Dr. Marty Ralph NOAA/ESRL/Physical Sciences Division 22 March 2011

1. Physical understanding and forecasting of extreme precipitation events and flooding: Atmospheric Rivers Dr. Marty Ralph NOAA/ESRL/Physical Sciences Division 22 March 2011 WGA/WSWC/CDWR/ Workshop on Impacts of Climate Change on Extreme Events/Severe Weather

2. Background • Research over the last decade has revealed that atmospheric rivers (AR) play a major role in extreme precipitation events on the U.S. West Coast. • Some evidence is also emerging that ARs can also contribute to extreme events in the Intermountain West. • This briefing • reviews the physical characteristics of ARs and how they produce extreme precipitation, and • illustrates the challenge of predicting heavy precipitation • Later presentations at this Workshop consider • climate change impacts on ARs (Dettinger), • impacts of ARs on Flood protection planning (Anderson), and • observations of ARs (Reynolds)

3. Background - HMT • NOAA’s Hydrometeorology Testbed (HMT) • Connects researchers, forecasters and forecast users • Has been researching and developing prototypes on extreme precipitation in California since 2003 • Builds on earlier experiments from 1997-2002 • Lessons learned from HMT have been documented in over 50 formal peer-reviewed technical publications • http://hmt.noaa.gov/pubs/ http://hmt.noaa.gov

4. Atmospheric rivers: Two recent examples that produced extreme rainfall and flooding These color images represent satellite observations of atmospheric water vapor over the oceans. Warm colors = moist air Cool colors = dry air ARs can be detected with these data due to their distinctive spatial pattern. In the top panel, the AR hit central California and produced 18 inches of rain in 24 hours. In the bottom panel, the AR hit the Pacific Northwest and stalled, creating over 25 inches of rain in 3 days. From Ralph et al. 2011, Mon. Wea. Rev.

5. cold air Enhanced vapor flux in Atmos. river warm air Warm, Humid cold air IWV > 2 cm Atmos. river 400 km • Observational studies by Ralph et al. (2004, 2005, 2006) extend model results: • Long, narrow plumes of IWV >2 cm measured by SSM/I satellites considered proxies for ARs. • These plumes (darker green) are typically situated near the leading edge of polar cold fronts. • P-3 aircraft documented strong water vapor flux in a narrow (400 km-wide) AR; See section AA’. • Airborne data also showed 75% of the vapor flux was below 2.5 km MSL in vicinity of LLJ.

6. Observations of many atmospheric rivers were composited and define the average width and strength of atmospheric rivers (from Ralph et. al. 2004). 415 km 75% 190 km Rainrate: 75% in 150 km 75% The average width of an AR is roughly 400 km in terms of water vapor, and 150-200 km in terms of clouds and precipitation. This is important partly because it defines the spatial scales for which coastal monitoring is needed.

7. When atmospheric rivers strike coastal mountains (Ralph et al. 2003) • Details (e.g., wind direction) of the atmospheric river determine which watersheds flood

8. Thresholds in water vapor and wind are key in determining heavy hourly rainfall • The next 4 graphs each show 8 winters of hourly observations from an atmospheric river observatory near Bodega Bay operated in HMT. • Over 18,000 hourly measurements of • Water vapor • Winds at 1 km above sea level • Coastal mountain rainfall

9. Winters: 2001-2009; 18347 hourly data points All data points Component of the flow in the orographic controlling layer directed from 230°, i.e., orthogonal to the axis of the coastal mtns Neiman et al. (2008), Water Management

10. Winters: 2001-2009 Any rain: >0 m/s; >1 cm

11. Winters: 2001-2009 Rain >5 mm/h: >6 m/s; >1.5 cm

12. Winters: 2001-2009 Rain >10 mm/h: >12.5 m/s; >2 cm Atmospheric river quadrant: Strongest IWV fluxes yield heaviest rains *Nearly 2/3 of tropospheric water vapor is in the lowest 2 km MSL. Hence, to first order, the IWV flux provides a close estimate of the low-level water-vapor transport into the coastal mountains.

13. Physical variables required for extreme precipitation (includes AR conditions) • Wind in the controlling layer near 1 km MSL • speed > 12.5 m/s • direction (determines location of rain shadow) • Water vapor content • vertically integrated water vapor (IWV) > 2 cm • Snow level • Above top of watershed

14. 16-Feb-04; p.m. comp. North coast • 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 South coast IWV >2cm: <1000 km wide 1000 km SSM/I Integrated water vapor (cm) 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 Neiman, P.J., F.M. Ralph, G.A. Wick, J. Lundquist, and M.D. Dettinger (2008), J. Hydrometeor., 9, 22-47. Approach: Developed a methodology for creating a multi-year AR inventory.

15. Flooding on California’s Russian River: Role of atmospheric rivers Ralph, F.M., P. J. Neiman, G. A. Wick, S. I. Gutman, M. D. Dettinger, D. R. Cayan, A. White Geophys. Res. Lett., 2006 • SSM/I satellite image shows • atmospheric river • Stream gauge data show • regional extent of high stream flow • Covering roughly 500 km of coast This paper showed that flooding on the Russian River is associated with atmospheric rivers (all 7 floods over 8 years). If a strong AR stalls for 12-36 hours, it can create flooding.

16. Flooding in Western Washington: The Connection to Atmospheric Rivers by Paul J. Neiman, Lawrence J. Schick, F. Martin Ralph, Mimi Hughes, and Gary A. Wick in review at Monthly Weather Review Of 48 annual peak daily flows on 4 watersheds, 46 were associated with the land-fall of atmospheric river conditions. The orientation of an atmospheric river strongly influences which specific watersheds receive the most precipitation and highest stream flow.

17. Histogram of AR intensity from Neiman et al 2008 catalog of AR events: The strongest value of vertically integrated vapor flux in each AR w/in 1000 km of coast. Dates from the 20 top 3-day precip. events between 1949-2007 (from the CDC 0.25x0.25 deg unified precip. dataset) in the Sierra from Wes Junker are also marked(http://www.hpc.ncep.noaa.gov/research/California_major_rains.htm). (11) 01/05/66 (12) 01/18/69 (14) 11/11/73 (08) 03/09/95 (20) 03/29/74 (04) 12/14/02 (06) 02/14/86 (09) 01/09/05 (10) 11/18/50 (18) 12/12/95 (19) 02/13/00 (03) 12/21/55 (07) 01/15/74 (02) 01/31/63 (05)12/30/05 (17) 01/01/97 (13) 12/18/81 (15) 02/06/60 (16) 01/22/70 (01) 12/22/64

18. HMT Findings used in NWS Training • Improved situational awareness • Advance lead time that a “big event” may be coming, a few days ahead • Details on locations, timing and strength improve as event nears, but precipitation amounts are generally underpredicted

19. Assessment of Extreme Quantitative Precipitation Forecasts (QPFs) and Development of Regional Extreme Event Thresholds Using Data from HMT-2006 and COOP Observers NWRFC CNRFC F. M. Ralph, E. Sukovich, D. Reynolds, M. Dettinger, S. Weagle, W. Clark, and P. J. Neiman Journal of Hydrometeorology (December 2010) GLA MAR UIL VER SKY Of the 20 dates with >3 inches of precipitation in 1 day, 18 were associated with ARs. SEA ABE SMP ENU CIN FRA OHA CUG AST LEE PDX DET The Forecasting Challenge SMI EUG CGR CNRFC CRL ILH SXT 4BK Mean Absolute error (in) HON BKL BRR ORO FAR TKE BLU HYS GEO VNO PCH SMF CZC FOL RIO BND 41 West Coast sites were used Forecasting large precipitation amounts is difficult On average forecasts are 50% less than observations TPK

20. An important cause of forecast errors: A mesoscale frontal wave can increase the duration of AR conditions, leading to a localized region of extreme precipitation Ralph et al., Mon. Wea. Rev. (2011; in press) Ralph et al., Geophys. Res. Lett. (2006) See also case shown in Neiman et al., Mon. Wea. Rev. (2004)

21. Atmospheric River Observatory (ARO) and Forecast Aid Forecast Example from near Big Sur from 19 March 2011 showing observations of the onset of AR conditions hitting Central California on 19 March 2010. Example from near Big Sur from 19 March 2011 showing forecast of AR conditions hitting Central California on 20 March 2010. Note that time increases from right to left in this display, which is a meteorological style.

22. Regions where AR impacts on extreme precipitation and flooding have been documented (or suggested by preliminary analysis) Glacier NP Nov 2006 Pac NW Floods & Water Supply Lee side of Sierras SW Utah Dec 2010 California Floods and Water Supply Southern AZ and San Juan Mtns. of CO Nov-Dec 2007

23. CalWater & HMT-West Observing Systems Winter 2010/2011 in California Three S-Prof precipitation profilers (NOAA/PSD) 449 MHz wind profiler G-1 Research aircraft for CalWater (DOE/PNNL) 1 Feb – 7 mar 2010 SKYWATER Radar C-band scanning radar (NOAA/PSD) Seven 915 MHz wind profilers (NOAA/PSD) Scanning Radar GPS IWV & balloon Sounding Systems

24. Atmospheric Rivers, Floods and the Water Resources of California by Mike Dettinger, Marty Ralph, , Tapash Das, Paul Neiman, Dan Cayan Water, 2011 (in Press) 25-35% of annual precipitation in the Pacific Northwest fell in association with atmospheric river events An average AR transports the equivalent of 7.5 times the average discharge of the Mississippi River, or ~10 M acre feet/day 35-45% of annual precipitation in California fell in association with atmospheric river events

25. 20-21 March 2011 Extreme Precipitation Event

26. 20-21 March 2011 Extreme Precipitation Event Several sites observed over 10 inches of rain in < 24 hours. 14-16 UTC 20 March 2011 Storm Total Rainfall through 5 AM PT 21 March 2011 (Preliminary; from NWS Oxnard WFO) SANTA BARBARA SOUTH COASTMARIA YGNACIO RIDGE........... 5.98MOUNT CALVARY..................... 5.63SANTA BARBARA..................... 4.46SANTA BARBARA(KSBA)............6.27MONTECITO HILLS................... 7.40SANTA BARBARA MOUNTAINS AND FOOTHILLSREFUGIO PASS...................... 7.21GIBRALTAR DAM...................11.73CACHUMA RESERVOIR...........11.13CELITE........................... 6.00RANCHO SAN JULIAN.................6.55SAN MARCOS PASS................10.19EL DESEO.........................…....10.74 VENTURA COUNTY MOUNTAINSNORTH FORK MATILIJA..............6.08WHITE LEDGE PEAK.................. 6.18NORDHOFF RIDGE.................... 9.22ROSE VALLEY......................... 10.32OLD MAN MOUNTAIN...............8.19

27. Conclusions • Extreme precipitation and flooding in the West Coast States (and likely in some areas of the Intermountain West) is often associated with Atmospheric Rivers. • While extreme precipitation is very difficult to predict accurately, research advances on ARs are the basis for development of new forecast tools and for forecaster training. • To understand how changes in climate will effect extreme precipitation and flooding in the region, it is important to examine conditions associated with ARs.

28. Thank You • HMT web page • http://hmt.noaa.gov • Atmospheric Rivers Information Page • http://www.esrl.noaa.gov/psd/atmrivers/ • CalWater web page • http://www.esrl.noaa.gov/psd/calwater/ • Marty.Ralph@noaa.gov