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Extreme (Hydroclimatic) Events in
Western Mountains
Michael Dettinger, USGS, SIO, La Jolla
Geoff Schladow, UC Davis, TERC
Marty Ralph, NOAA/ESRL/PSD
Paul Neiman, Allen White, NOAA/ESRL/PSD
Dan Cayan, USGS/SIO
and others
NOAA’s California-Nevada Applications
(fall and
Snowmelt Peaks
(spring and summer)
Front Range
(summer &
Ralph et al., Obs Vision for Western States
Water Council, 2011
1. What are atmospheric rivers? Why should we
2. Examples of AR impacts in the Tahoe basin
3. What changes can we anticipate in 21st Century
storms & floods?
Atmospheric Rivers
Landfalling atmospheric rivers
Altitude MSL (km)
Orographic cloud
and precipitation
“Controlling layer”
(upslope winds)
-Lateral structure from satellite data (~400 km width & 2000 km long)
-Vertical structure from airplanes & radar (intense jet of vapor transport
between 1 – 2 km above sea level; 10-20 Mississippis)
Last night’s atmospheric rivers
Just how BIG are we talking about?
Largest 3-day storm totals in >30 yrs COOP records
Ralph & Dettinger, BAMS, 2012
Atmospheric rivers as resources & hazards
(days 0 to +1), 1998-2008
• 87% of declared floods of Russian River
since 1948 have been atmospheric rivers (ARs), (Ralph et
al 2006; Florsheim & Dettinger, in review)
• In Washington, 46 of 48 annual peak daily flows have
been associated with ARs (Neiman et al 2011)
• Responses of daily flows in American & Merced Rivers
to ARs are typically order of magnitude larger than to
other storms
(Dettinger 2004, 2005)
Dettinger et al. 2011
Arizona atmospheric-river storm, Jan 2010
Vertically integrated water vapor imagery
(c) 20 Jan 2010 p.m.
(d) 21 Jan 2010 p.m.
AR #3
AR #3
(d) 00Z 22 Jan
Total water vapor
Neiman et al., tentatively
accepted, JHM
Arizona atmospheric-river storm, Jan 21-23
Neiman et al., tentatively
accepted, JHM
Southwest atmospheric-river
storm, December 2010
Ralph & Dettinger, BAMS, 2012
Southwest atmospheric-river storm,
December 2010
800% of normal Dec in
Southern California
(670 mm/26 in)
Flooding, Laguna Beach, 12/21/2010
Bountiful, UT, 12/24/2010
Almost 400% of normal
Dec in SE Utah
(430 mm/17 in)
Ralph & Dettinger, BAMS, 2012
Since 1950…
Why ARs matter…
•ARs as West Coast “drought busters” (33-74%)…
• ARs & Mojave River aquifer recharge (57%)…
• ARs & San Francisco Bay salinity variations (50-83%)
• ARs & Sacramento Rvr floodplain inundation (78%)
• ARs & Central Valley levee breaks (81%)…
© Waters
Why ARs matter…
Atmospheric rivers, and the extreme events that they
bring to the West, are particularly influential in many
“environmental” contexts
Thus, understanding, forecasting & projecting ARs is
of direct importance to westerners
Western systems—ecological, geomorphic,
hydrologic, infrastructural, … --have all adapted in the
context of ARs and their particular properties.
Understanding that particular context and how we
have modified it with structures etc should help us to
better manage & restore elements & services in this
new world we’ve created.
© Waters
Lake Tahoe: Storms, inflows, sediment loading
& clarity
Area: 500 km**2
Altitude: 1897 m asl
Residence time: 650 yr
Lake Tahoe: ARs as big warm storms
On average, AR storms at Tahoe are warmer by 2ºC and
wetter by 85% than wet days in general.
Lake Tahoe: Warm storms & ARs
Of 38 days with Tmin > 0ºC & Precipitation > 5 cm
at Tahoe City, WY1948-2010
Not AR
All such days
AR days among them
Lake Tahoe: Lake Level Jumps & ARs
76 days when Lake level rises by
> 5 cm/day since 1958
84% ARs
AR storm
Not Ar
700 mb Vapor Transport
Rates, 15 largest 1-day lake
Lake Tahoe sediment loading
Upper Truckee River at South Lake
Total sediment transports by high-load days
vs other days, 1971-92
High load days
Daily Suspended Sediment Loads
from Upper Truckee River
All other days
Upper Truckee River at South Lake
Total suspended sediments
transported to lake
Number of days
Low-load days
High-load days
High-load days
Flow Contributions to Lake
Other flows
High-load flows
The total amount (18% of all transport) of suspended sediments
transported to the Lake by the Upper Truckee on the days with loads >
150 tons/day is out of proportion to the flows on those days (4% of all
flows) & far out of proportion to the number of such days (0.5% of days).
Sediment transport & warm storms
In WY1981-1992, there were 9 days with > 5 cm of
precipitation & Tmin > 0C occurred at Tahoe City.
Total sediments transported
6% of
Warm rain sed loads
Rest of loads
Numbers of days
Warm rains
Rest of days
On average, those
9 warm rain days yielded 242
tons/day of suspended sediments, whereas the
12-yr average daily sediment loads were 8 tons/day…30
times as much.
Lake Tahoe: ARs & sediment loading
Upper Truckee River at South Lake
Total sediment transports by 36 high-load vs other days,
and by AR high-load vs other high-load days
Daily Suspended Sediment Loads
from Upper Truckee River
Other days
High-load AR days
Other high-load
Numbers of days
in each category
Lake Tahoe clarity
TERC, person. comm., 2012
Anomalous 500 mbar Levels
Anomalous 700 mbar Temperatures
Anomalous Integrated Water
Vapor Transports
Composites of
conditions on the 5
days preceding each
of 10 days with
largest anomalous
secchi depths, 19672012
Anomalous depths are secchidepths minus 4th order longterm trend and 5th order
seasonal cycle
Sudden changes in Lake Tahoe clarity
Largest Measurement-to-Measurement Changes
in Lake Clarity, WY1968-2011 (>10 m!)
Of 15 occasions with largest msmt-msmt CLARITY INCREASES,
ALL 15 were preceded by stormy conditions, and…
11/15 were preceded by ARs.
73% ARs
Of 15 occasions with largest msmt-msmt CLARITY DECREASES,
ALL 15 were preceded by high pressures, as offshore blocking
patterns or overhead.
Lake Tahoe: AR storms & clarity
However, on the longer (seasonal-annual)
scales, more ARs are associated with
Lake Tahoe: AR storms & clarity
r = 0.55
Lake Tahoe: AR storms & clarity
Secchi Depths from UC Davis TERC
& AR/PE Counts from Dettinger et al., Water, 2011
Future of Western storms & floods?
(fall and
Snowmelt Peaks
(spring and summer)
ARs ?
Snowmelt ?
Monsoons ?
(summer &
Atmospheric Rivers & Climate Change
Water Vapor &
Low-Level Winds
By end of 21st Century, most GCMs (in a
7-member A2-emissions ensemble) yield:
• More atmospheric vapor content, but
weakening westerly winds
Net increase in “intensity” of extreme
AR storms
Water Vapor &
Low-Level Winds
• Warmer ARs (+1.8 C)  snowline raised
by about 1000 feet on average
• Lengthening of AR seasons (maybe?)
Dettinger, JAWRA, 2011
Atmospheric Rivers & Climate Change
Intensities of ARs from different directions
San Jacinto Mtns
Santa Ana Mtns
San Gabriel Mtns
Dettinger, JAWRA, 2011
Snow Mediated Floods & Climate Change
Observed (1949-2003)
Observed (past 85 yrs)
Trends in Annual-Peak Flows
Hirsch & Ryberg, HSJ, 2011
McCabe et al., BAMS, 2006
Snow Mediated Floods & Climate Change
Projected (1951-2099)
• Wintertime
flood flows
increase in
frequency &
• Springsnowmelt high
flows decline
CalDWR media slide
Das et al., 2011, Climatic Change
Projected Floods in Sierra Nevada
16 GCMs, A2 emissions
Northern Sierra Nevada
Southern Sierra Nevada
Change in mean
annual flow
Change in mean
annual flow
Drier avg
Change in flood flows with various return periods
Return period
Das et al., in prep
Monsoon storms/floods?
Vapor Transports are a missing link!
• Understanding atmospheric rivers will be crucial for
addressing floods, water resources & a variety of
environmental processes around the Western US
• Forecasting ARs is basic to forecasting/managing key
environmental processes
• The ability to follow vapor as it is carried, aloft, into the
West (by ARs or monsoons) is a critical gap in our ability to
track, diagnose and predict important storms and floods.
• Although the science is still nascent, more intense storms,
higher snowlines, and resulting greater flood risks are being
projected for the 21st Century.
• Dettinger, M.D., 2005, A long-term (50 yr) historical perspective on flood-generating winter storms in the American River basin: Proc. 2005
California Extreme Precipitation Symposium, 62-73.
• Dettinger, M.D., 2011, Climate change, atmospheric rivers and floods in California—A multimodel analysis of storm frequency and
magnitude changes: Journal of American Water Resources Association, 47, 514-523.
• Dettinger, M.D., Ralph, F.M., et al.., 2012, Design and quantification of an extreme winter storm scenario for emergency preparedness and
planning exercises in California: Natural Hazards, 60, 1085-1111.
• Das, T., Dettinger, M., Cayan, D., and Hidalgo, H., 2011, Potential increase in floods in California’s Sierra Nevada under future climate
projections: Climatic Change, 109, 71-94.
• Florsheim, J., and Dettinger, M., in review, Intentional levee breaks promote lowland floodplain biogeomorphic processes: as chapter in
Hudson, P., and Middelkoop, H. (eds.), Geomorphology and management of embanked floodplains—North American and European fluvial
systems in an era of global environmental change, Springer-Verlag, 15 p.
• McCabe GJ, Clark MP, Hay LE (2007) Rain-on-snow events in the Western United States. BAMS, 1–10
• Neiman, P.J., Ralph, F.M., Moore, B.J., Hughes, M., Mahoney, K.M., and Dettinger, M.D., in revision, The landfall and inland penetration of a
flood-producing atmospheric river in Arizona—Part I, Observed synoptic-scale and hydrometeorological characteristics: J. Hydromet., 44 p.
• Neiman, P.J., L.J. Schick, F.M. Ralph, M. Hughes, and G.A. Wick, 2011: Flooding in western Washington: The connection to atmospheric
rivers. J. Hydromet., 12, 1337-1358.
• Porter, K., et al., 2011, Overview of the ARkStorm scenario: U.S. Geological Survey Open-File Report 2010-1312, 183 p.
• Ralph, F.M., and Dettinger, M.D., 2011, Storms, floods and the science of atmospheric rivers: Eos, 92(32), 265-266.
• Ralph, F.M., and Dettinger, M.D., 2012, Historical and national perspectives on extreme west-coast precipitation associated with
atmospheric rivers during December 2010: Bulletin of the American Meteorological Society, 93, 783-790.
• Ralph, F.M., Neiman, P.J., Wick, G., Gutman, S., Dettinger, M., Cayan, D., and White, A.B., 2006, Flooding on California’s Russian River—
Role of atmospheric rivers: Geophysical Research Letters, 33 (L13801), 5 p.
• Ralph, F.M., P.J. Neiman, and G.A. Wick, 2004: Satellite and CALJET aircraft observations of atmospheric rivers over the eastern NorthPacific Ocean during the winter of 1997/98. Mon. Wea. Rev., 132, 1721-1745.
• Ralph, F.M., Dettinger, M.D., White, A., Reynolds, D., Cayan, D., Schneider, T., Cifelli, R., et al., 2011, A vision of future observations for
western US extreme precipitation events and flooding—Monitoring, prediction and climate: Report to Western States Water Council, 74 p.
• Neiman, P.J., F.M. Ralph, A.B. White, D.A. Kingsmill, and P.O.G. Persson, 2002: The statistical relationship between upslope flow and rainfall
in California’s coastal mountains: Observations during CALJET. Mon. Wea. Rev., 130, 1468-1492.
• Thorncroft, C.D., Hoskins, B.J., and McIntyre, M.E., 1993, Two paradigms of baroclinic-wave life-cycle behavior: Quarterly Journal of Royal
Meteorological Society, 119, 17-35.
• White, A., Anderson, M., Dettinger, M., Ralph, M., Hinajosa, A., and Cayan, D., 2012, A 21st Century observing network for California: 16th
Annual Symposium on the Advancement of Boundary Layer Remote Sensing, Boulder, CO, 4 p.
• Zhu, Y., and R.E. Newell, 1998: A proposed algorithm for moisture fluxes from atmospheric rivers. Mon. Wea. Rev., 126, 725-735.
Surface Wind
Anomaly Correlations with Regional Reanalysis Values
Dettinger, Clim Chg, 2012
Downscaled GFDL Trends in Downward
Longwave Radiation at Sfc
B1 emissions
A2 emissions
Watts / m2 / century
Tahoe gets more
downward longwave
Downscaled GFDL Trends in Downward
Shortwave Radiation at Sfc
B1 emissions
A2 emissions
Watts / m2 / century
Modest changes in solar
insolation at Tahoe
Downscaled GFDL Trends in
Near-Surface Wind Speeds
A2 emissions
B1 emissions
Percent of historical / century
Modest changes in
winds at Tahoe
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