Tropical Cyclones - (ARC) Centre of Excellence for Climate System

Tropical Cyclones
Elizabeth Ritchie
School of Physical, Environmental, and Mathematical Sciences
UNSW – Canberra
E.Ritchie@adfa.edu.au
Outline
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Climatology
Mature Tropical Cyclone Structure
Movement
Intensification Mechanisms
Genesis
Extratropical Transition
Landfall Impacts
Climatology
1851-2006
WPAC
CPAC
EPAC
NATL
NIO
SIO
Australian
Region
SPAC
Climatology
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On average 80 tropical
cyclones (> 17m/s surface
sustained winds) form globally
every year (Gray 1968)
as few as 69 and as many as
103 have formed in one year.
Formation locations are tied to
warmest ocean temperatures
Regional seasonality is linked
to the regional seasonal
variation of ocean
temperatures
Maximum activity in late
summer, early autumn when
the ocean heat content is
maximum
Climatology
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NORTHERN HEMISPHERE (Jul-Sep)
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North Atlantic & Gulf of Mexico:
• peak in activity ~ 10 Sep
• ~ 12.1/yr [1,25]
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Eastern & Central North Pacific:
• Densest TC activity (per km2)
• 15-16/yr
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Western North Pacific:
• Most active basin
• TCs in all months
• ~26/yr
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North Indian Ocean:
• Deadliest TCs (Bay of Bengal)
• ~4/yr
Climatology
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SOUTHERN HEMISPHERE (Jan-Mar)
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Australian/SEIO:
• ~11/yr
• Double peak in activity – lull
as monsoon moves over
Australian continent
• Variability from ENSO
•
SPAC:
• 7-8/yr
• Mostly west of dateline except
in El Niño periods
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SWIO:
• ~9/yr
• Rain records held by TCs
making landfall in LaReunion.
Mature Tropical Cyclone Structure
Hurricane Katrina (2005)
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Standard features of a mature
tropical cyclone
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From satellite:
• Eye
• Eyewall
• Rainbands
• Cirrus outflow
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From radar:
• Eye
• Eyewall
• Spiral Rainbands
• Convective Structure
• Moat Regions
TC Yasi (2011)
Mature Tropical Cyclone Kinematic Structure
Hurricane Gilbert (1988)
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First-order approximation:
• Axisymmetric (k=0)
• Asymmetries superposed (k>0)
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Cylindrical Coordinate system centred on the TC centre.
Azimuthally average
Decompose the wind field into tangential and radial
components.
Radial-height cross-sections
•
Composite Cyclone
ࢂ࢘
ࢂࣅ
ࢂ
Mature Tropical Cyclone Kinematic Structure
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Primary Circulation
• Tangential component of wind
• Strongest component
• Near-zero at the centre
• Increase linearly to radius of
maximum winds (RMW) just
outside the eyewall
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Hurricane Gilbert (1988)
Composite Cyclone
Decrease exponentially outside RMW
Secondary maximum in principle rainbands
Maximum winds near the surface
Fairly constant up to about 500 hPa (barotropic) then drop off rapidly with height
Cyclonic to upper levels begin to spiral out and turn anticyclonic at 100-200 km
radius in upper-levels.
Mature Tropical Cyclone Kinematic Structure
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Secondary Circulation - comprises
radial and vertical components
• Much weaker than primary
circulation
• Strong inflow in boundary layer
(response to friction)
• Weaker, deep layer of inflow
through middle levels
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Concentrated upper-level, near-tropopause outflow
Saturated ascent in the eyewall
Weak descent in moat regions and at far radius
Inflowing air loses angular momentum through friction, so after it rises in the
eyewall and turns outward, it swirls anticyclonically beyond 100-200 km radius
Secondary circulation supplies AM and thermal energy, which intensifies the
primary circulation and maintains it against frictional dissipation
Mature Tropical Cyclone Kinematic Structure
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TCs come in many different sizes and shapes
Note that the entire wind field of TC Tracy (cat
4 TC) almost fits inside the RMW of
Supertyphoon Tip
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Size doesn’t tell the whole story as Tracy, in
spite of its midget size, wiped out the heart of
Darwin with a direct hit
Extreme sizes:
Midget Cyclone Tracy: 100 km
Super Typhoon Tip: 2220 km
Average TC Kerry: 550 km
Mature Tropical Cyclone Thermal Structure
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In cylindrical coordinates:
•
The Thermal wind equation can be written:࢜ࣅ ૛
૒઴
൅ ࢌ ࢜ࣅ ൌ
࢘
૒࢘
•
ࣔ
Which can be re-written by talking ࣔࢠ of both sides and substituting
ࣔ઴
ࣔࢠ
ൌ
ࡾࢀ
ࡴ
as
૛࢜ࣅ
૒࢜ࣅ
ࡾ ૒ࢀ
൅ ࢌ
ൌ
ࡴ ૒࢘
࢘
૒ࢠ
•
For ࢜ࣅ with z , T with r . That is, the TC is warmest at the centre warmcore system.
Mature Tropical Cyclone Thermal Structure
Furthermore …
૛࢜ࣅ
૒࢜ࣅ
ࡾ ૒ࢀ
൅ ࢌ
ൌ
࢘
૒ࢠ
ࡴ ૒࢘
ฺ
ࣔࢀ̱
૛࢜ࣅ
ࣔ࢜ࣅ ࣔ࢘ ࡴ
൅ࢌ
࢘
ࣔࢠ
ࡾ
Scale Analysis: L ~ 100 km, f ~ 5 x 10-5 s-1, U ~ 50 m s-1.
ࢾࢀ ̱
૛ࢁ
൅ࢌ
ࡸ
ࢁࡴࡸ
ࡴࡾ
̱
૛ ൈ ૞૙
൅ ૞ ൈ ૚૙ି૞
૚૙૙૙૙૙
૞૙ ൈ ૚૙૙૙૙૙
̱ ૚ૠԨ
૛ૡૠ
Mature Tropical Cyclone Thermal Structure
Hurricane Hilda (1964)
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Perturbation ૚૟Ԩ
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interpolated from
flight-level data
above and below
(dashed lines)
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Low surface
pressure at center
hydrostatically
balanced by warm
core aloft
Hawkins and Rubsam 1968
Mature Tropical Cyclone Thermal Structure
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In General:
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Highest temperature perturbation
located near 200-300 hPa
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Cold anomaly above
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Thermal wind balance tells us that the
strongest winds must be near the surface
and decrease with height – as we have
already observed
Mature Tropical Cyclone Kinematic Structure
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Recall: The Rossby number can be used to assess the relative importance of the
Coriolis force:
Centrifugal Acceleration
‫ݒ‬ఒ ଶ Τ‫ݎ‬
‫ݒ‬ఒ
ܴ଴ ൌ
ൌ
݂‫ݎ‬
݂ ‫ݒ‬ఒ
Coriolis Acceleration
The flow is in:
gradient balance if
R0 ~ 1
- TC outer region
‫ݒ‬ఒ ଶ
ͳ ߲‫݌‬
൅ ݂ ‫ݒ‬ఒ ൌ
ߩ ߲‫ݎ‬
‫ݎ‬
cyclostrophic balance if
R0 >> 1
- TC core region (inside RMW)
geostrophic balance if
R0 << 1
- Environment.
݂ ‫ݒ‬ఒ ൌ
‫ݒ‬ఒ ଶ
ͳ ߲‫݌‬
ൌ
ߩ ߲‫ݎ‬
‫ݎ‬
ͳ ߲‫݌‬
ߩ ߲‫ݎ‬
Movement
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Generally initially steered by
deep tropospheric easterly
winds – so move to the west
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If in the deep tropics then
westward movement is typical
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If slightly further poleward, then
can get steered around the
subtropical high and recurve into
the midlatitude westerlies –
head back to the east.
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In NIO tracks tend to be directly
northward
Movement
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And sometimes they just do seem
to do really weird things.
Movement
Some basic controls on motion include:• Environmental Steering:
• to first order this is a deeplayer mean of the
environmental winds within
about 1000 km
• Interactions with other
weather systems:
• can consider weather systems
as vortices and approximate
vortex interactive behaviour
Movement
• The Earth vorticity gradient:
• Puts a northwest (southwest)
motion on the TC in the Northern
(Southern) hemisphere
• Convective asymmetries in the
૚ૠԨ
TCs own circulation:
• low-level convergence associated
with strong convection induces
positive vorticity tendencies. TC
moves toward positive vorticity
tendencies – small track
deflection
Tropical Cyclone Intensification
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It is fairly well accepted that tropical cyclones rely on ocean fluxes as their energy
source – this is why activity is maximised in the late summer early autumn
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The primary circulation
is maintained/intensified
against the effects of
frictional dissipation
because of the high ࣂࢋ
air brought in via the
secondary circulation,
lifted in convection and
then expelled in the
outflow layer.
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As this air flows in the inflow branch, angular momentum is lost to friction, and heat
is lost to adiabatic expansion. If this energy is not replaced somehow, then the air
that is lifted in deep convection does not add much energy to the mid-to-upper
troposphere.
Tropical Cyclone Intensification
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As the air parcels adiabatically expand and cool in the inflow layer, they pick up
energy from sensible and latent heat fluxes from the warm ocean
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When they arrive at the
eyewall they have very
high ࣂࢋ values – above
the ambient tropics
This air is then lifted in
deep convection. The
latent heat release that
occurs just offsets
adiabatic expansion and
adds little heat to the
upper atmosphere
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The high ࣂࢋ air is lifted nearly moist adiabatically and for values of ~365K can
produce a central pressure of ~960 hPa.
Tropical Cyclone Intensification
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A second mechanism kicks in at about 33 m/s – eye formation
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Back of the envelope
calculations suggest that
a subsidence of
w ~ -2 cm/s
can produce a net
warming of
൅ૡǤ ૟૝ ι࡯Ȁࢊࢇ࢟
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This subsidence also acts
to lower the maximum
warm core from the
tropopause (100 hPa) to
200-300 hPa as we saw
in TC Hilda.
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The additional warming results in much lower surface
central pressures via hydrostatic balance than would
otherwise be possible.
Tropical Cyclogenesis
Stages of Intensity
Tropical
Disturbance
???
1.
2. Tropical
Depression
< 17 m/s
Genesis:
A mixture of large-scale
(environmental), mesoscale,
and convective-scale
processes interact
3. Named Tropical
Cyclone
17 - 33 m/s
4. Intense/Severe
Tropical Cyclone
> 33 m/s
Intensification/Steady-state maintenance:
WISHE
Tropical Cyclogenesis
30 N
20 N
10 N
90 W
75 W
60 W
45 W
Tropical Cyclogenesis
Large-scale conditions necessary for genesis (Gray 1968):
Thermodynamic Conditions:
x Sufficient ocean thermal energy
x Enhanced mid-tropospheric relative humidity
Thermodynamic
conditions for
deep convection
x Conditional instability
Dynamic Conditions
x weak vertical shear at the genesis location
x at least 5° latitude away from the equator
x Enhanced lower tropospheric relative vorticity
Deep Convection
Non-zero Coriolis
Pre-existing disturbance
x Transient large-scale circulations can enhance the environment for
genesis – MJO, Equatorial waves (Kelvin, Rossby, inertia-gavity)
Tropical Cyclogenesis
Types of “parent” disturbances:
x Monsoon trough – provides
horizontal shear vorticity
environment
x Monsoon gyre – provides
both horizontal shear vorticity
and rotational vorticity
x ITCZ – shear vorticity – can
become unstable and
spontaneously break down
Tropical Cyclogenesis
Types of “parent” disturbances:
x Easterly waves – weak rotational
vorticity strongest at 600-700 hPa
x Common in the North Atlantic
x African easterly waves are
triggered by an instability on the
low-level jet that forms as a
result of the reversal of the largescale PV gradient over the Sahara
x Wavelength: ~1500-2000 km
Tropical Cyclogenesis
n=1 Equatorial Rossby Wave:
x Triggered by large-scale persistent
heating in the tropics
x westward phase speed of ~ 5 m/s
x Dispersive
x Wavelength: 2000-4000 km
x associated with twin TC developments
straddling the equator
Tropical Cyclogenesis
Two-stage process to genesis:
• Stage 1: Preconditioning of the
environment.
• Stage 2: Mesoscale development.
Tropical Cyclogenesis
• Key questions:
1. Reduction of the Rossby radius of
deformation, LR
the kinematic response to a cumulus cloud is to excite gravity
waves that disperse the energy to LR. The rotational wind
response to the heating in the cloud is at the scale of LR.
Mean tropical Rossby Radius is ~2000 km whereas the TC core
is < 100 km.
Scale problem
Tropical Cyclogenesis
• Key questions:
2. Disorganised convection to
organised convection (Ooyama 1960s)
for an individual air mass cumulus
cloud, the downdrafts on the edge of
the cloud, and finally within the cloud
itself offset, and may even dominate,
the updraft effect that produced
boundary layer convergence and a spin
up of low-level vorticity.
Tropical Cyclogenesis
There is both a kinematic and thermodynamic cooperation that is
the key to this question.
- Convection acts to moisten the lower and middle levels of the
atmosphere
- As the convection becomes more long-lived in the form of MCSs
then a dynamic response occurs within the saturated midlevels of
the MCS – a midlevel vortex (100-200 km) (Chen and Frank 1993)
- Acts as an organizing feature for more convection- continued
moistening reduces the downdrafts so that surface convergence
is maintained and gradually enhanced (number of refs)
- Process can happen very quickly (within 24 hours) or as a pulsing
process over several days depending on the supporting
background environment
Tropical Cyclogenesis
Example: TS Ofelia (1993) - Flight centered on 0000 UTC 23 July 1993
Potential Vorticity along 16 㼻N
Divergence along 16 㼻N
Tropical Cyclogenesis
TS Ofelia (1993)
500 mb
700 mb
850 mb
Tropical Cyclogenesis
TS Ofelia (1993) -
Flight centered on 0800 UTC 24 July 1993
Potential Vorticity along 17 㼻N
Relative Vorticity along 17 㼻N
Tropical Cyclogenesis
Re-initiation of
Convection …
TS Ofelia (1993) Subsequent
to 2nd flight…
Voila … TS Ofelia!
Extratropical Transition
STY Chataan 2002
0709/00Z
0709/12Z
0710/12Z
SLP Rising
973 mb
0711/12Z
TC Dissipating
982 mb
STY Phanfone 2002
0818/12Z
969 mb
0819/12Z
SLP Rising
0820/12Z
985 mb
ETC Intensifying
00821/00Z
~ 975 mb
Extratropical Transition
These systems generate heavy seas and very large waves (speed of
translation resonates with maximum winds to generate large swell)
→ impacts shipping, for example…
• The QEII encountered a rogue 100’ wave in an ET
(Hurricane Luis) system in 1995.
• The “1991 Halloween Storm”
(Hurricane Gladys and strong midlatitude trough off U.S. east
coast in 1991) – loss of a fishing trawler and all on board.
• The 1998 Sydney-Hobart yacht race
Sub-tropical system and rapidly developing trough between Victoria
and Tasmania. Loss of 7 yachts, 57 sailors rescued, 6 died. 44 out
of 115 completed the course. 40 m/s (90 mph) winds and over 80’
waves.
Landfall and Impacts
Tropical Cyclones are socially and economically
the most destructive natural phenomena on the globe
Considering all natural hazards, they account for*:
• 14% of natural disasters
• 26% of natural disaster economic damage
Considering all meteorological hazards, they account for:
• 48% of meteorological disasters
• 69% of human impact of meteorological disasters
Finally, 77% of the disasters associated with TCs in the last decade (EM-DAT):
• were associated with heavy rain, flooding, and associated impacts
* Data for 1990-2013 (Guha-Sapir et al. 2013; World Bank 2014, International Disaster
Database, 2014)
Landfall and Impacts
Definition of landfall:
when the center of the storm moves across the
coast.
- in strong TCs this is when the eye moves over
land.
> 63 m/s
> 50 m/s
- The severest wind impacts of these systems
are concentrated near the eyewall.
Landfall and Impacts
BUT … damaging winds can extend more than
a hundred kilometres from the eye
Storm surge can extend several hundred
kilometres
Tornadoes develop typically in outer
rainbands as they come on shore.
Large amounts of rain from the
core and rainbands can continue to
fall far inland of landfall. If
combined with steep terrain,
significant flooding and mudslides
can develop
> 63 m/s
> 50 m/s
> 33 m/s
> 25 m/s
> 17 m/s
Landfall and Impacts
Wind
Storm surge
Rain
Landfall and Impacts
Flooding
TC Oswald (2013) – Australia
TY Saola (2012) – Philippines Cyclone Nargis (2008) – Myanmar
Land/Mudslides
H. Mitch (1998) – Nicaragua
H. Mitch (1998) – Nicaragua
TS Manuel (2013) – Mexico
Landfall and Impacts
Landfall and Impacts
Global Landfalling TC Impacts
*1991 and 2008 excluded
Thank You!!!
Elizabeth Ritchie
School of Physical, Environmental, and Mathematical Sciences
UNSW – Canberra
E.Ritchie@adfa.edu.au