Microwave satellite water vapor image showing an atmospheric river making landfall on the U. S. West coast on February 5,
2015, interacting with mountains and a mid-latitude cyclone. Approximate locations of tropical moisture exports and a warm
conveyor belt (WCB) feeding moisture into the cyclone are marked. From Dettinger, M.D., F.M. Ralph, and D. Lavers, 2015:
Setting the stage for a global science of atmospheric rivers, Eos, 96, doi:10.1029/2015EO038675.
www.ca-nv-awwa.org 27
THE TERM ATMOSPHERIC RIVER
(AR) was coined at the Massachu-setts
Institute of Technology in the
1990s to describe corridors of concentrat-ed
water vapor transport that occur in
the earth’s atmosphere. At least 2,000 km
(~1,200 miles) long and roughly 600-800
km (400-500 miles) wide, ARs cover less
than 10 percent of the earth’s circumfer-ence
but typically account for 90 percent
of atmospheric water vapor transport
(Zhu and Newell, 1998). Three to five of
these narrow plumes are present within a
hemisphere at any given time.
ARs have a central role in the global
water cycle and are a cause of extreme
precipitation events in western coastal
regions of North America, Europe, and
elsewhere. Significantly, 30 to 50 percent
of annual precipitation on the West Coast
of the United States occurs in just a hand-ful
of AR events (Dettinger et al 2011,
Ralph et al 2013).
An average AR transports as water va-por
the equivalent of the discharge of the
Mississippi River as liquid, or 20 million
acre-feet in a day. In some of the most
extreme ARs, the water vapor transport
is enhanced by the fact that they entrain
(draw in) water vapor directly from the
tropics (Bao et al 2006, Ralph et al. 2011).
It is now recognized, for example, that the
well-known “pineapple express” storms
(a term that has been used on the U.S.
West Coast for many years) correspond
to a subset of ARs that have a connection
to the tropics near Hawaii.
California exhibits the greatest year-to-
year precipitation variability in the
country. This variability is caused by the
presence, or absence, of a few AR-related
large storms. The good news is that in a
large fraction of the West Coast, ARs pro-vide
the bang that breaks droughts, espe-cially
in winter. The effect of these infre-quent
but large events is being quantified
for the California Department of Water
Resources (DWR), using the Northern Si-erra
Eight Station Index, which measures
precipitation in the northern Sierra Neva-da.
(Ralph et al. 2016) One goal is to iden-tify
the top 10 precipitation events in a
given water year. In water year 2016, over
50 percent of total precipitation was due
to that year’s top 10 events, each of which
was associated with an atmospheric river
that made landfall in northern California.
(The summary for the 2016 water year is
available at cw3e.ucsd.edu/?p=5662).
Formation and Life Cycle
Although atmospheric rivers are im-portant
features in global water and heat
cycles, the mechanisms behind their for-mation
and life cycle remain an active
topic of research. ARs are generally lo-cated
in the warm sectors of mid-latitude
cyclones but can stretch across an entire
ocean basin. ARs do not cause cyclones
but typically occur in conjunction with
cyclones although not always. In addi-tion
to water vapor swept up by the cy-clone’s
cold front, the source of an AR’s
high concentration can include long-dis-tance
water vapor transport originating
in the tropics and collection of other at-mospheric
moisture along its path (Ralph
et al. 2011, Dacre et al. 2015).
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