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

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 générique signifiant dépression tropicale)

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  1. 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.

  2. 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)

  3. 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

  4. 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

  5. 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

  6. 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

  7. 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

  8. 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

  9. Chap. 5.5Tropical storms over North Atlantic Source : D’après Atkinson, 1971. 9 per year

  10. Chap. 5.5Tropical storms over Eastern North Pacific 17 per year Source : D’après Atkinson, 1971.

  11. Chap. 5.5Tropical storms over Western North Pacific Source : D’après Atkinson, 1971. 27 per year

  12. Chap. 5.5Tropical storms over Southwest Pacific Source : D’après Atkinson, 1971. 5 per year

  13. 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 ?

  14. 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

  15. Chap. 5.5Tropical storms over Southwest Indian Ocean Source : D’après Atkinson, 1971. 13 per year

  16. Chap. 5.5Tropical storms over Southeast Indian Ocean Source : D’après Atkinson, 1971. 10 per year

  17. - 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)

  18. 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)

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