The importance of the Chiricahua Gap is that it represents the primary pathway in which low altitude atmospheric water vapor is transported across the Continental Divide.

Motivated by identification of the Chiricahua Gap, upper-air observations from a wind profiling radar in Tucson, model reanalyses (Climate Forecast System Reanalysis), and gridded daily precipitation data (NCEP Stage-IV) are used to construct a case study and 15-year climatology to link summer monsoon rainfall events in southeast Arizona to low-altitude water vapor transport within the Chiricahua Gap. The results show that 76% of the wettest summer monsoon days in southeast Arizona during 2002-2016 occurred in conditions of low-altitude easterly water vapor transport in the Chiricahua Gap on the previous day. This result highlights how low-altitude water vapor associated with the wettest summer monsoon days in southeast Arizona originates from the east side of the Continental Divide, which differs from previous studies published since the 1970s. Much of the recent scientific literature points to southwesterly surges of low-altitude water vapor from over the Gulf of California as the primary driver of rainfall over southern Arizona during the summer monsoon. The current study by F. M. Ralph and T. J. Galarneau shows that the source region of low-altitude water vapor in southeast Arizona during the summer monsoon is potentially more complex, and is significantly influenced by source regions east of the Divide.

The paper is an example of CW3E expanding its research to examine the dynamics of the North American monsoon. Because monsoon is an important source or water for the US southwest and can cause flooding events, particularly flash floods, better understanding and improving forecasts of the North American monsoon is and important component of CW3E achieving its goal of revolutionizing the physical understanding, observations, weather predictions, of extreme events in Western North America and their impacts on floods, droughts, hydropower, ecosystems and the economy.

Figure 1: Terrain height (shaded in m) over Arizona, New Mexico, western Texas, and northern Mexico. Key terrain features are labeled in black. The location of Tucson, Arizona, is labeled by the black-filled circle. Low-altitude easterly water vapor transport through the Chiricahua Gap is shown by the blue arrows. This figure is modified from Fig. 1b in Ralph and Galarneau (2017).

CW3E Field Team Beats the Heat, Installs Meteorology and Hydrology Instruments in Russian River Watershed

CW3E Field Team Beats the Heat, Installs Meteorology and Hydrology Instruments in Russian River Watershed

September 6, 2017

A group of CW3E graduate students, postdocs, and staff worked to install soil moisture, meteorology, and streamflow instruments in the Lake Mendocino watershed August 28 – September 1. Taking extra precautions and shifting work schedules due to California’s triple-digit heat wave, the team installed three soil moisture and surface meteorology arrays and a stream gauge on ranchlands representative of the hilly topography draining into Lake Mendocino. CW3E thanks the landowners who have volunteered to have instruments installed on their properties, as well as Steve Turnbull of the U.S. Army Corps of Engineers for participating in the installations. Two more soil moisture and meteorology arrays and three more stream gauges are planned to be installed in the watershed prior to the 2017-18 AR season for a total of six soil moisture and meteorology arrays and six stream gauges. The data from these sites will be used to better understand AR meteorological and hydrologic impacts in this region and improve streamflow forecasts on the Russian River.

The field team after completion of the Potter Valley North site: Lindsey Jasperse, Steve Turnbull, Will Chapman, Maryam Asgari-Lamjiri, Douglas Alden, Anna Wilson and Xin Zhang. Not pictured: Julie Kalansky and Brian Henn

CW3E Outreach at Local Elementary School

CW3E Outreach at Local Elementary School

May 15, 2017

CW3E’s Brian Kawzenuk, Chad Hecht, and Anna Wilson recently visited La Costa Meadows Elementary School to discuss some of the unique observations and tools that meteorologists use to study the atmosphere. Over 1000 students from kindergarten to fifth grade joined to observe and discuss meteorological ground instrumentation and a weather balloon launch. Discussion focused on how the instrumentation works, what they measure, how the observations can be useful to understanding the atmosphere, and how this knowledge and research can be used to improve forecasting and water resource management. During the demonstration students were actively engaged, asked numerous questions, and two assisted in releasing the weather balloon.

After the balloon launch, fourth and fifth grade students were invited to a more in depth discussion and presentation on meteorology, which focused on several aspects that are related to their science curriculum. Topics covered included data gathered from radiosonde launches, radar and satellite observations, storm systems and fronts, and forecasting and atmospheric models. This discussion gave students the opportunity to discuss and ask more questions about the atmosphere and how it is studied. Overall, the event proved to be a valuable experience for both students and staff.

Left: Anna Wilson and Chad Hecht prepare a radiosonde and weather balloon at La Costa Meadows Elementary School. Right: Brian Kawzenuk describes the process of performing a weather balloon launch while Chad and Anna prepare to hand off the balloon and radiosonde to two students.

CW3E Launches Radiosonde with Potter Valley Elementary School Students

CW3E Launches Radiosonde with Potter Valley Elementary School Students

April 16, 2017

CW3E’s Reuben Demirdjian and Anna Wilson recently conducted a demonstration radiosonde launch at the Potter Valley Fire Department for the Potter Valley Elementary School in the Russian River Watershed. The 5th and 6th grade classes, taught by Lori Clark and Merri Emerson respectively, were very enthusiastic participants in the demonstration launch. During the event the students showed their interest while asking a seemingly endless supply of questions about how the information is collected and why it it is useful. The students wrote a message on the to be launched radiosonde’s plastic exterior (Go Bearcats!), and released the balloon. Topics covered included the research done with the collected data, how the information gained from our experiments might help their strong agricultural community, why the balloon expands as it rises, what the sensors attached to the balloon measure, and how far the balloon might go (atmospheric layers were discussed!). Reuben and Anna, along with CW3E’s Douglas Alden, were also able to interact with the same classes at the start of the 2017 field season in January, when they deployed ground instrumentation including a vertically pointing radar, optical disdrometers, rain gauges, and more instrumentation at the Potter Valley Fire Department. The instrumentation at the Potter Valley Fire Department site has collected a valuable dataset over this historic winter season and CW3E is grateful for the support and collaboration and looks forward to a strong relationship going forward.

CW3E graduate student Reuben Demirdjian gets ready to hand off the radiosonde and balloon to two capable Potter Valley Elementary School students to sample a vertical profile of the atmosphere; Potter Valley Elementary School students watch the balloon ascend.

Where was the most extreme precipitation yesterday in the West?

Where was the most extreme precipitation yesterday in the West?

January 10, 2017

Sign up to receive an automated “R-Cat Extreme Precipitation Alert” email from CW3E showing he most extreme precipitation events over the previous 3 days (only on the rare days when there is extreme precipitation). The attached map shows the locations of 52 such reports from the storm that hit the West from 7-10 January 2017.

The maximum three-day precipitation during the weekend event was 521 mm (20.51 inches) at a location called Strawberry Valley on the western slopes of the Sierra Nevada Mountains, about 1,161 meters (3,807 feet) above sea level, near Interstate 80.

That made this an “R-Cat 4” extreme precipitation event on the CW3E’s scale. This is the top magnitude possible and is very rare. “R-Cat” stands for “Rainfall Category,” a simple scaling system invented by CW3E’s Marty Ralph and Mike Dettinger (see brief journal article here or here.

The landfall of a very strong long-duration atmospheric river (AR) (see second figure) followed by a second AR in California over the last few days produced extreme precipitation over much of Central and Northern California. This event was identified and reported in real-time by a new tool developed by David Pierce and Marty Ralph at CW3E that automatically monitors rain gauges across the Western U.S. and sends an email alert when extreme precipitation events occur to anyone signed up for the service. The service is free and is intended to provide information to interested individuals in a timely manner.

To subscribe to this automated CW3E R-Cat Extreme Precipitation Alert via email: just email a message with subject “subscribe” to

The alerts use a simple new method to identify extreme events, which was published after analysis of decades of daily rainfall showed that 3-day precipitation totals were the most logical choice to characterize events that can have the broadest and largest impacts, especially in the Western U.S. The categorization method is based on 3-day observed precipitation totals (rain and/or the liquid equivalent of snow that fell), where “R-Cat” is short for “Rainfall Category:”

R-Cat 1: 200-299 mm (roughly 8-12 inches) / 3 days

R-Cat 2: 300-399 mm (roughly 12-16 inches) / 3 days

R-Cat 3: 400-499 mm (roughly 16-20 inches) / 3 days

R-Cat 4: more than 500 mm (more than roughly 20 inches) / 3 days

Historically these R-Cat events have occurred nationally with the following average annual frequencies (based on a network of several thousand rain gages that each had to have at least 30 years of daily observations; note that the R-Cat Alert tool does not require sites to have had 30 years of data, so more sites are likely to be found meeting the R-Cat criteria than in the earlier detailed analysis):

R-Cat 1: 48 episodes involving a total of 173 rain gauge sites that exceed the R-CAT1 threshold per year

R-Cat 2: 9 episodes involving 23 rain gauge sites that exceeded the R-CAT2 threshold

R-Cat 3: 2 episodes involving 4 rain gauge sites that exceeded the R-CAT3 threshold

R-Cat 4: 1 episode Involving 2 rain gauge sites that exceeded the R-CAT4 threshold

For comparison, the number of R-Cat 3 or 4 events annually roughly matches the average number of major hurricanes that occur annually in the Atlantic (Cat 3, 4, 5 combined) and the number of the most extreme tornadoes that occur (EF-4 and 5 combined).

Notably, in light of the events of last weekend, it is useful to note that, in the Western US between 1948 and 2010, 44 of the 48 occasions when RCAT3 or RCAT4 conditions were reached coincided with the arrival of an atmospheric-river storm.

CW3E partners with California Department of Water Resources, California Geological Survey, US Geological Survey, and the Western Regional Climate Center to assess post-fire debris flow hazards in northern California

CW3E partners with California Department of Water Resources, California Geological Survey, US Geological Survey, and the Western Regional Climate Center to assess post-fire debris flow hazards in northern California

December 2, 2016


Atmospheric River knowledge and tools support post-fire debris flow hazard mitigation and fast-response studies of debris flow-meteorology linkages

An important consequence of the recent record drought in parts of California is the occurrence of major wildfires. The Butte, Valley and Soberanes fires occurred in the last 18 months and have been some of the largest in California history. These tragic burns caused many adverse impacts at the time, and continue to create natural hazards due to the increased risk of damaging debris flows that can occur after the rains return.

California’s burned steeplands are prone to hazardous debris flows during winter storms. Wildfires remove vegetation and alter soil properties, increasing the likelihood of debris flows, even for relatively low intensity storms. When rainfall of sufficient intensity and duration impacts recently burned steeplands, landslides and surface runoff can mobilize ash, rocks, and other material into debris flows that devastate life and property.

California’s Department of Water Resources (DWR) is sponsoring work to examine the role of Atmospheric Rivers on flooding and landslide occurrence and magnitude. The project is led by the Center for Western Weather and Water Extremes (CW3E) at Scripps Institution of Oceanography and includes a team of experts from Scripps, California Geological Survey (CGS) and the U.S. Geological Survey (USGS) Landslide Hazards Team.

Within these burn areas, the geology team, led by Jeremy Lancaster of CGS, is deploying sensors and making measurements in the burn areas when conditions warrant. Doing so requires making decisions on whether to make observations at a study site following a storm event. In support of this, CW3E Graduate Student researchers Nina Oakley and Meredith Fish are using new knowledge of weather systems capable of producing intense precipitation, especially Atmospheric Rivers, to evaluate the potential for high-intensity precipitation over the Soberanes Fire, Butte Fire, and Valley Fire burn areas to advise Lancaster. Key to these preparations and day-to-day decisions are the new Atmospheric River forecasting tools at CW3E. Additionally, post-storm, CW3E scientists will compile meteorological data relevant to the storm event such as maximum precipitation intensity, storm total precipitation, radar imagery, an evaluation of Atmospheric River variables, and any information unique to that storm. For events that produce a debris flow response, a more in-depth case study will be conducted combining both geologic and atmospheric information.

Figure 1: Map of three burn areas that we propose to assess: Soberanes, Valley, and Butte wildfires.

Synthesis of the information collected through these storm and debris flow response logs will provide insight to post-fire debris flow triggering rainfall thresholds across northern California and the meteorological conditions that produce such rainfall. This integrated approach of meteorologists and geologists working together to address the post-fire debris flow issue will help advance our knowledge of these potentially hazardous events. This knowledge will be incorporated into landslide/debris flow hazard outlooks that factor in both landscape conditions (e.g., fire) and meteorology (e.g., extreme precipitation from Atmospheric Rivers)

Figure 2: Debris flow deposits stopped by cement barriers outside the Big Sur Lodge in California. This event was triggered by rain falling on burned steeplands in 2009, near an area now burned again by the Soberanes wildfire. (credit: David Longstreth, CGS).

Figure 3: During Fall 2016, USGS and CGS researchers install a rain near Pfeiffer Falls in the Soberanes Fire burn area to measure the rainfall intensities that trigger post-fire debris flows.

Contacts: Jeremy Lancaster (CGS), Nina Oakley (DRI and CW3E), John Stock (USGS), F.M. Ralph (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

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.

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

Test Beds Linking Research and Forecasting

Test Beds Linking Research and Forecasting

September 10, 2013

TestBeds Linking Research and Forecasting

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.

Publication Notice: Chemical properties of insoluble precipitation residue particles

CW3E Publication Notice

Chemical properties of insoluble precipitation residue particles

Jessie Creamean posing for a photo while clearing snow from the top of the NOAA trailer at Sugar Pine Dam after the storm on 2/25/11.

This article provides an in-depth analysis of resuspended residues from precipitation samples collected at a remote site in the Sierra Nevada Mountains in California during the 2009-2011 winter seasons. These residues may be used as a benchmark for classification of insoluble precipitation. Knowledge of the precipitation chemistry of insoluble residues coupled with meteorological and cloud microphysical measurements will ultimately improve our understanding of the link between aerosols, clouds, and precipitation.

This paper represents a significant milestone from the CalWater experiment, which is led by members of UCSD/Scripps’ new Centers on aerosols (CAICE) and extreme events (CW3E), as well as NOAA, DOE, NASA, USGS. It also highlights the multi-disciplinary research stimulated by CalWater, and the partnerships between key researchers across organizations. The lead author, Jessie Creamean, received her PhD in atmospheric chemistry from UCSD under Kim Prather using CalWater data, and is now bringing that expertise to a primarily meteorological group in NOAA as she pursues emerging topics in aerosol-precipitation interactions in collaboration with CW3E scientists.

A personal use copy of the article is available here.