1/20 1. tropical cyclones: fundamentals and basic...
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• Definitions and naming
• Life cycle
• Structure
• Processes
• Broadscale influences
1. Tropical Cyclones: Fundamentals and basic processes
Image courtesy of BoM/JMA
Should you use these resources please acknowledge the Bureau of Meteorology.
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A low pressure system that forms over warm waters having organised convection and gales near the cent re
+ Australia: extending more than half way around the system centre and persisting for at least six hours.
What is a TROPICAL CYCLONE ?
Modis Image of Pam courtesy of NASA
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Text• Click to edit Master text styles
– Second level
Tropical Cyclones
Tropical CycloneSevere Tropical Cyclone
Tropical StormHurricane
Tropical StormTyphoon
Same thing … different names
From http://earthobservatory.nasa.gov/Newsroom/NewImages/images.php3?img_id=17447
Hurricane, Typhoon, Severe Tropical Cyclone (sustained winds >= 64 knots).
(Non-severe) tropical cyclone, tropical storm (winds >= 34 knots, < 64 knots)
Tropical cyclone is generic term for Tropical Revolving Storm
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Cyclone Names: by regionAustralia; SPAC (FMS); PNG; BMKG; JMA (NWPAC)
http://www.bom.gov.au/cyclone/about/names.shtml http://www.vmgd.gov.vu/vmgd/index.php/forecast-division/tropical-cyclone
http://severe.worldweather.org/tc/au/tcname.html http://severe.worldweather.org/tc/sp/tcname.html
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The life cycle of a cyclone:genesis, maturing, weakening, decay
Every cyclone is unique!
Hurricane Earl Aug 2010Image courtesy of CIMSShttp://tropic.ssec.wisc.edu/storm_archive/montage/atlantic/2010/EARL10-notrack.gif
brief Vs long, weak Vs strong, small Vs big,impacts
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The life cycle of a cyclone
Examples CIMSS
March 2015 http://tropic.ssec.wisc.edu/archive/data/stettner/11MAR15/11MAR15.html
What do you notice?
Hires Himawari shows variations over shorter time scales
ST Noul (May15) http://cimss.ssec.wisc.edu/goes/blog/wp-content/uploads/2015/05/150509-
10_himawari8_visible_band3_STY_Noul_anim.gif
Yasi http://www.bom.gov.au/cyclone/history/yasi-satellite.shtml
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Anatomy of a tropical cycloneinner and outer circulations
Eye
Eye
wall
Rain
bands
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Technical parameters
R64
Radius of maximum winds(RMW)
R48
R34
Intensity: max wind, central pressureSize: Gale radius, ROCI (POCI)
Radius of Outer Closed Isobar (ROCI)
R34 asymmetry
Image courtesy of BoM/JMA
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Cat. Max wind Wind Impact No. (kn)1 34-47
2 48-63
3 64-85
4 86-106
5 > 106
“Destructive” winds Moderate
SEVERE Major
“Very destructive” Tracy, Yasi*
EXTREME Monica
“Damaging” winds Minor
Australian Intensity Scale
http://www.bom.gov.au/cyclone/about/#severity
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Importance of Size
R34: 'midget' <60nm; ave 80-100nm; large >120nmEye diameter: ~5-20nmRMW: 5-30nm;
Size: warning area, duration, waves, surge, spin dow n rate
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Simplified: Cyclones as heat engines
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Cyclones as heat engines
IN, UP and OUT
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TC Meteorology Key Terms
IN, UP and OUT
Convergence &Vorticity (IN), Convection (UP), Outfl ow (OUT)
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The 3 Dimensional Wind StructureWhich one has the highest winds?
VT VR ω
Tangential wind Vertical wind Radial wind
tangential radial vertical
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Idealised picture:intensification of winds at low levels
Basic principle: conservation of absolute angular momentum:
r
v
If r decreases, v increases!
Spin up requires radial convergence
12
Mv fr
r= −
M
M
212M rv fr= + f = Coriolis parameter
r = radiusv = tangential wind
Modified from R. Smith, Aspects of TC dynamics: Part 1 the Boundary Layer
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More realistic picture –effect of friction
FD = Frictionally driven inflowThe Planetary Boundary Layer is a momentum sink, Absolute Angular Momentum is not conserved
FD
rFr
P
r
vfv
dt
du +∂∂−+=
ρ12
Acceleration of the radial wind
fv = Coriolis force onrotating wind
v2/r = centrifugalacceleration
Acceleration due toradial Pressure gradient
Fr = small scale mixing
Frictionally driven secondary circulation
r
Top of boundary layer
Surface
Modified from R. Smith, Aspects of TC dynamics: Part 1 the Boundary Layer
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Circulations at different levels (streamlines and isotachs)
Low levels (Gradient-850 hPa)Boundary Layer- large scale inflow- convergence not uniform- max winds near core
Mid levels (700-400 hPa)- the ‘steering’ level
Upper levels
(100-300 hPa)
-cyclonic core for strong TC
- peripheral outflow as anticyclonic (peripheral ridge)
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upward spiralling air in the core spreads out with height (it diverges) & slows.
Cyclonic movement decelerates, so 0 tangential velocity ~ 200km from the centre of the TC.
Anticyclonic upper air movement builds a peripheral ridge (Ri)
Away from the core winds are the prevailing (environmental) upper winds.
Upper level behaviour –the anticyclonic outflow
Vorticity = rotation of air around a vertical axis.
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The inner & outer regions of a TC
Inner region (0-100 km) – Convection dominates•Large absolute vorticity, small radius of rotation
•Coriolis effect small – cyclostrophic balance
•Inertially very stable (will resist changes in radial displacement of winds by the environment)
•Very symmetric (does not interact much with surroundings)
•Winds adjust to changes in the mass field (heating/cooling, convergence/divergence will lead to changes in the wind).
Outer region (100-600 km) environmental infl.•smaller absolute vorticity, larger radius•Coriolis effect significant – gradient balance.•not so symmetric – influenced by environmental flow (eg monsoon, STR)•mass adjusts to the wind field
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Summary
• Defined TCs and naming convention
• Simplified view of TC engine: IN-UP-OUT
• Key terms convergence, convection, vorticity, outflow
• The strongest winds are tangential winds, and are located in the eyewall and within the boundary layer
• Complex dynamics and processes within TCs