Severe Weather: Hurricanes

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Severe Weather Hurricanes Jim
Kossin Cooperative Institute for Meteorological
Satellite Studies University of
WisconsinMadison Madison, WI kossin_at_ssec.wisc.edu
http//www.ssec.wisc.edu/kossin
National Press Foundation, Understanding Violent
Weather Program, 12 March 2007
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• 80-90 tropical cyclones (hurricane/typhoon/cyclon
  e) globally each year.
• One of the most costly natural hazards in terms
  of life and property.
• North Atlantic, East / West / South Pacific,
  North / South Indian Oceans.
• Less than 15 occur in North Atlantic.
• Wind, storm surge, fresh water flooding,
  (lightning, tornadoes)

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Operational hurricane forecasting challenges
Primary focus

• Where is it going? (cross-track)
• When will it arrive? (along-track)
• How strong will it be when it gets there?

Other

• Big storm (Katrina) or small storm (Charley)?
• How much storm surge? Tides? Coastal topography?
• Rain amounts?

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Present skill (track and intensity)
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How does the environment control
hurricanes? Genesis warm SST, low shear, , an
incipient vortex. Intensity change SST, shear,
, landfall, Saharan dust. Track larger-scale
circulation patterns. Need to predict the
environment that the hurricanes are traveling
through. (Requires good track prediction) Suppose
we could predict the environment and track
perfectly. Could we then have perfect intensity
predictions?
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The challenge Hurricane-scale processes control
/ modify intensity

• Eye / Eyewall exchanges (meso-vortices)
• Spiral-band (rain-band) processes (RAINEX)
• Eyewall replacement cycles (concentric eyewall
  cycles)

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Model simulation of eye / eyewall exchange
(mixing)
Hurricane Alberto (2000)
(figure adapted from Kossin and Eastin 2001,
Kossin et al. 2002)
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Hurricane Isabel near local sunrise on 12 Sep
2003.
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? Hurricane Isabel
Model simulation ?
(figure adapted from Kossin and Schubert 2001,
2004)
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Why do we care? Small-scale mixing affects
intensity change
weakening events
mixing events
(figure adapted from Kossin et al. 2006)
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Eyewall replacement cycles
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Eyewall replacement cycles usually cause rapid
intensity swings. Particularly problematic as
storms approach land (Hurricane Andrew 1992).
(figure adapted from Willoughby et al. 1982)
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Longer-term forecasting challenges
How are hurricanes affected by climate and
climate change? (How is the climate affected by
hurricanes?) What changes in the level of
hurricane activity can we expect during the next
5, 10, 50, 100 years? Are there cycles and / or
trends? Are the cycles / trends natural or
man-made?
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How do we meaningfully measure the changes?

• Frequency (how many?)
• Intensity (how strong?)
• Duration (how long-lasting?)
• Location (where are they? More / less landfall?)

Accumulated Cyclone Energy (ACE) Power
Dissipation Index (PDI) Number of Cat 4-5
storms Systematic (i.e. not random) track changes
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Direct relationship between SST and hurricane
intensity
The theory of Potential Intensity (PI, MPI)
suggests that, all other things being equal, an
increase of underlying SST will lead to an
increase in the maximum intensity that a
hurricane can achieve.
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Relationship between SST and hurricane activity
(figure adapted from Emanuel 2005)
Reflects changes in frequency, intensity, and
duration
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Changes in frequency of the most intense
hurricanes
(figure from Webster et al. 2005)
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Relationship between SST and hurricane location /
track
(figure from Kossin and Vimont 2007)
The relationship with SST is part of a more
general relationship with the Atlantic Meridional
Mode (AMM). The AMM describes large-scale
circulation patterns that go beyond the
limitations of Potential Intensity theory.
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Data issues
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A Brief History of the Global Hurricane Record
Age of the Weather Satellites
Post WW-II Aircraft Reconnaissance
1950s
1970s
Sporadic measurements. Mostly Atlantic. West
Pacific until 1987. Maximum intensity rarely
measured. Storm counts are better.
Occasional measurements. Serendipitous
sources. Maximum intensity rarely measured. Storm
counts may be low.
Hourly measurements. Almost global. The counts
are good.
The existing record is inconsistent by its nature
and its construction.
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Data reanalysis Variability and increases in
the Atlantic verify well. Global trends may be
inflated.
(figure adapted from Kossin et al. 2007)
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• Why are Atlantic hurricanes apparently reacting
  more markedly to warming SST than storms in other
  ocean basins?
• The answer may lie in the way SST is related to
  other factors that affect hurricanes
• All climatic factors cooperate in the Atlantic.
  They are either all favorable or all unfavorable.
• This is not true in other basins. One factor
  might be favorable while another is unfavorable.
  The factors offset each other.

(from Vimont and Kossin 2007)
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Changes in the long-term records Human-induced
variability and increases, natural cycles, or
both?
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The Atlantic Multi-decadal Oscillation
(AMO) Hypothesis 1 The AMO is a natural cycle
related to periodic changes in the thermohaline
circulation (a.k.a. the Atlantic conveyor belt).
This natural cycle is superimposed on a smaller
man-made trend. Under this hypothesis, SST is
expected to eventually decrease to a long-term
cooler regime. Hypothesis 2 The signal known
as the AMO is actually just a superposition of
two human-induced signals anthropogenic
greenhouse gas warming and sulphate aerosols.
Under this hypothesis, SST is expected to
continue its present rate of human-induced
increase with no natural cycle to help offset it.
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Summary Hurricane forecasting faces many
challenges in both an operational setting and
toward long-term risk assessment. Operational
intensity forecasts are challenged by the broad
spectrum of scales that matter (environment to
hurricane scale). Long-term forecasting is
challenged by our present lack of understanding
of the relationships between hurricanes and
climate change. Questions How will frequency,
intensity, and tracks change? Could the effects
of increasing SST be offset by more frequent
eyewall replacement cycles and/or mixing
events? Why is the Atlantic changing so
profoundly? Is the present high Atlantic activity
just a phase of a cycle or will it continue
indefinitely?
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Relationship between the AMM and the 3 factors
comprising hurricane activity (frequency,
duration, intensity)
Correlations of raw / low-pass / high-pass time
series. Bold ? significant.
Raw time series
AMM
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An explanation for the variability of duration
AMM
There is a systematic shift of the mean tropical
cyclogenesis region to the southeast (northwest)
during positive (negative) phases of the AMM.
Since storms generally track westward to
northwestward, a southeast shift allows storms to
last longer before reaching hostile environments
(land, cold SST, high shear).