1. 5.6.1 Hurricane : introduction
Chap 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

2. 5.6.2 Hurricane structure
Source :
Cyclone Edwina, le 28/01/93 à 0146 UTC;
Satellite NOAA 10
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

3. 5.6.2 Hurricane structure D Spiral bands Eyewall In low troposphere :
Vr < 0
Northern Hemisphere
Source : from Stormfury, 1970
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

4. 5.6.2 Hurricane structure H
Source : from Stormfury, 1970
Northern Hemisphere
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

5. 5.6.2 Eyewall structure : center hurricane
: radial and vertical section
Source : Cyclone Edwina, le 28/01/93 à 0146 UTC;
Satellite NOAA 10
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

6. 5.6.2 Eyewall structure Source : from Jorgensen, 1984
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

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

8. 5.6.2 Eye structure Source : from Jorgensen, 1984
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)

9. 5.6.2 Eye structure D Weather associated :
Source : from Jorgensen, 1984 D
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
Chap 5.6.3

10. 5.6.2 Hurricane structure : radar
eye
eyewall
Spiral bands
Source : from Burpee and Marks, 1984

11. 5.6.2 Structure du cyclone tropical
Radial wind
Source : from Gray, 79
Tangential wind
Anticyclonic
circulation
Radial outflow
Bell-shaped
Cyclonic circulation
Radial inflow
Source : from Burpee and Marks, 84.
Horiz.
wind
som. chap.5

12. 5.6.2 Hurricane structure : a balanced fluid
For hurricanes, equations are written in cylindrical co-ordinates (r, λ ,z) around the cyclone centre (axis-symmetric flow).
For a rotating fluid, the wind can be divided into r λ z
: relative tangential wind component
: relative radial wind component
Radial equation of motion (above PBL)
Outward
Acceleration
null
Centrifugal
force
Coriolis
Force
Pressure
force
Outer eyewall (R>40 km)
= gradient-wind balanced
Inner eyewall (R<40 km)
= cyclostrophic balanced

13. 5.6.2 Hurricane structure : a balanced fluid
Outer eyewall (R>40 km)
= gradient-wind balanced z
Northern
Hemisphere r
Pressure
force λ
Coriolis
Force
Centrifugal
Force
Chap

14. 5.6.2 Hurricane structure : a balanced fluid
Inner eyewall (R<40 km)
= cyclostrophic balanced z
Northern
Hemisphere r
Pressure
force λ
Centrifugal
Force

15. 5.6.2 Hurricane structure : spirals bands
Source : image from satellite NOAA10 of 28/01/93 at 0146TU. Cyclone Edwina over Indian Océan
‘Spiral bands’

16. 5.6.2 Hurricane structure : upper outflow
Source : image issue du satellite
à défilement NOAA 10

17. 5.6.2 Hurricane structure : radar
Circular eye
Source : d’après Burpee et Marks, 84
Elliptic eye

18. 5.6.2 Hurricane structure : hot core
+15° at 250 Hpa
Source : from Hawkins and Rubsam, 68.
+3° at surf.
Vertical cross-section of temperature anomaly for hurricane Inez

19. 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 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, Mon. Wea. Rev., 96, pp
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
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.