Transcript Tropical Cyclones, pt. 2 • Review of structure • Climatological questions • Dangers – Wind, storm surge, flooding, tornadoes.

Tropical Cyclones, pt. 2
• Review of
structure
• Climatological
questions
• Dangers
– Wind, storm
surge, flooding,
tornadoes
Why are TCs named?
Tropical cyclones are named to provide ease of communication between
forecasters and the general public regarding forecasts, watches, and warnings.
Since the storms can often last a week or longer and that more than one can be
occurring in the same basin at the same time, names can reduce the confusion
about what storm is being described.
For more info, visit: http://www.aoml.noaa.gov/hrd/tcfaq/B1.html
Radial profile of TC winds
Wind speed of Hurricane Anita (1977). Note the exponential
increase from the eye, out to a “radius of maximum winds” of ~
30 km, then exponential decrease toward the periphery of the
tropical cyclone.
Source: Holland (1981)
Hurricane Fran
Category 3
Mitch (1998) Statistics
•
•
•
•
9000+ deaths in Nicaragua & Honduras
Minimal central pressure: 905mb
Max sustained winds: 155kt (180mph)
2nd strongest October hurricane ever
recorded
• 7th strongest hurricane ever in Atlantic
Hurricane Wilma
Category 5
Hurricane Gilbert
Compare to Wilma & Mitch (location)
Cyclone Monica
23 April 2006
Most intense TC in 2006. But, just how intense?
Cyclone Monica
23 April 2006
Most intense TC in 2006. But, just how intense?
Australia (Darwin): 905 hPa, sustained surface winds 135 kts (10-min)
Joint Typhoon Warning Center (Hawaii): 879 hPa; 145 knots (1-min)
Dvorak (Wisconsin) satellite estimate: 869 hPa; 170 kts (1-min)
How intense
is this
western
Pacific
typhoon?
How intense
is this
western
Pacific
typhoon?
NOAA CI number
7.0 (140kts, 898mb)
JMA: 925mb; 95
kts (10-min speed;
108 kts 1-min
speed)
Different landfall intensities
JTWC: 110kt G 140 kt 1-min sustained = Cat 4
*JMA: 75 kt 10-min mean ~ 84 kt 1-min sustained = CAT 2
CWB: 74 kt G 93 kt 10-min mean ~ 83 kt G 104 kt = CAT 2
HKO: 90 kt 10-min mean ~ 101 kt = CAT 3
Only one can be correct. But which is it? And will we ever know?
Most intense Atlantic hurricanes
Intensity is measured solely by central pressure
Rank
Hurricane
Season
1
Wilma
2005
882* mb (hPa)
2
Gilbert
1988
888 mb (hPa)
3
"Labor Day"
1935
892 mb (hPa)
4
Rita
2005
895 mb (hPa)
5
Allen
1980
899 mb (hPa)
6
Katrina
2005
902 mb (hPa)
Camille
1969
905 mb (hPa)
Mitch
1998
905 mb (hPa)
9
Ivan
2004
910 mb (hPa)
10
Janet
1955
914 mb (hPa)
7
Min. pressure
Most intense global TCs
Rank
Name
Pressure
Location
Year
1
2
2
2
2
2
7
8
9
9
11
11
11
14
14
14
14
14
19
19
21
22
22
24
25
Typhoon Tip
Typhoon Gay
Typhoon Ivan
Typhoon Joan
Typhoon Keith
Typhoon Zeb
Typhoon June
Typhoon Forrest
Typhoon Ida
Typhoon Nora
Typhoon Rita
Typhoon Yvette
Typhoon Damrey
Typhoon Vanessa
Typhoon Angela
Typhoon Faxai
Cyclone Zoe
Typhoon Chaba
Typhoon Violet
Hurricane Wilma
Typhoon Irma
Typhoon Mike
Cyclone Daryl-Agnielle
Hurricane Gilbert
Labor Day Hurricane
870 mb
872 mb
872 mb
872 mb
872 mb
872 mb
875 mb
876 mb
877 mb
877 mb
878 mb
878 mb
878 mb
879 mb
879 mb
879 mb
879 mb
879 mb
882 mb
882 mb
884 mb
885 mb
885 mb
888 mb
892 mb
Western Pacific
Western Pacific
Western Pacific
Western Pacific
Western Pacific
Western Pacific
Western Pacific
Western Pacific
Western Pacific
Western Pacific
Western Pacific
Western Pacific
Western Pacific
Western Pacific
Western Pacific
Western Pacific
South Pacific
Western Pacific
Western Pacific
Atlantic
Western Pacific
Western Pacific
South Indian
Atlantic
Atlantic
1979
1992*
1997*
1997*
1997*
1998*
1975
1983
1958
1973
1978
1992*
2000*
1984
1995*
2001*
2002**
2004*
1961
2005
1971
1990
1995*
1988
1935
How is TC intensity determined?
1. Ground-based
observations (ships, ocean
buoys, surface stations,
and Doppler radar)
2. Aircraft reconnaissance
(only for the Atlantic basin
though; West-Pacific from
3. Satellite estimates
- most common method of
estimation (b/c most TCs
do not reach land or pass
over buoys).
- Based on historical
relationships between wind
and minimum pressure
Dvorak technique flow chart
Dvorak technique flow chart, continued
Minimum Pressure
T-Number
1.0 - 1.5
2
2.5
3
3.5
4
4.5
5
5.5
6
6.5
7
7.5
8
Take the current intensity (T-number) from the
Dvorak technique and translate that into wind
and pressure estimates
Winds
(knots)
25
30
35
45
55
65
77
90
102
115
127
140
155
170
(millibars)
Atlantic
NW
Pacific
---1009
1005
1000
994
987
979
970
960
948
935
921
906
890
---1000
997
991
984
976
966
954
941
927
914
898
879
858
Note: The pressures shown for the NW Pacific
are lower as the pressure of that whole
environment is lower as well.
Tropical cyclone climatology
Many current studies examining the links
between global climate change and
changing Atlantic and global TC activity
Numbers of weak (category 1, 2) hurricanes have remained generally
steady over the last 35 yrs, but numbers of category 4 & 5 hurricanes
have dramatically increased. Is this due to global warming, better
observing technology, or a combination of factors? Simple answer is
that we just don’t know for sure.
Global datasets are not perfect!
One example: southern hemisphere
was very poorly observed (no land, few
ships) before 1977. In 1977, GMS-1
satellite launched, resulting in great
increase in number of TCs detected
annually. Still important to include the
Southern Hemisphere in the global
datasets because the SH does account
for between 25% and 35% of global TC
activity!
Total number of TCs has remained relatively constant over past 35 years.
Longevity of TCs has exhibited some variability in this same period, peaking in
early 1990s and decreasing to present.
Source: Webster, P. J., G. Holland, J. Curry, and H.-R. Chang, 2005: Changes in Tropical Cyclone Number, Duration, and Intensity
Warming Environment. Science, 309, 1844-1846.
in a
Total numbers of hurricanes has exhibited strong year-to-year variability
(especially West Pacific WPAC] and Southern Hemisphere [SH}). However,
biggest interest is in the marked increase in very strong hurricanes (Categories
4 + 5) in the past 15 years.
Source: Webster, P. J., G. Holland, J. Curry, and H.-R. Chang, 2005: Changes in Tropical Cyclone Number, Duration, and Intensity in a
Warming Environment. Science, 309, 1844-1846.
Similar to the Webster et al. study on the previous slide, Emanuel confirmed
the increasing destructiveness (measured by a Power Dissipation Index,
PDI, which is very sensitive to higher wind speeds) of Atlantic and West
Pacific hurricanes in the last 15 years.
Source: Emanuel, K. A, 2005: Increasing
destructiveness of tropical cyclones over the past 30
years. Nature, 436, 686-688.
Some conclusions from
global warming & tropical
cyclones:
1- global sea surface temperatures
have warmed in the last 35 yrs
2- numbers of named storms has
remained relatively constant in last 35
yrs (~ 80 storms per yr)
3- numbers of intense storms (cat 3-45) has increased in the last 35 yrs
4- computer simulations of high carbon
dioxide (the main global warming
culprit) seem to indicate not more
hurricanes, but instead stronger
hurricanes (see figure at right)
5- large degree of uncertainty is
associated with all of these findings
(from SST values, to numbers of
storms in the past 35 yrs, to numbers
of storms expected in next 80 yrs)
Results from a global climate model
simulation: a shift of the mean toward more
intense, and a shift of the variance toward
more frequent intense events
Hurricane Dangers
• Straight-line winds
– Winds that circulate around the low
• Storm surge
– Sea water pushed onshore by strong winds
• Tornadoes
– Frictional drag enhances vertical shear
– Generally weak (F0 to maybe F2)
• Inland rain
– The deadliest hurricane killer in the last 30 yrs*
* Katrina’s deaths heavily skewed the statistics
Straight Line Winds
• Strongest
winds on
right side
– With respect
to storm motion
• Hurricane
Katrina
• 90 kt wind
• 965mb pressure
• Departing FL
into GoM
• Hurricane
Katrina
• 100 kt wind
• 941mb pressure
• Category 3
intensity
– Notice the storm
is asymmetric
• Hurricane
Katrina
• 150 kt wind
• 902 mb pressure
– 6th lowest ever in
Atlantic
• Notice the incredible
symmetry of the
inner-most winds
– At this point most of
the surge is being
generated (water is
being “piled up” north
& northeast on the
right side of Katrina)
• Hurricane Katrina
• 110 kt wind
• 920 mb pressure
• Interaction with Louisiana
& Mississippi
– Note surface winds are
disrupted over land
• Note Katrina’s winds are
quite asymmetric
– Notice direction of winds
are pushing gulf waters
towards Lake
Pontchartrain
– New Orleans barely has
hurricane-force winds
• Hurricane Ivan
at landfall
(Alabama / Florida
border)
• 110kt wind
• 943mb pressure
• Note eye just east
of Mobile Bay
• Note asymmetry in
wind field
Example from
hurricane Edouard
(1996) showing the
cooling effects of a
hurricane
Strong hurricane winds
act to mix the warm
surface water with
cooler water from
below
Shows the need to not
only have warm sea
surface temperatures,
but also to have a
sufficiently deep layer
of warm water to
minimize this mixing
(>50 meters)
Storm Surge
• Increase in ocean level
– direct wind-driven water
– uplift enhanced by atmospheric pressure drop
• sea level will rise 1 centimeter for every 1 millibar decrease in atmospheric
pressure
– so a 900 mb hurricane will have 1 meter of surge associated with pressure drop
• Coastline shape has an effect
– Concave: enhances
• Bays, inlets (Appalachia Bay, FL)
– Convex: diminishes
• Headlands
• Evacuations are primarily to avoid storm surge
• Katrina’s 1,830+ deaths attributed mostly to storm surge
flooding (drownings resulting from breaking of dikes &
levees)
Storm Surge
Storm Surge Simulation
• High terrain
• Steep slope
• Less surge
danger
• Low terrain
• Gentle slope
• More surge
danger
Each county develops
evacuations based on expected
storm surge conditions
Obviously stronger storms will
have more surge that goes
farther inland – thus more people
need to be evacuated
States and counties have locally identified
evacuation routes for their coastal citizens
Problems occur when:
1) Hurricane strengthens rapidly before landfall &
more people need to evacuate (and others get
scared & want to evacuate)
2) Hurricane approaches parallel to coastline –
need to evacuate larger areas due to
uncertainty
3) Infrastructure doesn’t support quick or efficient
evacuations (i.e., Houston during Rita in 2005)
Katrina’s 27-foot storm surge
Waveland, Mississippi
Inland Flooding
• Tropical cyclones are prolific rain producers
– Typical hurricane drops 6” to 10” along its track
• If storm is slow, or moves over mountainous
terrain … rainfall is enhanced
– TS Allison, 2001
• 30+” in Houston, $6 billion in damage
– Hurr. Mitch, 1998
• 10,000+ killed in Honduras / Nicaragua
– Jeanne, 2004
• 1,500+ killed in Haiti
– Floyd, 1999
Hurricane Floyd
Hurricane Floyd at Landfall
Inland Flooding: Hurricane Floyd
Inland Flooding: Hurricane Floyd
Hurricane Floyd vs. May 3 Tornadoes
• 56 people died in the US due to Floyd
– 48 (or 86%) due to drowning in inland freshwater
flooding
– Vehicle deaths: 31 people (25 men)
Hurricane Floyd in NC
vs.
May 3rd Tornadoes in OK
$3 billion in damage
…
$1.2 billion in damage
7,000 homes destroyed
…
2,314 homes destroyed
56,000 homes damaged
…
7,428 homes damaged
56 deaths
…
36 deaths
Tornadoes
• Occur in land-falling tropical cyclones
– Typically in spiral bands and right-front
quadrant
• Very difficult to predict … need:
– Vertical shear
• Remember – vertical shear is *detrimental* to a
tropical cyclone!
– Higher-than-normal instability
Examples from 2005:
Katrina
Examples from 2005:
Rita
Examples from 2005:
Rita
Examples from 2004:
Charley
Examples from 2004:
Charley
Examples from 2004:
Frances
Examples from 2004:
Frances
Examples from 2004:
Frances
Examples from 2004:
Frances
Examples from 2004:
Ivan
Examples from 2004:
Ivan
Examples from 2004:
Ivan
Tropical cyclone fundamentals
• TC originates over tropical oceans and is driven
principally by heat transfer from the ocean
– Almost always develop over open ocean water whose temp > 26C
• Behave as an approximately axisymmetric vortex
– Wind max ~10-100km from center, then decaying as r -1/2
– Max upward vertical velocity ~5-10 ms-1
• In eye, sinking air ~ 5-10cms-1
• Boundary Layer Equations:
Source:
Smith 2003
Basic energetics
• Recognized early on (Riehl 1950 and
Kleinschmidt 1951) that TC energy source is
heat transfer from ocean
– Charney and Eliassen (1964) caused 15-yr “hiccup”
by proposing an alternate energy source [ CISK ]
• Current research has reverted to earlier theories (and
expanded them), e.g., Emanuel’s (1986) air-sea interaction
• Flux of momentum (into the sea)
• Flux of enthalpy (from the sea)
Resulting wind equation:
• Surface wind is function of:
– Ratio between transfer coefficients (to be revisited
shortly)
– Thermodynamic efficiency
– Enthalpy disequilibrium from ocean to air
• This disequilibrium forms the core of air-sea interaction
theory!
Interesting side note . . .
• Dissipative heating
– Sink of energy from the boundary layer
• If vertically integrate this equation to obtain
the kinetic energy dissipation …
– Average TC dissipates 3 x 1012 W
• Equal to rate of power consumption in the US in
the year 2000!
Positive feedback mechanism
• Increased wind 
greater enthalpy flux
Fk  increased
winds  greater
flux, etc etc
• Limit on intensity:
when dissipation
(which grows as
cube of velocity)
reaches equilibrium
with enthalpy flux
Surface drag coefficient CD
• CD is a function of:
– Z0, U10
– wave age
• ratio of local friction
velocity to phase
speed of dominant
spectral component
– wave steepness
• As CD decreases,
Vmax increases
• Commonly-used linear formulation
based on Large and Pond (1982):
Sea state in very high wind speeds
Source: Emanuel
2003
Courtesy NWS
Chicago
• Role of sea
spray in the
hurricane
boundary layer
– Not understood
– Never been
studied (until
very recently)
46 ms-1
Source: Powell 2003
55 ms-1
GPS dropwindsondes (1998-pres):
vertical wind profiles
• Resembles “log wind profile” in lowest 100-200 m
• Decreases w/height above ~300 m
Source: Franklin et al. 2003
Vertical wind profile
(as a percentage of the 700mb flight-level wind)
• Notes:
– Wind max occurs between
300 and 600 m and
decreases above that
• Coincident with thermal
wind balance of a warmcore cyclone (Bluestein
vol. 1, p. 187-88)
– Mitch, the strongest
cyclone of the group, had
lowest wind max
• Agrees with theory of
reduced roughness length
in high wind environments
Source: Franklin et al. 2003