1. 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 http://www.nhc.noaa.gov

2. 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, 22/01/2002 image infrarouge colorée. Source : Météo-France

3. Chap. 5.5 Tropical cyclone : OMM classification échelle Beaufort et 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 mn over USA and their survey zones (Atlantic N. and Pacific N.) and 10 mn elsewhere 7B 34 8/9B 10/11B 12B 48 64 91 115 Tropical depression Tropical storm Severe Tropical storm HurricaneSevere 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

4. Which difference between extra-tropical cyclone and tropical cyclone ? 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 Chap. 5.5 Tropical cyclone 5.6.2 :struct. hurricane Source : Merrill, 93

5. 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

6. Chap. 5.5 Tropical cyclone These 6 conditions necessary for initiation of 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

7. 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

8. 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

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

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

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

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

13. 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.

14. 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

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

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

17. 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

18. 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