the impacts of climate change on northwest windstorms cliff... · 14/05/2019 · pacific cyclone...
TRANSCRIPT
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The Impacts of
Climate
Change on
Northwest
Windstorms
Cliff Mass
Atmospheric Sciences
University of
Washington
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1993 Inauguration Day Storm
Although the Pacific Northwest does not
experience tropical storms or hurricanes,
we do experience very strong Pacific
cyclones, with winds equal to category two
or three hurricanes
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Native Americans knew about NW windstorms
and had several legends regarding their origin.
The Thunderbird
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The force of southerly storms was evident to every eye; large
and extensive woods being laid flat by their power, the
branches forming one long line to the North West,
intermingled with the roots of innumerable trees, which have
been torn from their beds and helped to mark the furious
course of their tempests.
John Meares,
1788, off of
Cape
Flattery of the
Olympics Peninsula
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Early Settlers
• As European settlers learned thatNorthwest windstorms were aforce to be reckoned with.
• In 1875, Seattle pioneer ArthurDenny noted "a strong gale,which threw down considerabletimber and overturned lightstructures, such as sheds andoutbuildings.”
Arthur Denny
Seattle
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Big Trees:
NW Storm
Force
Multiplier
A minor
windstorm
Easter Sunday,
April 97
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Compared to this…
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Most Northwest Windstorms are
Produced by Midlatitude Cyclones
with low central pressures
• A cyclone is an area of low pressure around which air circulates in a
counterclockwise direction (in the northern hemisphere)
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Midlatitude
Cyclones
• The lower the central pressure the stronger the winds.
• Typical winter low 990-1000 hPa. Moderate windstorms 980-990 hPa. Strong ones, 950-980hPa
• Deep lows have strong pressure gradients, which result in the strong winds.
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Weather Satellites Imagery of
Our Storms Reveal The Huge
Scale of Our Major Storms
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Tropical Cyclones: Much
Smaller
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Chanukah Eve Storm: 18-h forecast for 10 PM December 14, 2006
L
Pressure Pattern Associated with the Chanukah Eve Storm
Of December 15, 2006
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The Most Extreme Northwest Windstorm: The
Columbus Day Windstorm of 12 October 1962
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Began as Typhoon Frieda and then moved
northward and transformed into a midlatitude
cyclone.
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Columbus Day Storm
• The most damaging windstorm to strike the PacificNorthwest in 150 years.
• An area from northern California to southern BritishColumbia experienced hurricane-force winds,massive treefalls, and power outages.
• In Oregon and Washington, 46 died and 317 requiredhospitalization as a result of the storm.
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Max Winds
(mph)
Columbus Day
Storm 1962
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Columbus Day 1962: At Cape Blanco there were
150 mph with gusts to 179! Strongest winds on
bluffs and windward slopes of coastal orography
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More Recent Storms
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Inauguration
Day
Storm
January 20,
1993
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The first major windstorm to be highly skillfully forecast….
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Winds of over 100 mph wereobserved at exposed sites inthe coastal mountains and theCascades, with speedsexceeding 80 mph along thecoast and in the interior ofwestern Washington.
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Chanukah Eve Storm of Dec. 2006
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• Winds gusted to 90 mph along the coast, 80mph in the eastern Strait of Juan de Fuca,and 70 mph over the Puget Sound lowlands.
• In the Cascades, winds reached 100 mph atSunrise on Mt. Rainier and 113 mph atChinook Pass.
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Pacific Cyclone Key Facts
• Each year, typically get several storms with
winds gusting to 30-50 mph.
• Every year, a storm reaching roughly 50-70
mph.
• Every ten years, 70-90 mph
• Every 30-50 years, 90+ in exposed locations.
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A Winter Season Phenomenon
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Northwest Windstorms and
Global Warming• Will there be more of them?
• Will they become more intense?
The Inauguration Day Storm
1993
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Northwest Windstorms
• The best science suggests that there will not be
an increasing trend.
• But first, what has been the trend over the past
half-century?
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Seattle City Light sponsored
study
• Started with global climate models
• Used regional, high-resolution models to look at
local impacts.
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Number of times above 90
percentile for 1970-2000 (DJF)
Just
offshore of
Washington
Coast
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Seattle
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Why SHOULD we expect
Northwest windstorms to
change?• Their energy source is the jet stream and the
strong horizontal temperature gradients
associated with it.
• Most global climate models suggest that the
temperature gradients will weaken at low levels
as the Arctic heats up more than the poles.
• But the temperature gradient increases in upper
troposphere.
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Hurricanes hitting the Northwest
under Global Warming?Forget it, even a warmed eastern Pacific will
be far too cold.
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Observed Trend at Sea-Tac
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Damaging Winds Can Also
Be Produced in other Ways
Gaps in Our Terran Produce
Strong Wins.
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Enumclaw and nearby foothills
locations can experience severe
windstorms…while calm winds
occur a dozen miles away
December 24, 1983
Winds over 118 mph
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December 24, 1983
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Why Enumclaw?
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Enumclaw Windstorm Pressure
Pattern
December 24, 1983
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Fraser River Gap Windstorms
December 28, 1990
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Max Winds, 28
Dec. 1990
Gusts
above 90
mph
Fraser River NE Gap Flow
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What will Global Warming Do
to Strong Gap Winds?• Probably weaken them
• Cold air in the interior of the continent results
in higher pressure that can help drive gap
winds
• Global warming is lessening the severity of the
cold air in the interior.
• Less strong offshore flow in gaps during
winter.
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Cold Wave Trends
The U.S. Environmental Protection Agency did a study of the
trend of cold wave days from 1948 to 2015
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An Aside: Forecasting of
Northwest Windstorms is
MUCH Better
• Better forecast models
• Satellite data around the
globe
• Higher resolution and
more.
• Before ~1993 we got most
wrong. Today, most
correct days in advance.
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Forecasts Need to Be Used
Better in Construction
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Seattle WindWatch
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Bottom Line
• There is no reason to expect increased extreme
winds over the region under global warming.
• Weather forecasting skill has improved, which
can be useful for protecting construction and
buildings in place.