1/23

• A bit about the course presenters…

• Objectives/Resources/learning material

• Discussion and Actions

• This is for you so make the most of it and engage

– ask when you are not sure!

Joe Courtney (left)

And

Andrew Burton (right)

Tropical Cyclones:

Introduction

2/23

• Definitions and naming

• Life cycle

• Structure

• Processes

• Broadscale influences

1. Tropical Cyclones:

Fundamentals and basic processes

Image courtesy of CIMSS

3/23

A low pressure system that forms over warm waters

having organised convection and gales near the centre

+ Australia: extending more than half way around the system centre and

persisting for at least six hours.

What is a TROPICAL CYCLONE /

CYCLONIC STORM?

Modis Image of Phailin 2013

courtesy of NASA

4/23

• Click to edit Master text styles

– Second level

Tropical Cyclones

Tropical Cyclone

Severe TC >63kn

Tropical Storm

Hurricane >63kn

Tropical Storm

Typhoon >63kn

Same thing … different names

From http://earthobservatory.nasa.gov/Newsroom/NewImages/images.php3?img_id=17447

Hurricane, Typhoon, Severe TC (SH), Very STC (NIO)(mean winds = 63 knots).

(Non-severe) tropical cyclone, tropical storm (winds >= 34 knots, < 64 knots)

Tropical cyclone is generic term for Tropical Revolving Storm

Depression 17-27kn

Deep Depression 28-33

Cyclonic Storm 34-47

Severe CS 48-63

Very SCS 64-89

Extremely SCS 90-119

Super CS >119

5/23

Source: http://www.wmo.int/pages/prog/www/tcp/Storm-naming.html

IMD: http://www.rsmcnewdelhi.imd.gov.in/images/pdf/cyclone-awareness/tc-names/tc-names.pdf

Cyclone Names?

6/23

The life cycle of a cyclone

Image courtesy of CIMSS http://tropic.ssec.wisc.edu/storm_archive/montage/atlantic/2010/EARL10-notrack.gif

Hurricane Earl Aug 2010

7/23

The life cycle of a cyclone

Examples CIMSS

March 2015 http://tropic.ssec.wisc.edu/archive/data/stettner/11MAR15/11MAR15.html

ST Noul (May15) http://cimss.ssec.wisc.edu/goes/blog/wp-content/uploads/2015/05/150509-10_himawari8_visible_band3_STY_Noul_anim.gif

Hudhud (Oct 2014) http://rammb.cira.colostate.edu/products/tc_realtime/loop.asp?product=1kmsrvis&storm_identifier=IO032014&starting_image=2014IO03_1KMSRVIS_201410090200.GIF&ending_image=

2014IO03_1KMSRVIS_201410131200.GIF

8/23

Anatomy of a tropical cyclone

Eye

Eye wall

Rain bands

Image courtesy of CIMSS

9/23

Technical parameters

R64

Radius of

maximum

winds

(RMW)

R48

R34

Intensity: max wind, central pressure

Size: Gale radius, ROCI (POCI)

Radius

of Outer

Closed

Isobar

(ROCI)

R34

asymmetry

Image courtesy of CIMSS

10/23

Importance of Size

R34: 'midget' <60nm; ave 80-100nm; large >120nm

Eye diameter: ~5-20nm

RMW: 5-30nm;

Size: warning area, duration, waves, surge, spin down rate

11/23

Simplified: Cyclones as heat engines

12/23

Cyclones as heat engines

IN, UP and OUT

13/23

TC Meteorology Key Terms

IN, UP and OUT

Convergence &Vorticity (IN), Convection (UP), Outflow (OUT)

14/23

The 3 Dimensional Wind Structure

Primary circulation Secondary circulations

VT VR

Tangential

Swirling

Azimuthal

Radial Vertical

15/23

Summary of TC circulations

http://apollo.lsc.vsc.edu/classes/met130/notes/chapter15/vertical_circ.html

height of 700 hPa

surface

Tangential wind Vertical wind Radial wind

16/23

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

212

M rv fr f = Coriolis parameter

r = radius

v = tangential wind

Modified from R. Smith, Aspects of TC dynamics: Part 1 the Boundary Layer

17/23

More realistic picture –

effect of friction

FD = Frictionally driven inflow

The Planetary Boundary Layer is a momentum sink, Absolute Angular Momentum is not conserved

FD

rFr

P

r

vfv

dt

du

12

Acceleration of the radial wind

fv = Coriolis force on

rotating wind

v2/r = centrifugal

acceleration

Acceleration due to

radial 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

18/23

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 (500 hPa)

- the ‘steering’ level

Upper levels (200 hPa)

-cyclonic core for strong

systems

- peripheral outflow as

anticyclonic (peripheral ridge)

Green = cyclonic

Red = anticyclonic curvature

19/23

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

20/23

Enhancing and inhibiting mechanisms

Enhancing mechanisms H = heating of upper part of eyewall

B = Buoyancy due to updrafts

C = Buoyancy/friction induced convergence

T = transfer of sensible heat from sea

SST = Sea Surface Temperature

SE = sloping eyewall

FC= freezing of updraft condensate

Inhibiting mechanisms Co = adiabatic cooling

Wu = warm and dry upper eye

F = inner core frictional dissipation

TME = upper level tangential momentum

export. (can also be an enhancing

mechanism)

E= eye wall entrainment

H H

B B C C Co Co T T SST SST

Wu SE SE

F F

TME

TME

TME

TME

E

FC FC

Diagram modified from http://apollo.lsc.vsc.edu/classes/met130/notes/chapter15/vertical_circ.html

E E

21/23

B B C C Co Co

T T SST SST F F

low levels : Energy driven by latent

heat release (condensation)

Enhancing mechanisms

B = Buoyancy due to updrafts

C = Buoyancy/friction induced

convergence

T = transfer of sensible heat from sea

SST = Sea Surface Temperature

Inhibiting mechanisms

Co = adiabatic cooling

F = inner core frictional dissipation

22/23

upward spiralling air in the eyewall

spreads out with height (it diverges).

Due to Coriolis force it loses cyclonic

vorticity and gains anticyclonic vorticity.

Cyclonic movement decelerates, so 0

tangential velocity ~ 200km from the

centre of the TC.

Anticyclonic upper air movement builds

a peripheral ridge (Ri)

At large distances from the centre, the

winds reaccelerate under the influence

of the prevailing upper winds.

Upper level behaviour –

the anticyclonic outflow

Vorticity = rotation of air around a vertical

axis.

23/23

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