Atmospheric Rivers and Extreme Winter Precipitation Events in the
Southwestern United States
Erick R. Rivera1*, Francina Dominguez1,2, and Christopher L. Castro1
1. Atmospheric Sciences, The University of Arizona, Tucson, AZ
2. Hydrology and Water Resources, The University of Arizona, Tucson, AZ
* E-mail: [email protected]
Method: We use a “bottom-up” approach by first evaluating extreme 3-day precipitation in the Verde River
Basin (VRB), then identifying the two most important AR patterns (referred to as Type 1 and Type 2,
respectively) using an extended EOF statistical analysis.
Science Question: What is the relationship between extreme winter precipitation in the Southwestern US and
penetrating atmospheric rivers (ARs)?
We performed a climatological characterization of atmospheric rivers (ARs) that
affect the Southwestern US and their role in generating extreme cool
precipitation in the Verde Rivera Basin (VRB) in Arizona for the period 19792011.
Atmospheric rivers (ARs) are filamentary water vapor plumes that cover about 10% of the
globe and are responsible for most of the meridional water vapor transport observed in the
extratropical atmosphere (Zhu and Newell 1998). These features are located in the warm
sector of major extratropical cyclones where a pre-cold front low-level jet (LLJ) is present
(Ralph et al. 2006).
Figure 1: Example of an AR event that produced extreme precipitation along the US
west coast, and exhibited spatial continuity with the tropical water vapor reservoir as
seen in SSM/I satellite observations of IWV. Taken from Ralph et al. (2011).
The VRB is strategically located to allow a virtually uninterrupted passage of
moisture from the ocean. In some cases, strong water vapor influx associated
to ARs and orographic lift combine to produce extreme precipitation episodes.
Example of Type 1 AR
Type 1 AR
In the US west coast,
orographically-enhanced cold
season extreme precipitation
events have been extensively
related to the occurrence of
landfalling ARs. However, the
effects of ARs that penetrate
further
inland
into
the
Southwest US are little
known.
We explored the connection between ARs and extreme winter
precipitation in the Verde River Basin (VRB) in the Southwestern
US. Our work had a “bottom-up” approach, as we first
identified extreme precipitation events in the VRB and then
evaluated the atmospheric conditions that lead to these
extreme events.
Figure 4: Composite (a) IVT (kg m-1 s-1), (b) cross section of water vapor flux (g kg-1
m s-1), and (c) precipitation (mm day-1) for the selected ten Type 1 ARs. The NWSE line from the Southwestern US to northwest Mexico shown in panel (a) is the
line for the cross section in panel (b).
Figure 2: Topographical map of the southwestern US with the Verde River Basin in central Arizona
delineated in blue.
We identified the two most important
AR patterns (referred to as Type 1
and 2, respectively) using an
extended EOF (EEOF) statistical
analysis. Results suggest that the
moisture
sources
for
these
Southwestern ARs are not always the
same. While Type 1 ARs form and
obtain a major part of their water
content in the midlatitudes, Type 2
ARs usually tap moisture from the
tropical reservoir in the eastern
Pacific.. Both types of ARs cross the
Peninsula of Baja California before
affecting the VRB.
Figure 3: Spatial structure of (a) EEOF 1, (b) EEOF 1 plus
the mean integrated vapor transport (IVT) field, (c) EEOF
2, and (d) EEOF 2 plus the mean IVT field for the 97
extreme cases (units of kg m-1 s-1). (e) Temporal behavior
of EEOFs 1 and 2 where the square and circle marks
indicate the selected cases for the composite analysis of
Type 1 and 2 Ars, respectively.
We analyzed two representative Type 1 and Type 2 AR events to further
illustrate the hydrometeorological impacts of the ARs in the Southwestern
US.
A composite analysis of Type 1
ARs suggests that the core of the
water vapor (40-50 m s-1) transport
into the VRB (111°W–113°W) is
concentrated approximately in the
first 1.5 km above the surface
(below 700 mb). We observe that
this mode resembles the long and
narrow water vapor corridor
associated with the landfalling ARs
that have been analyzed in
previous studies over the Western
US, specifically in California.
The interaction between the local
topography and Type 1 ARs can
lead to precipitation rates that can
exceed 30 mm/day in various
portions of the basin. In addition,
the coast of southern California
and the Sierra Nevada receive as
much as 50-60 mm/day during the
occurrence of these episodes.
Type 2 AR
The second AR pattern shows a
meridionally-oriented mode of water
vapor transport into several parts of
Arizona, New Mexico and southern
Utah. The composites for Type 2
ARs show very similar precipitation
intensities and spatial distributions
as compared to those of Type 1
ARs, except for the lower rates in
the Sierra Nevada. This is due to
the meridional orientation of Type 2
ARs.It is important to note that
this type of ARs had not been
identified in previous studies,
which focus on the western coast of
the US, and nonetheless can lead
to extreme precipitation events of
magnitudes similar to those that
occur in portions of California
during the cool season.
Figure 6: (a) SSM/I integrated water vapor (IWV) (cm), (b) 500-mb
geopotential height (gpm) and 850-mb winds (m s-1), (c) cross section of
water vapor transport (g kg-1 m s-1), and (d) 3-day accumulated precipitation
(mm) for Case 1.
A specific example of Type 2 AR
occurred during 19-21 February 1993
(Case 2). A 500-mb trough off the coast
of California and strong normallyoriented moisture transport from the
tropical eastern Pacific characterized
this event. Over the central north part of
the VRB, the 3-day precipitation
exceeded 100 mm and for other parts of
the basin the accumulations reached
40-50 mm. The White Horse Lake
SNOTEL station reported as much as
5.0 inches of snow during the period of
influence of the AR event. Peak
discharge at a gauging site in the lower
Verde River (Camp Verde) was 95 m3/s
(1.5 US million gallons per minute).
The extreme events in central Arizona
during 21-23 January 2010 (Case 1), are
a specific example of Type 1 ARs. SSM/I
data indicates that during these events,
the associated moisture corridor had
core IWV values up to about 4 cm.
Extreme 3-day precipitation associated
with the January 2010 events show
accumulations above 100 mm in the
eastern and southeastern parts of the
VRB and 3-day snow accumulations
greater than or equal to 5 inches. In the
lower Verde River, the mean discharge
measured during the day of most intense
precipitation was 227 m3/s (3.6 million
US gallons per minute).
Example of Type 2 AR
Figure 7: Same as Figure 6 but for Case 2.
Since ARs may account for a large percentage of winter precipitation in several watersheds
of the Southwestern US, we hypothesize that some of the increase in intensity of future
extreme events projected by regional climate models (Dominguez et al. 2012) may be due
to changes in the intensity of the impacting ARs. This will be the focus of future studies.
Acknowledgements: This work is supported by DOE (DE-SC0001172) and NSFEAR (1038938).
Figure 5: Same as Figure 4 but for Type 2 ARs.
Literature cited
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