Rivers in the sky!!!
(Atmospheric Rivers [ARs])
SIO 209: Weather Discussion
Atmospheric Rivers (ARs)
• Rivers in the sky!
• Narrow regions in the atmosphere (~400-600km
wide), extending thousands of kilometers long.
• Responsible for carrying most of the horizontal
moisture (water vapor) transport outside of the
tropics.
• Integrated water vapor greater than 2cm
• Wind speeds greater than 12.5m/s in lowest 2km
• About 3-5 ARs on the globe at any given time
http://www.esrl.noaa.gov/psd/atmrivers/questions/
EOS Transactions, AGU: Ralph and Dettinger, 2011
Atmospheric Rivers (ARs)
• Most are weak systems but they provide beneficial rain
or snow that is essential to the water supply
• Strongest case of ARs can cause major flooding when
they make landfall.
• All major floods in Russian River since 1997 have been
due to ARs.
• Forecast for potential heavy rain (5-7days in advance)
• Well-known example: Pineapple Express
– Brings moisture from the tropics near Hawaii to western
coast of U.S.
Water Vapor content
SSM/I: Special Sensor
Microwave Imager
Vertically
integrated
water vapor
transport
Dettinger et. al, 2011, Water
GOES-West (Infrared)
GOES-Global Water Vapor Channel
Total Precipitable Water
http://tropic.ssec.wisc.edu/real-time/mimic-tpw/global/main.html
Moisture Flux as on 12 UTC, 12 October 1991
~90% of poleward water vapor flux outside of tropics is conducted
by ARs in less than 10% of zonal circumference
Zhu and Newell, 1998
General Location of ARs features
• Generally found in warm sectors
of mid-latitude cyclones, ahead of
the cold front
• Continually form, move and
evolve with storms in mid-latitude
storm tracks
http://www.islandnet.com/~see/weather/almanac
/arc2009/alm09dec.htm
http://apollo.lsc.vsc.edu/classes/met130/notes/ch
apter12/warm_cold_conveyor.html
(a) SLP analysis (observations) ,
wind flags=25m/s, full barbs=5m/s, half
barbs=2.5m/s
(b) NOAA GOES infrared satellite image
Ralph et al., 2004, Monthly Weather Review
Cross-section of an atmospheric river
Ralph et al., 2004, Monthly Weather Review
Variability of California’s precipitation
Dettinger et. al, 2011, Water
Dettinger et. al, 2011, Water
Dettinger et. al, 2011, Water
Oct. 13-14, 2009
EOS Transactions, AGU: Ralph and Dettinger, 2011
26 inches of rain in 2 days for Dec 2010
17ft of snow in the Sierra Nevada from Dec 17-22, 2010
Wikipedia: Atmospheric Rivers