Chap. 5.5 Tropical cyclones (TC) (terme générique signifiant dépression tropicale)

• 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.

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

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

Dina, 22/01/2002image infrarouge colorée.Source : Météo-France

Chap. 5.5 Tropical cyclone : OMM classification

échelle Beaufortet vitesse du vent (kt)

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 mnover USA and their survey zones (Atlantic N. andPacific N.) and 10 mn elsewhere

7B

34

8/9B 10/11B 12B

48 64 91 115Tropical

depression

Tropical

storm

Severe

Tropical

storm

Hurricane Severe

Hurricane

Very

Severe

Hurricane

• 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

Baptism threshold (only lettre of Alphabet

Which difference between extra-tropical cyclone and tropical cyclone ?

• Extra-tropical cyclone : storm system that primarly getsits energy from horizontal gradient temperature. They arecalled 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

Chap. 5.5 Tropical cyclone

5.6.2 :struct. hurricane

Source : Merrill, 93

Chap. 5.5 Tropical cyclone

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

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 thermodynamical conditions :

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 cyclone

These 6 conditions necessary for initiationof tropical storm explain the spatial distribution

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

Source : d’après Gray, 1979

Mean vertical shear between 850 and 200 hPa in august

‣ The strong vertical shear (20-40 kt) over Central Pacific prevents initiation of Tropical Storm

Chap. 5.5 Tropical cyclone

contents chap.5

Source : d’après Gray, 1968

Chap. 5.5 Tropical 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

9 per year

Source : D’après Atkinson, 1971.

Chap. 5.5 Tropical storms

over Eastern North Pacific

17 per year

Source : D’après Atkinson, 1971.

Chap. 5.5 Tropical storms

over Western North Pacific

27 per year

Source : D’après Atkinson, 1971.

Chap. 5.5 Tropical storms

over Southwest Pacific

5 per year

Source : D’après Atkinson, 1971.

Chap. 5.5 Tropical storms

over North Indian Ocean

4.5 per year

Why not TC in july-august while SST is the highest ?

Source : D’après Atkinson, 1971.

Chap. 5.5 Tropical storms

over North Indian Ocean

Mean vertical shear between 850 and 200 hPa in august

‣ strong vertical shear > 40 kt over North Indian Ocean in august (SW monsoon flow in surface, Tropical Easterly Jet at 200 hPa) prevents initiations of tropical storms

Source : d’après Gray, 1968

Chap. 5.5 Tropical storms

over Southwest Indian Ocean

13 per year

Source : D’après Atkinson, 1971.

Chap. 5.5 Tropical storms

over Southeast Indian Ocean

10 per year

Source : D’après Atkinson, 1971.

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 (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