Chap. 5.6 Hurricanes

5.6.1 Hurricane : introduction

5.6.2 Hurricane structure

5.6.3 Hurricane : theory

5.6.4 Forecasting of hurricane

Sommaire chap.5

sommaire

5.6.2 Hurricane structure

• Cloudy rolling of deep organized convection , with nearly circular shape, which can extent to 1000 km of diameter, generally associated with an eye, without frontal system, and occurred in the summer hemisphere

• Cyclone with hot core

• Balanced fluid (Rossby radius of 20 km)with - cyclostrophic equilibrium < 40 km

- gradient wind balanced > 40 km

Source :Cyclone Edwina, le 28/01/93 à 0146 UTC;Satellite NOAA 10

• In low troposphere : The airstream is spiraling inwards the cyclone centre with :

- a tangential flow cyclonic : > 0 - a radial inflow whence convergence : < 0)

• Convection is organized into, long, narrow rainbands (5 to 50 km large), known as ‘spiral bands’ (visible through RADAR)

• Speed velocity, rain, Hu, increase as and when the airstream approaches the eyewall of the hurricane

Spiral bandsSpiral bands

rv

Vr < 0Vr < 0

v

Eyewall

0v0v

D

5.6.2 Hurricane structure

Northern Hemisphere

Source : from Stormfury, 1970

0rv0rv

5.6.2 Hurricane structure

H0v

• From a certain distance of the cyclone centre (from 10 to 40 km), the airstream is suddenly rejected upwards = initiation of the eyewall

• In upper troposphere ⒈one part of the airstream is spiraling outward the eyewall with :

- a tangential flow anticyclonic : < 0 - a radial outflow whence divergence : > 0

⒉the other part of the airstream is subsiding inward the eyewall originating the eye

v

rv

Northern Hemisphere

Source : from Stormfury, 1970

Eyewall : maximum intensity of hurricane with ‣ circle of CB with top around 12 to 15 km and

torrential rain‣ maximum of horizontal and vertical wind

: radial and vertical section

: center hurricane

5.6.2 Eyewall structure

Source : Cyclone Edwina, le 28/01/93 à 0146 UTC; Satellite NOAA 10

• Horizontal scale of the eyewall ~ 10 to 20 km large

• The eyewall tilts outwards with altitude : ‣ the max. of horizontal wind is tilted outwards ‣ the max. of vertical wind is tilted outwards (-7 to –9 m/s) ‣ the precipitations fall outwards the cone

• Bigger the eye is, more tilted the cone is

5.6.2 Eyewall structure

Source : from Jorgensen, 1984

Eye

5.6.2 Eye structure

Source : Cyclone Edwina, le 28/01/93 à 0146 UTC; Satellite NOAA 10

• Diameter of the eye ~ 30 to 60 km (mini 8 km , max. 200 km)

• strong subsidence (~ + 3 m/s) especially along the inward side of the eyewall and between PBL - upper tropo

• subsidence explains that eye temperature is warmer than the surrounding environment (10°C at 12km, 0 to 2°C at surface)

5.6.2 Eye structure

Source : from Jorgensen, 1984

• Weather associated : ‣ minimum of mean-sea level pressure observed at center ‣ no rain and light wind ‣ variable cloudiness : sky clear with cirrus but sometimes overcast with low clouds (Sc/St) when the low-level is moist with a thermal inversion above

• Shape of the eye : circular or elliptic : no relation or very light correlation established between size of the eye and intensity of hurricane

• Speed phase of the eye ~ 20 km/h

D

5.6.2 Eye structure

Chap 5.6.3

Source : from Jorgensen, 1984

eyewall

eyewall

eye

eye

Spiral bands

5.6.2 Hurricane structure : radar

Source : from Burpee and Marks, 1984

Radial wind Tangential wind

Horiz.wind

Bell-shapedCyclonic circulation

Anticycloniccirculation

Radial inflow

Radial outflow

som. chap.5

5.6.2 Structure du cyclone tropical

Source : from Burpee and Marks, 84.

Source : from Gray, 79

• For a rotating fluid, the wind can be divided into

For hurricanes, equations are written in cylindrical co-ordinates (r, λ ,z) around the cyclone centre (axis-symmetric flow).

rλ

z

e

re

01

0

2

r

pfv

r

v

t

vr

• Radial equation of motion (above PBL)

OutwardAccelerationnull

CoriolisForce

Pressureforce

Centrifugal force

rrevevv

: relative tangential wind component

: relative radial wind componentrvv

Outer eyewall (R>40 km)= gradient-wind balanced

01

0

2

r

pfv

r

v

Inner eyewall (R<40 km) = cyclostrophic balanced

r

p

r

v

0

2 1

5.6.2 Hurricane structure : a balanced fluid

r

λ

z

e

re

Outer eyewall (R>40 km)= gradient-wind balanced

r

pfv

r

v

0

2 1

5.6.2 Hurricane structure : a balanced fluid

Centrifugal Force

ieF

ieF

CoriolisForce

chF

chF

Pressureforce

pF

pF

NorthernHemisphere

V

Chap. 5.6.3

r

λ

z

e

re

Inner eyewall (R<40 km)= cyclostrophic balanced

r

p

r

v

0

2 1

5.6.2 Hurricane structure : a balanced fluid

Pressureforce

pF

pF

NorthernHemisphere

V

Centrifugal Force

ieF

ieF

‘Spiral bands’

5.6.2 Hurricane structure : spirals bands

Source : image from satellite NOAA10 of 28/01/93 at 0146TU. Cyclone Edwina over Indian Océan

5.6.2 Hurricane structure :upper outflow

Source : image issue du satelliteà défilement NOAA 10

Circular eye

Elliptic eye

5.6.2 Hurricane structure : radar

Source : d’après Burpee et Marks, 84

Vertical cross-section of temperature anomaly for hurricane Inez

+3° at surf.

+15° at 250 Hpa

5.6.2 Hurricane structure : hot core

Source : from Hawkins and Rubsam, 68.

References

- Burpee, R. W., and F. D. Marks, Jr., 1984 : Analyses of digital radar data obtained from coastal radars during Hurricanes David (1979), Frederic (1979), and Alicia (1983). Preprints, 10th Conference on Weather Forecasting and Analysis, Clearwater beach, Fla., American Meteorological Society, Boston, 7-14

-Gray, W. M., 1979 : ‘Hurricanes, their formation, structure , and likely role in the tropical circulation’. Pp. 155-218 in : Shaw, O. B., ed., Meteorology over the tropical oceans, Conference, August 1978, Royal Meteorological Society, Bracknell, 278 pp.

- Hawkins, H. F., and D. T. Rubsam, 1968 : Hurricane Hilda, 1964, II : Structure and budgets of the hurricane on October 1, 1964. Mon. Wea. Rev., 96, pp. 617-636.

- Jorgensen David P., 1984 : ‘Mesoscale ans convective-scale characterictics of mature hurricane , Pt.2, Inner core structure of hurricane ‘Allen’ (1980)’. J. of Atm. Sci., vol. 41, n°8, pp. 1287-1311.

- Merrill, R. T., 1993 : ‘Tropical Cyclone Structure’ –Chapter 2, Global Guide to Tropical Cyclone Forecasting, WMO/Tropical Cyclone- N°560, Report N° TCP-31, World Meteorological Organization; Geneva, Switzerland

- STORMFURY, 1970 : Projet international sur les cyclones tropicaux. NOAA Technical Memorandum ERL NHRL N° 95, 57 pages : Experiments which provide theorical guidance for project STORMFURY are summarized.