presntation_schick_2010 atmospheric rivers
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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, CO2University of Washington, Seattle, WA
3U.S. Geological Survey, Scripps Institution of Oceanography, La Jolla, CA4U.S. Army Corp of Engineers, Seattle, WA
5National Center for Atmospheric Research, Boulder, CO6Oregon Climate Service, Oregon State University, Corvallis, OR
Presented at:The 2010 Mountain Climate Research Conference
Blue River, Oregon. 8 June 2010
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Outline
1. Brief Review of ARs2. ARs as extreme weather events
3. Long-term impacts of ARs
4. Concluding RemarksA few acronym definitions:AR = atmospheric river IWV = integrated water vaporLLJ = low-level jet MSL = above mean sea levelSWE = snow water equivalent APDF = annual peak daily flow
NARR = North American regional reanalysis
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.
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Zhu & Newell (1998) concluded in a 3-year ECMWF model diagnostic study:1) 95% of meridional water vapor flux occurs in narrow plumes in <10% of zonal circumference.2) There are typically 3-5 of these narrow plumes within a hemisphere at any one moment.3) They coined the term “atmospheric river” (AR) to reflect the narrow character of plumes.4) ARs constitute the moisture component of an extratropical cyclone’s warm conveyor belt.
5) ARs are very important from a global water cycle perspective.
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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.
IWV > 2 cmAtmos. river
cold air
warm air
400 km
Enhanced vapor fluxin Atmos. river
coldair
warmair
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Global reanalysis melting-levelanomaly (hPa; rel. to 30-y mean)
Melting level ~4000 ft (1.2 km) abovenormal across much of the PacNW
during the landfall of this AR
Pacific Northwest Landfalling AR of early November 2006Neiman et al. (2008a)
~30”rain
SSM/I satellite imageryof integrated water vapor (IWV, cm)
This AR is also located near the leadingedge of a cold front, with strong vaporfluxes (as per reanalysis diagnostics)
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Hydroclimatic analysis for the AR of 5-9 November 2006
Greatest 3-day precip.
totals during the periodbetween 5-9 Nov. 2006
>700 mm (28”)
>600 mm (24”)
Historical Nov. ranking for
the max. daily streamflowbetween 5-9 Nov. 2006
plus highmelting level
equals
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Courtesy of Doug Jones , Mt Hood NF
Aftermath of flooding and a debris flow on the White River Bridge in Oregon
High-Impact Consequences!
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• SSM/I satellite image shows AR.
• Frontal wave stalls AR on coast,causing prolonged heavy rains.• Stream gauge rankings for 17-
Feb-04 show regional extent ofhigh streamflow covering ~500 kmof coast.
• All 7 flood events on the RussianRiver between 1997-2006 weretied to land-falling ARs.
Russian River, CA Flooding250 mm of rain in 2 days
(Ralph et al. 2006)
Russian River flooding: Feb. 2004photo courtesy of David Kingsmill
AR stalls:heavy rains& flooding
Frontalwave
inflection
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Given these results: What are the long-term hydrometeorological impacts oflandfalling ARs in western North America? Neiman et al. (2008b)
SSM/I Integrated water vapor (cm)
16-Feb-04;
p.m. comp.
IWV >2cm:<1000 km wide
IWV >2cm:>2000 km long
• 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
• Focus on cool season when most
precip falls in western U.S., and onthe north-coast domain
1000 km
Approach: We developed a methodology for creating a multi-year AR inventory.
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• 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 easternPacific 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 Reanalyses – focus on North Coast Winter
IWV (cm) Daily rain (mm)Composite mean SSM/I axes
W i n t e r ( D J F )
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• Strong vapor transport intersects coastline during winter, withmaximum on the warm side of the cold front.
• Transport originating from low latitudes
Composite Mean Reanalysis IVT (kg s-1 m-1) – North Coast winter
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• Wintertime ARs are associatedwith trough/ridge couplet in the
mid-troposphere (~2-6 km MSL).
Composite Reanalysis Fields– North Coast WinterNorth-coast
5 0 0 h P a Z M e a n
( m )
9 2 5 h P a T A n o
m a l y ( C )
Wintertime ARs are associated withanomalous warmth at low levels.
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Composite Winter Reanalysis Soundings at North Coast
Flow strengthenswith height...
...in pre-cold-frontalwarm-advection shear
Moisture decreaseswith height
Mountain top height
Max. moisture flux atmtn top... favors mtnprecip. enhancement.
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1.8x the average snow accumulation
Sierras (119 sites)
snow pillow sites>1.5 km MSL
(~5000 ft)
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 withlandfalling Atmospheric Rivers produced:
2.0x the average precipitation
*Qualitatively similar to, but quicker to explain than, north-coast results.
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Seattle
HansonDam
Sauk
Queets
Satsop
Now let’s turn the problem on its head: What causes the largest annualrunoffs on major watersheds in western Washington? (Neiman et al. 2010)
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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]
APDF dates for WY 1998-2009
Green River Sauk River
Satsop River Queets River
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 instreamflow that are an order of magnitude larger than those from non-AR storms
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Ranked APDFs for WY1980-2009 (NARR period)
Green River Sauk River
Satsop River Queets River
The APDFs occur most often Nov. - Jan.
T
o p
1 0
T
o p
1 0
Dates from the top-10 APDFs are used to create composite analyses from theNorth American Regional Reanalysis (NARR; Mesinger et al. 2006) to assess the
composite meteorological conditions that produced flooding in each of the 4 basins
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Basin altitude attributes above gauges, and mean NARR top-10 melting-level altitudes**(300 m below 0°C altitude)
snow
rain
snowrain
snow
rain
snow
rain
mean mean
mean mean
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Anomalously high melting levels + heavy precip = floods
NARR Composite Mean 2-day Precipitation (mm) for top-10 APDFs
(a) Green (b) Sauk
(c) Satsop (d) Queets
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NARR Composite Mean Integrated Vapor Transports (kg s-1 m-1) for top-10 APDFs
(a) HHDW1 (b) SAKW1
(c) SATW1 (d) QUEW1
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NARR Composite Mean Geopotential Heights (m) at 925 hPa for top-10 APDFs
(a) Green (b) Sauk
(c) Satsop (d) Queets
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Seattle
Green
Queets
Sauk
Satsop
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NARR Composite Mean profiles at coast for top-10 APDFs
weakstability
weakstability
Max. orographic forcingin AR conditions at
coast. Minimal terrainlift would place it inweakest stability –
optimal for heavy precip!
Green
Queets
SatsopSauk
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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 anomalouslywarm and moist with weak static stability and strong onshore flow, resulting inorographically enhanced precipitation, high melting levels, and flooding.
Because ARs contribute significantly to precipitation, reservoir and snowpackreplenishment, and flooding in western North America, they represent a keyphenomenon linking weather and climate.
The highly 3-D character of the terrain in western Washington yields basin-specific impacts arising from landfalling ARs (i.e., strong dependence on flowdirection).
Next steps include quantifying the role of ARs in the global climate system andestimating the modulation of AR frequency and amplitude (and associatedextreme precipitation and flooding upon landfall) due to projected climatechange. Mike Dettinger is delving into this research (Dettinger et al. 2009).
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Thank you!Crater Lake from Watchman Peak (©2009 Paul Neiman)
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References
Dettinger, M.D., 2004. Fifty-two years of “pineapple-express” storms across the west coast of North America. U.S.
Geological Survey, Scripps Institution of Oceanography for the California Energy Commission, PIER Energy-Related
Environmental Research. CEC-500-2005-004, http://www.energy.ca.gov/2005publications/CEC-500-2005-004/CEC-500-
2005-004.PDF, 15 p.
Dettinger, M.D., H. Hidalgo, T. Das, D. Cayan, and N. Knowles, 2009: Projections of potential flood regime changes inCalifornia: California Energy Commission Report CEC-500-2009-050-D, 68 p.
http://www.energy.ca.gov/2009publications/CEC-500-2009-050/CEC-500-2009-050-D.PDF.
Kalnay, E., and Coauthors, 1996: The NCEP/NCAR 40-year reanalysis project. Bull. Amer. Meteor. Soc., 77, 437-471.
Mesinger, F., and Coauthors, 2006: North American Regional Reanalysis. Bull. Amer. Meteor. Soc., 87, 343-360.
Neiman, P.J., F.M. Ralph, G.A. Wick, Y.-H. Kuo, T.-K. Wee, Z. Ma, G.H. Taylor, and M.D. Dettinger, 2008a: Diagnosis of an
intense atmospheric river impacting the Pacific Northwest: Storm summary and offshore vertical structure observed with
COSMIC satellite retrievals. Mon. Wea. Rev., 136, 4398-4420.
Neiman, P.J., F.M. Ralph, G.A. Wick, J. Lundquist, and M.D. Dettinger, 2008b: Meteorological characteristics and overland
precipitation impacts of atmospheric rivers affecting the West Coast of North America based on eight years of SSM/I
satellite observations. J. Hydrometeor ., 9, 22-47.
Neiman, P.J., L.J. Schick, P.J. Neiman, F.M. Ralph, M. Hughes, and G.A. Wick, 2010: Flooding in western Washington: The
connection to atmospheric rivers. J. Hydrometeor ., 11, to be submitted.
Ralph, F.M., P.J. Neiman, and G.A. Wick, 2004: Satellite and CALJET aircraft observations of atmospheric rivers over the
eastern North-Pacific Ocean during the winter of 1997/98. Mon. Wea. Rev.,132
, 1721-1745.Ralph, F.M., P.J. Neiman, and R. Rotunno, 2005: Dropsonde Observations in Low-Level Jets Over the Northeastern Pacific
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Ralph, F.M., P.J. Neiman, G.A. Wick, S.I. Gutman, M.D. Dettinger, D.R. Cayan, and A.B. White 2006: Flooding on
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