Chap. 5.5 Tropical cyclones (TC) (terme g

1
Chap. 5.5 Tropical cyclones (TC) (terme
générique signifiant dépression tropicale)
Source daprès 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
• Tropical cyclone reaching 17m/s are called
  tropical storm
• and are baptized.

2
Chap. 5.5 Tropical 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. 5.5 Tropical 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
Tropical depression Tropical storm Severe Tropical storm Hurricane Severe Hurricane Very Severe Hurricane
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
Chap. 5.5 Tropical cyclone
Which difference between extra-tropical cyclone
and tropical 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. 5.5 Tropical cyclone
6 conditions necessary for the development of
tropical storms (Gray, 79)
3 thermodynamical conditions
1. SSTgt26.5 (80F) 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.
Hugt70 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 Slt12 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
Source daprès Gray, 1979

• No initiation
• over land
• inside the equatorial zone (5N/5S)
• over S. Atl. and Pacific SE (no ITCZ and SST
  too cold)
• why no initiation over Central Pacific ? Answer,
  next slide

7
Chap. 5.5 Tropical cyclone
Mean vertical shear between 850 and 200 hPa in
august
Source daprès Gray, 1968
? The strong vertical shear (20-40 kt) over
Central Pacific
prevents initiation of Tropical Storm
contents chap.5
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
Source Daprès Atkinson, 1971.
9 per year
10
Chap. 5.5 Tropical storms over Eastern
North Pacific
17 per year
Source Daprès Atkinson, 1971.
11
Chap. 5.5 Tropical storms over Western
North Pacific
Source Daprès Atkinson, 1971.
27 per year
12
Chap. 5.5 Tropical storms over Southwest
Pacific
Source Daprès Atkinson, 1971.
5 per year
13
Chap. 5.5 Tropical storms over North Indian
Ocean
Source Daprès Atkinson, 1971.
4.5 per year Why not TC in july -august while
SST is the highest ?
14
Chap. 5.5 Tropical storms over North Indian
Ocean
Mean vertical shear between 850 and 200 hPa in
august
Source daprès Gray, 1968
? strong vertical shear gt 40 kt over North
Indian Ocean in august (SW monsoon flow in
surface, Tropical Easterly Jet at 200 hPa)
prevents initiations of tropical storms
15
Chap. 5.5 Tropical storms over Southwest
Indian Ocean
Source Daprès Atkinson, 1971.
13 per year
16
Chap. 5.5 Tropical storms over Southeast
Indian Ocean
Source Daprès Atkinson, 1971.
10 per year
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 Forecasters guide to
  tropical meteorology. USAF Air Weather Service,
  Technical Report N240, 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- N560,
  Report N TCP-31, World Meteorological
  Organization Geneva, Switzerland