Storms, Floods, and the Science of Atmospheric Rivers

Abstract

Imagine a stream of water thousands of kilometers long and as wide as
the distance between New York City and Washington, D. C., flowing toward
you at 30 miles per hour. No, this is not some hypothetical physics
problem—it is a real river, carrying more water than 7-15
Mississippi Rivers combined. But it is not on land. It's a river of
water vapor in the atmosphere. Atmospheric rivers (ARs) are narrow
corridors of water vapor transport in the lower atmosphere that traverse
long swaths of the Earth's surface as they bind together the atmospheric
water cycle (Figure 1). The characteristic (indeed defining) dimensions
of these ARs are (1) integrated water vapor (IWV) concentrations such
that if all the vapor in the atmospheric column were condensed into
liquid water, the result would be a layer 2 or more centimeters thick;
(2) wind speeds of greater than 12.5 meters per second in the lowest 2
kilometers; and (3) a shape that is long and narrow, no more than
400-500 kilometers wide, and extending for thousands of kilometers,
sometimes across entire ocean basins.

Full text

VOLUME 92 NUMBER 32
9 AUGUST 2011
PAGES 265–272
EOS, TranSacTiOnS, amErican GEOphySical UniOn
Eos, Vol. 92, No. 32, 9 August 2011
Imagine a stream of water thousands of
kilometers long and as wide as the distance
between New York City and Washington,
D.C., flowing toward you at 30miles per
hour. No, this is not some hypothetical phys-
ics problem—it is a real river, car rying more
water than 7–15Mississippi Rivers combined.
But it is not on land. It’s a river of water
vapor in the atmosphere.
Atmospheric rivers (ARs) are narrow cor-
ridors of water vapor transport in the lower
atmosphere that traverse long swaths of the
Earth’s surface as they bind together the
atmospheric water cycle (Figure1). The char-
acteristic (indeed defining) dimensions of
these ARs are (1)integrated water vapor
(IWV) concentrations such that if all the
vapor in the atmospheric column were con-
densed into liquid water, the result would be
a layer 2 or more centimeters thick; (2)wind
speeds of greater than 12.5 meters per second
in the lowest 2kilometers; and (3)a shape
that is long and narrow, no more than 400–
500 kilometers wide, and extending for thou-
sands of kilometers, sometimes across entire
ocean basins.
Research during the past decade ha s doc-
umented the importance of ARs to the over-
all workings of the midlatitude global water
cycle. Moreover, their presence and charac-
teristics are at the root of the most extreme
precipitation and flooding in areas where
these features encounter mountains. At the
same time, ARs make important contribu-
tions to how much snow and water will be
available in these regions. Thus, understand-
ing their behavior may be the key to deter-
mining how changing climate patterns influ-
ence extreme precipitation and floods. Over-
all, the need to understand ARs opens up a
new set of grand challenges for water cycle,
water supply, and flood prediction science.
Observations of Atmospheric Rivers
Zhu and Newell [1998] helped coin
the term “atmospheric river” based on its
narrow ness and importance to the water
cycle. They found that at any given time, an
average of more than 90% of the total pole-
ward atmospheric water vapor transport
through the middle latitudes is concentrated
in four to five narrow regions that total less
than 10% of the circumference of the Earth
at that latitude. These features are gener-
ally located in the warm sectors of midlati-
tude cyclones, ahead of cold fronts. They
continually form, move, and evolve with
storms in the midlatitude storm tracks, some-
times drawing tropical water vapor and heat
directly into the middle latitudes [e.g., Stohl
etal., 2008; Ralph etal., 2011].
Since the seminal work of Zhu and Newell
[1998], the prevalence and role of ARs in the
water cycle and in continental weather have
become ever more clear, partly because of
the advent of microwave remote sensing
from polar- orbiting satellites, especially the
Special Sensor Microwave Imager (SSM/I),
which provides frequent global measure -
ments of IWV over the Earth’s oceans, mea-
surements that previously were available
above only the relatively few sites where
weather balloons and related instruments
were deployed. The imager works very well
over oceans and, since its spatiotemporal
coverage became adequate in about 1998,
has focused growing attention on ARs (see
Figure1a) in ways that previous water vapor
data could not.
In the years since then, a growing number
of field experiments and related studies have
PAGES 265–266
Storms, Floods, and the Science
of Atmospheric Rivers
BY F. M. RALPH AND M. D. DETTINGER
Fig. 1. Analysis of an atmospheric river (AR) that hit California on 13–14October 2009. (a)A
Special Sensor Microwave Imager (SSM/I) satellite image from 13–14October showing the AR
hitting the California coast; color bar shows, in centimeters, the amount of water vapor present
throughout the air column at any given point if all the water vapor were condensed into one
layer of liquid (vertically integrated water vapor). (b)Rain gage data for 12:00 UTC on 14Octo-
ber 2009 showing the total amount of precipitation (in inches) that occurred over the previous
24hours. (c)Discharge for Nacimiento River (site indicated by red triangles in other panels);
data are from U.S. Geological Survey stream gage 11148900. (d)Statewide streamflow historical
ranking of 14October 2009, compared to discharges on the same day of the year recorded by
gages with more than 30years of data.

Eos, Vol. 92, No. 32, 9 August 2011
explored the physical characteristics and
effects of ARs, focused mostly over the east-
ern Pacific Ocean and western North Amer-
ica (a bibliography of AR- related research
papers and many additional resources on
ARs are available at http:// www .esrl .noaa
.gov/ psd/ atmrivers/). Research aircraft obser-
vations in two ARs above the eastern North
Pacific in the winter s of 1998 and 2005 [Ralph
etal., 2005, 2011] showed that they trans-
ported water vapor at about 13–26cubic
kilometers per day, a rate equivalent to 7.5–
15times the average daily discharge of the
Mississippi River into the Gulf of Mexico.
A Closer Look at Rainfall
From Atmospheric Rivers
Because ARs transport so much water
vapor, they represent a significant source of
precipitation to coastal regions. For exam-
ple, a recent numerical model study [Smith
etal., 2010] estimated that roughly 20 –40%
of the water vapor transported a shore by
an AR crossing over northern California
was rained out there. This rainout happens
because when ARs make landfall on the
West Coast of North America (as well as on
other continents [e.g., Stohl etal., 2008]),
they are forced up and over coastal moun-
tains, where they cool and condense large
part s of their heavy burden of vapor [e.g.,
Neiman etal., 2008; Leung and Qian, 2009].
In a recent example, an AR event pro-
duced more than 410 millimeters (16.5
inches) of rainfall at one site in coa stal Cal-
ifornia on 14–15October 2009 (Figure 1).
This particular AR had a very long fetch,
spanning most of the North Pacific (Fig-
ure1a), and upon making landfall depos-
ited more than 200 millimeters of rain along
a swath of coastal California several hun-
dred kilometers wide (Figure1b). Significant
streamflow resulted, including a 5- meter rise
in water level on the Nacimiento River over
12hours (Figure1c), with the flows crest-
ing at 525 cubic meters per second (18,600
cubic feet per second). Record- high daily
streamflows (for that date of year) were also
observed at many other stations in central
and northern California (Figure 1d).
It should be noted that this peak flow of
the Nacimiento River exceeded the annual
peak flow in 28 of the past 40years and did
so in spite of the very dry conditions preced-
ing this storm. This event exhibits key attri-
butes found in other extreme ARs [e.g., Nei-
man etal., 2008; Ralph etal., 2011], including
very large IWV values, indications of entrain-
ment of tropical water vapor (from the west-
ern Pacific in this case, incorporating rem-
nants of a western Pacific typhoon), and the
fact that it stalled over parts of the West Coast
in ways that amplified the storm’s impacts.
Historically, AR storms have been the
sources of many (and, in some areas, most)
floods in the Pacific coast states. For exam-
ple, all storms that resulted in declared
flood conditions on the Russian River of cen-
tral California from 1998 to 2005 arose from
landfalling ARs [Ralph etal., 2006]; similar
relations appear to exist between ARs and
major flooding in most rivers from Califor-
nia to Washington State. In addition to their
roles as producers of extreme storms and
flood hazards, it is important to mention that
ARs also are major contributors to western
(especially California) water supplies [Det-
tinger etal., 2011; Guan etal., 2010]. Indeed,
the half dozen or so ARs per year that make
landfall in California have contributed an
average of one third to one half of all the
state’s precipitation, with a single typical AR
storm yielding perhaps 2.5 –5 cubic kilome-
ters of precipitation, or roughly 10% of the
annual average runoff of the entire Sacra-
mento River basin.
Studies of Atmospheric Rivers
From the West Coast
The dual roles of ARs as hazards and
water resources in many coastal regions may
become a more pressing issue under anthro-
pogenic climate change, which may alter
both hazardous and productive aspects of
these storms [Dettinger, 2011]. For example, in
view of the havoc that these storms wreak on
the Pacific coast states, understanding and
predicting them on time scales from days to
decades, and at spatial scales from mountain
ranges like the Sierra Nevadas and Cascades
to indiv idual river basins, present major chal-
lenges for West Coast meteorologists, clima-
tologists, and hydrologists. Although research
to address the roles of ARs elsewhere is
mostly just beginning, AR research has been
vigorous and productive on the West Coast
for more than a decade.
Over the past decade several studies led by
the National Oceanic and Atmospheric Admin-
istration (NOAA) (see http:// hmt .noaa .gov/)
have explored the inner workings of ARs and
the effects they produce, through intense field
campaigns and the use of new meteorologi-
cal and hydrometeorological sensors includ-
ing radar and sounding assets, research air-
craft, and other remote sensing tools as well
as numerical models. As understanding of the
scales and impacts of ARs has grown, scien-
tific efforts have expanded to include other
agencies on federal, state, and local levels,
including the U.S. Geological Survey (USGS),
the U.S. Army Corps of Engineers, NASA, Cali-
fornia’s Department of Water Resources and
the California Energy Commission, and local
agencies around San Francisco Bay and in
fire- scarred areas of southern California. By
now, a wide- ranging collection of studies are
currently under way on the West Coast to elu-
cidate various aspects of AR phenomena and
their impacts on the West Coast.
One study is the Hydrometeorolog y
Test bed - West (HMT- West), led by the Phys-
ical Sciences Division of NOAA’s Earth
System Research L aboratory. HMT- West
includes long- term geographically focused
field research, as well as innovative moni-
toring and modeling to improve scientific
understanding and short- term prediction
of extreme precipitation events and flood-
ing associated with ARs. Efforts have been
focused around the Russian and American
river basins of central California.
Another is the Enhanced Flood Response
and Emergency Preparednes s (EFREP)
program led by California’s Department of
Wate r Re sour ces, NOA A, and Scripps Inst itu-
tion of Oceanography. EFREP seeks to fos-
ter development and deployment of state-
wide monitoring, modeling, and decision
support programs that make key findings
from HMT- West ope rati onal, for better dete c-
tion, monitoring, and prediction of ARs and
their impact s. A key component is a “picket
fence” of four evenly spaced coa stal obser-
vatories to monitor ARs, statewide observa-
tional networks of soil moisture and IWV,
and 10snow- level radars, all with associated
decision support capabilities.
The CalWater project, led by the Califor-
nia Energy Commission, NOAA, and Scripps,
completed a major field campaign last win-
ter that is providing data for research initia-
tives to address details of the interactions of
ARs with topography, aerosols, and air pollu-
tion. Another goal is to critically assess ARs
in climate models to quantify several key
uncertainties in climate projections of pre-
cipitation for California [Dettinger, 2011].
A major project led by the USGS Multi-
Hazards Demonstration Project, called
ARkStorm, has developed a storm emergency
scenario being used in hazards assessments
and activities aimed at improving emergency
preparedness and public awareness of the
potential for catastrophic AR storms in Cali-
fornia. Their scenario, based on the most
recent AR science, rivals the largest storms
and floods in California’s history and allows
researchers to explore possible responses to
historic levels of flooding, landslides, wind
damage, water pollution, and attendant infra-
structure and economic disruptions.
Outside of California, in 2009 a major
storm damaged an Army Corps of Engi-
neers dam near Seattle that protects a heav-
ily developed area from flooding. After that,
dam operators could not use the full flood
storage capacity of the reservoir (although
repairs have now restored much of this
capacity). To mitigate the risk of flood dam-
ages from 2009 to 2011, NOAA and the
Corps applied concepts and tools for bet-
ter monitoring of ARs that had been devel-
oped in California. AR-related obser vations
were deployed to Washington State to pro -
vide actionable information on AR storms
approaching the area above the dam.
Finally, the Winter Storms and Pacific
Atmospheric Rivers (WISPAR) project, jointly
led by NOAA, NASA, and Nor throp Grum-
man, performed field experiments in early
2011 using a high- altitude, long- endurance
drone aircraft, the Global Hawk, to make off-
shore observations of several ARs over the
Pacific Ocean. The field campaign included
deploying a newly developed dropsonde s ys-
tem to document in detail the structure of
water vapor transport in ARs.

Eos, Vol. 92, No. 32, 9 August 2011
Details of these and other investigations
are available at http:// www .esrl .noaa .gov/
psd/ atmrivers/.
A Scientific Challenge
As is illustrated here, and as was high-
lighted in a special session on ARs at the
2010 AGU Fall Meeting (see http:// hmt .noaa
.gov/ news/ 2011/ 012511 .html), ARs have
become a focus of research and devel-
opment aimed at better physical under-
standing, monitoring, short- term forecasts
and warnings, and climate projections.
Recognizing ARs is key to forecasting
extreme precipitation and flooding in the
Pacific coast states and is now included
in forecaster training by NOA A and other
agencies.
Because of the vast amounts of water that
they transport and deliver, ARs are proba-
bly just as important in many other regions
of the globe where they have been less stud-
ied. For example, recent flood catastrophes
in Nashville, Tenn. (May 2010), and North and
South Carolina (October 2010) were associ-
ated with ARs making landfall from over the
Gulf of Mexico and Atlantic. The central role
of ARs in water cycle dynamics outside the
tropics, increasing pressure on limited water
resources, and changing exposures to flood
risks due to development and climate changes
all demand improved scientific understanding
and forecasts of ARs. Providing those improve-
ments makes ARs a grand challenge for water
cycle science, with important implications for
flooding and water supply.
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Author Information
F. M. Ralph, Physical Sciences Division, Earth Sys-
tem Research Laboratory, NOAA, Boulder, Colo.;
E- mail: marty.ralph@ noaa .gov; and M.D. Dettinger,
USGS and Scripps Institution of Oceanography,
LaJolla, Calif.
A scientist works late to finish up yet
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To help scientists, research programs,
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New Education and Outreach Opportunities
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