Chap 5 5 tropical cyclones tc terme g n rique signifiant d pression tropicale
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Chap. 5.5 Tropical cyclones (TC) (terme générique signifiant dépression tropicale). Source : d’après Chris Landsea et le site internet de la NOAA http://www.nhc.noaa.gov. Tropical cyclone with maximum sustained winds of less than 17 m/s (34kt, 39 mph) are called ‘tropical depression’

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Chap 5 5 tropical cyclones tc terme g n rique signifiant d pression tropicale
Chap. 5.5Tropical cyclones (TC) (terme générique signifiant dépression tropicale)

Source : d’après Chris Landseaet le site internet de la NOAA http://www.nhc.noaa.gov

  • Tropical cyclone with maximum sustained winds of less

  • than 17 m/s (34kt, 39 mph) are called ‘tropical depression’

  • Tropical cyclone reaching 17m/s are called ‘ tropical storm’

  • and are baptized.


Chap 5 5 tropical cyclone
Chap. 5.5Tropical cyclone

Dina, 22/01/2002

image infrarouge colorée.

Source : Météo-France

Dina, 22/01/2002

  • If winds reach 33 m/s (64 kt, 74 mph), tropical cyclones

  • are called :

    • ‘hurricane’ (N. Atlantic, NE Pacific east of dateline,

    • South Pacific east of 160E)

    • ‘typhons’ (NW Pacific west of dateline)

    • ‘severe tropical cyclone’ (SW Pacific west of 160E,

    • SE Indian Ocean east of 90E)

    • ‘severe cyclonic storm’ (N. Indian Ocean)

    • ‘tropical cyclone’ (SW Indian Ocean)


Chap 5 5 tropical cyclone omm classification
Chap. 5.5Tropical cyclone : OMM classification

Sustained wind speed are used for classification

of tropical cylones;

Threshold are the same all over the world, but the

mean of the wind is realized over a period of 1 mn

over USA and their survey zones (Atlantic N. and

Pacific N.) and 10 mn elsewhere

échelle Beaufort

et vitesse du vent (kt)

7B

8/9B

10/11B

12B

64

34

115

91

48

Baptism threshold (only lettre of Alphabet

  • This scale is used around the world except for hurricanes

  • of N. Atlantic North. and NE Pacific where they use

  • SAFFIR scale.

  • Each year, 85 Tropical storm occurred whose 9 over

  • northern Atlantic


Chap 5 5 tropical cyclone1

Which difference between extra-tropical cyclone

and tropical cyclone ?

Chap. 5.5Tropical cyclone

Source : Merrill, 93

  • Extra-tropical cyclone : storm system that primarly gets

  • its energy from horizontal gradient temperature. They are

  • called mid-latitudes or baroclinic storms and low pressure

  • systems are associated with cold fronts, warm fronts, and

  • occluded fronts

  • Tropical cyclones, in contrast, typically have little to no

  • gradient horizontal temperature across the storm at the

  • surface and winds are derived from release of energy due

  • to cloud/rain formation from the warm moist air

5.6.2 :struct. hurricane


Chap 5 5 tropical cyclone2
Chap. 5.5Tropical cyclone

6 conditions necessary for the development of tropical storms (Gray, 79)

3 thermodynamical conditions :

1. SST>26.5° (80°F) over 50 m. at least (summer, early fall)

2. Atmosphere is conditionnaly unstable (at least, at the early stage of the TC); so no occurrence with trade inversion

3. Hu>70 % between 700 and 500 hPa

3 dynamical conditions :

4. Coriolis force; outside of 3-5° latitude (usually between

5 and 25°)

5. A strong disturbance = weak low with cyclonic circulation

6. Very little vertical shear (surface easterlies and

upper tropospheric easterlies)

usually S<12 m/s between surface and upper troposphere


Chap 5 5 tropical cyclone3
Chap. 5.5Tropical cyclone

These 6 conditions necessary for initiation

of tropical storm explain the spatial distribution

Source : d’après Gray, 1979

  • No initiation

  • over land

  • inside the equatorial zone (5°N/5°S)

  • over S. Atl. and Pacific SE (no ITCZ and SST too cold)

  • why no initiation over Central Pacific ? Answer, next slide


Chap 5 5 tropical cyclone4
Chap. 5.5Tropical cyclone

Mean vertical shear between 850 and 200 hPa in august

Source : d’après Gray, 1968

‣ The strong vertical shear (20-40 kt)over Central

Pacific prevents initiation of Tropical Storm

contents chap.5


Chap 5 5 tropical cyclone5
Chap. 5.5Tropical cyclone

Annual frequency of Tropical storm

Sources : Gray 68, Allard 84, Basher 95, Holland 84a, Holland 84b, Holland 84c, McBride 81a, McBride 82


Chap 5 5 tropical storms over north atlantic
Chap. 5.5Tropical storms over North Atlantic

Source : D’après Atkinson, 1971.

9 per year


Chap 5 5 tropical storms over eastern north pacific
Chap. 5.5Tropical storms over Eastern North Pacific

17 per year

Source : D’après Atkinson, 1971.


Chap 5 5 tropical storms over western north pacific
Chap. 5.5Tropical storms over Western North Pacific

Source : D’après Atkinson, 1971.

27 per year


Chap 5 5 tropical storms over southwest pacific
Chap. 5.5Tropical storms over Southwest Pacific

Source : D’après Atkinson, 1971.

5 per year


Chap 5 5 tropical storms over north indian ocean
Chap. 5.5Tropical storms over North Indian Ocean

Source : D’après Atkinson, 1971.

4.5 per year

Why not TC in july

-august while SST

is the highest ?


Chap 5 5 tropical storms over north indian ocean1
Chap. 5.5Tropical storms over North Indian Ocean

Mean vertical shear between 850 and 200 hPa in august

Source : d’après Gray, 1968

‣ strong vertical shear > 40 ktoverNorth Indian Ocean

in august (SW monsoon flow in surface, Tropical Easterly

Jet at 200 hPa) prevents initiations of tropical storms


Chap 5 5 tropical storms over southwest indian ocean
Chap. 5.5Tropical storms over Southwest Indian Ocean

Source : D’après Atkinson, 1971.

13 per year


Chap 5 5 tropical storms over southeast indian ocean
Chap. 5.5Tropical storms over Southeast Indian Ocean

Source : D’après Atkinson, 1971.

10 per year


References 1

  • - Allard, R. A., 1984 : ‘A climatology of the characteristics of tropical cyclones in the Northeast Pacific during the period of 1966-1990’. Master of Science Thesis, Texas Tech. Univ., Lubbock, TX, 106 p.

  • Atkinson, G. D., 1971 : Forecaster’s guide to tropical meteorology. USAF Air Weather Service, Technical Report N°240, 364 p.

  • Basher, R. E. and Zheng X. Z., 1995 :Tropical cyclones in the Southwest Pacific : Spacial patterns and relationships to Southern Oscillation and sea surface temperature’. J. Climate, Vol.8, p.

  • 1249-1260

  • Gray, W. M., 1968 : ‘Global view of the origin of tropical disturbances and storms’. Mon. Wea. Rev., Vol. 96, p .669-700

  • Gray, W. M., 1979. Hurricanes : Their formation, structure and likely role in the tropical circulation. In ‘meteorology Over the Tropical Oceans’ (D. B. Shaw, ed.), p. 151-218. Royal Meteorological Society, London.

  • Holland, G.J., 1984a : ‘On the climatology and structure of tropical cyclones in the Australian/Southwest Pacific Region. I. Data and tropical storms. Austra. Meteor. Mag., 32, p.1-16

  • Holland, G.J., 1984b : ‘On the climatology and structure of tropical cyclones in the Australian/Southwest Pacific Region. II. Data and tropical storms. Austra. Meteor. Mag., 32, p.17-32

  • Holland, G.J., 1984c : ‘On the climatology and structure of tropical cyclones in the Australian/Southwest Pacific Region. III. Data and tropical storms. Austra. Meteor. Mag., 32, p.33-46

References (1)


References 2

  • McBride, J.L., 1981a :’observational analysis of tropical cyclone formation. Part I. Basis definition of data sets. J. Atmos. Sci., Vol.38, p. 1132-1151

  • McBride, J. L. and T. D. Keenan, 1982 : ‘Climatology of tropical cyclone genesis in the Australian region’. J. Climate., Vol.2, p.13-33

  • 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

References (2)