Landfalling Impacts of Atmospheric Rivers: from Extreme Events to Long-Term Consequences

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. Brief Review of ARs 2. ARs as extreme weather events 3. Long-term impacts of ARs 4. 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) 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. 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

cold warm IWV > 2 cm air air Atmos. river

400 km Pacific Northwest Landfalling AR of early November 2006 Neiman et al. (2008a)

SSM/I satellite imagery Global reanalysis melting-level of integrated water vapor (IWV, cm) anomaly (hPa; rel. to 30-y mean)

~30” rain

This AR is also located near the leading Melting level ~4000 ft (1.2 km) above edge of a cold front, with strong vapor normal across much of the PacNW fluxes (as per reanalysis diagnostics) during the landfall of this AR Hydroclimatic analysis for the AR of 5-9 November 2006

Greatest 3-day precip. Historical Nov. ranking for totals during the period the max. daily streamflow between 5-9 Nov. 2006 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) • 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 of high streamflow covering ~500 km of coast. • All 7 flood events on the Russian River between 1997-2006 were AR stalls: tied to land-falling ARs. Frontal heavy rains wave & flooding inflection

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.

• Inspect 2x-daily SSM/I IWV 16-Feb-04; satellite composite images p.m. comp. • 8 water years Oct97-Sep05: • Identify IWV plumes >2 cm (0.8”): >2000 km long by <1000 km wide. IWV >2cm: • AR landfall at north- or south-coast >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

IWV (cm) Composite mean SSM/I axes Daily rain (mm) Winter (DJF)

• 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 North-coast • Wintertime ARs are associated with trough/ridge couplet in the mid-troposphere (~2-6 km MSL). 500 hPa Z Mean (m)

C ) Wintertime ARs are associated with anomalous warmth at low levels. 925 hPa T Anomaly Anomaly ( 925 hPa T Composite Winter Reanalysis Soundings at North Coast

...in pre-cold-frontal Flow strengthens warm-advection shear with height...

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 Ranked APDFs for WY1980-2009 (NARR period)

Green River Sauk River Top 10 Top Top 10 Top

Satsop 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

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

(a) Green (b) Sauk

Seattle

Queets

Green

Satsop NARR Composite Mean profiles at coast for top-10 APDFs

Green Queets Satsop Sauk

weak stability

weak Max. orographic forcing stability 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 basin- specific 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. 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 in California: 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 Ocean from CALJET-1998 and PACJET-2001: Mean Vertical-Profile and Atmospheric-River Characteristics. Mon. Wea. Rev., 133, 889-910. 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 California’s Russian River: The Role of Atmospheric Rivers. Geophys. Res. Lett., 33, L13801, doi:10.1029/2006GL026689. Zhu, Y., and R.E. Newell, 1998: A proposed algorithm for moisture fluxes from atmospheric rivers. Mon. Wea. Rev., 126, 725-735.