Landfalling Impacts of Atmospheric Rivers:
From Extreme Events to Long-term Consequences
Paul J. Neiman1, F.M. Ralph1, G.A. Wick1, M. Hughes1,
J. D. Lundquist2, M.D. Dettinger3, D.R. Cayan3, L.W. Schick4,
Y.-H. Kuo5, R. Rotunno5, G.H. Taylor6
1NOAA/Earth
System Research Lab./Physical Sciences Div., Boulder, CO
2University of Washington, Seattle, WA
3U.S. Geological Survey, Scripps Institution of Oceanography, La Jolla, CA
4U.S. Army Corp of Engineers, Seattle, WA
5National Center for Atmospheric Research, Boulder, CO
6Oregon Climate Service, Oregon State University, Corvallis, OR
Presented at:
The 2010 Mountain Climate Research Conference
Blue River, Oregon. 8 June 2010
Motivation: Atmospheric rivers (ARs) generate devastating floods, and also
replenish snowpacks and reservoirs, across the semi-arid West. Hence, it
is crucial to understand this key phenomenon, both as a major weather
producer and as one that contributes significantly to climate-scale impacts.
Outline
1.
2.
3.
4.
Brief Review of ARs
ARs as extreme weather events
Long-term impacts of ARs
Concluding Remarks
A few acronym definitions:
AR = atmospheric river
IWV = integrated water vapor
LLJ = low-level jet
MSL = above mean sea level
SWE = snow water equivalent
APDF = annual peak daily flow
NARR = North American regional reanalysis
Zhu & Newell (1998) concluded in a 3-year ECMWF model diagnostic study:
1)
2)
3)
4)
5)
95% of meridional water vapor flux occurs in narrow plumes in <10% of zonal circumference.
There are typically 3-5 of these narrow plumes within a hemisphere at any one moment.
They coined the term “atmospheric river” (AR) to reflect the narrow character of plumes.
ARs constitute the moisture component of an extratropical cyclone’s warm conveyor belt.
ARs are very important from a global water cycle perspective.
Observational studies by Ralph et al. (2004, 2005, 2006) extend model results:
1) Long, narrow plumes of IWV >2 cm measured by SSM/I satellites considered proxies for ARs.
2) These plumes (darker green) are typically situated near the leading edge of polar cold fronts.
3) P-3 aircraft documented strong water vapor flux in a narrow (400 km-wide) AR; See section AA’.
4) Airborne data also showed 75% of the vapor flux was below 2.5 km MSL in vicinity of LLJ.
5) Moist-neutral stratification <2.8 km MSL, conducive to orographic precip. boost & floods.
cold air
Enhanced vapor flux
in Atmos. river
warm air
IWV > 2 cm
Atmos. river
cold
air
warm
air
400 km
Pacific Northwest Landfalling AR of early November 2006
Neiman et al. (2008a)
SSM/I satellite imagery
of integrated water vapor (IWV, cm)
Global reanalysis melting-level
anomaly (hPa; rel. to 30-y mean)
~30”
rain
This AR is also located near the leading
edge of a cold front, with strong vapor
fluxes (as per reanalysis diagnostics)
Melting level ~4000 ft (1.2 km) above
normal across much of the PacNW
during the landfall of this AR
Hydroclimatic analysis for the AR of 5-9 November 2006
Greatest 3-day precip.
totals during the period
between 5-9 Nov. 2006
Historical Nov. ranking for
the max. daily streamflow
between 5-9 Nov. 2006
plus high
melting level
equals
>600 mm (24”)
>700 mm (28”)
High-Impact Consequences!
Aftermath of flooding and a debris flow on the White River Bridge in Oregon
Courtesy of Doug Jones , Mt Hood NF
Russian River, CA Flooding
250 mm of rain in 2 days
(Ralph et al. 2006)
Frontal
wave
inflection
AR stalls:
heavy rains
& flooding
• SSM/I satellite image shows AR.
• Frontal wave stalls AR on coast,
causing prolonged heavy rains.
• Stream gauge rankings for 17Feb-04 show regional extent of
high streamflow covering ~500 km
of coast.
• All 7 flood events on the Russian
River between 1997-2006 were
tied to land-falling ARs.
Russian River flooding: Feb. 2004
photo courtesy of David Kingsmill
Given these results: What are the long-term hydrometeorological impacts of
landfalling ARs in western North America?
Neiman et al. (2008b)
Approach: We developed a methodology for creating a multi-year AR inventory.
16-Feb-04;
p.m. comp.
• Inspect 2x-daily SSM/I IWV
satellite composite images
• 8 water years Oct97-Sep05:
• Identify IWV plumes >2 cm (0.8”):
>2000 km long by <1000 km wide.
• AR landfall at north- or south-coast
IWV >2cm:
>2000 km long
• Focus on cool season when most
precip falls in western U.S., and on
the north-coast domain
IWV >2cm:
<1000 km wide
1000 km
SSM/I Integrated water vapor (cm)
Composite Mean Reanalyses – focus on North Coast Winter
Composite mean SSM/I axes
Daily rain (mm)
Winter (DJF)
IWV (cm)
• The daily gridded NCEP–NCAR reanalysis dataset (2.5 x 2.5 ; Kalnay et al.
1996) was used to create composite analyses during AR conditions – 29 dates.
• Composite reanalysis IWV plume oriented SW-NE from the tropical eastern
Pacific to the coast.
• Composite plume situated ahead of the polar cold front.
• Wintertime ARs produce copious precip along coast, & frontal precip offshore.
• Reanalysis composites accurately depict the positions of the IWV plume and
precip. bands observed by the SSM/I composites... denoted by dotted lines.
Composite Mean Reanalysis IVT (kg s-1 m-1) – North Coast winter
• Strong vapor transport intersects coastline during winter, with
maximum on the warm side of the cold front.
• Transport originating from low latitudes
Composite Reanalysis Fields– North Coast Winter
925 hPa T Anomaly ( C)
500 hPa Z Mean (m)
North-coast
• Wintertime ARs are associated
with trough/ridge couplet in the
mid-troposphere (~2-6 km MSL).
Wintertime ARs are associated with
anomalous warmth at low levels.
Composite Winter Reanalysis Soundings at North Coast
Flow strengthens
with height...
...in pre-cold-frontal
warm-advection shear
Max. moisture flux at
mtn top... favors mtn
precip. enhancement.
Moisture decreases
with height
Mountain top height
Normalized Daily Precipitation and ΔSWE in CA* during DJF
Compared to the average of all precipitation days in the Sierra Nevada
(observed by rain gauges and snow pillows), those days associated with
landfalling Atmospheric Rivers produced:
2.0x the average precipitation
1.8x the average snow accumulation
snow pillow sites
>1.5 km MSL
(~5000 ft)
Sierras (119 sites)
*Qualitatively similar to, but quicker to explain than, north-coast results.
Now let’s turn the problem on its head: What causes the largest annual
runoffs on major watersheds in western Washington? (Neiman et al. 2010)
Sauk
Seattle
Queets
Hanson
Dam
Satsop
Annual peak daily flows (APDFs) and atmospheric river (AR) events for WY1998-2009
AR
non-AR
[determined from 2x-daily SSM/I IWV satellite imagery]
Green River
Sauk River
Satsop River
Queets River
APDF dates for WY 1998-2009
46 of 48 annual peak daily flows in last 12 years at the 4 sites due to AR landfalls
Results consistent with Dettinger (2004) in CA: ARs yield daily increases in
streamflow that are an order of magnitude larger than those from non-AR storms
Green River
Top 10
Top 10
Ranked APDFs for WY1980-2009 (NARR period)
Satsop River
Sauk River
Queets River
The APDFs occur most often Nov. - Jan.
Dates from the top-10 APDFs are used to create composite analyses from the
North American Regional Reanalysis (NARR; Mesinger et al. 2006) to assess the
composite meteorological conditions that produced flooding in each of the 4 basins
Basin altitude attributes above gauges, and mean NARR top-10 melting-level altitudes*
*(300 m below 0°C altitude)
snow
snow
rain
rain
mean
mean
snow
snow
rain
rain
mean
mean
NARR Composite Mean 2-day Precipitation (mm) for top-10 APDFs
(a) Green
(b) Sauk
(c) Satsop
(d) Queets
Anomalously high melting levels + heavy precip = floods
NARR Composite Mean Integrated Vapor Transports (kg s-1 m-1) for top-10 APDFs
(a) HHDW1
(b) SAKW1
(c) SATW1
(d) QUEW1
NARR Composite Mean Geopotential Heights (m) at 925 hPa for top-10 APDFs
(a) Green
(b) Sauk
(c) Satsop
(d) Queets
Sauk
Seattle
Queets
Green
Satsop
NARR Composite Mean profiles at coast for top-10 APDFs
Green
Queets
Satsop
Sauk
weak
stability
weak
stability
Max. orographic forcing
in AR conditions at
coast. Minimal terrain
lift would place it in
weakest stability –
optimal for heavy precip!
Concluding Remarks
 Atmospheric rivers (ARs) are long, narrow corridors of enhanced water vapor
transport responsible for most of the poleward vapor flux at midlatitudes.
 Lower-tropospheric conditions during the landfall of ARs are anomalously
warm and moist with weak static stability and strong onshore flow, resulting in
orographically enhanced precipitation, high melting levels, and flooding.
 Because ARs contribute significantly to precipitation, reservoir and snowpack
replenishment, and flooding in western North America, they represent a key
phenomenon linking weather and climate.
 The highly 3-D character of the terrain in western Washington yields basinspecific impacts arising from landfalling ARs (i.e., strong dependence on flow
direction).
 Next steps include quantifying the role of ARs in the global climate system and
estimating the modulation of AR frequency and amplitude (and associated
extreme precipitation and flooding upon landfall) due to projected climate
change. Mike Dettinger is delving into this research (Dettinger et al. 2009).
Thank you!
Crater Lake from Watchman Peak (©2009 Paul Neiman)
References
Dettinger, M.D., 2004. Fifty-two years of “pineapple-express” storms across the west coast of North America. U.S.
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Environmental Research. CEC-500-2005-004, http://www.energy.ca.gov/2005publications/CEC-500-2005-004/CEC-5002005-004.PDF, 15 p.
Dettinger, M.D., H. Hidalgo, T. Das, D. Cayan, and N. Knowles, 2009: Projections of potential flood regime changes in
California:
California
Energy
Commission
Report
CEC-500-2009-050-D,
68
p.
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