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Chap. 5.6 Hurricanes 5.6.1 Hurricane : introduction 5.6.2 Hurricane structure 5.6.3 Hurricane : theory 5.6.4 Forecasting of hurricane Sommaire chap.5 sommaire.

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Presentation on theme: "Chap. 5.6 Hurricanes 5.6.1 Hurricane : introduction 5.6.2 Hurricane structure 5.6.3 Hurricane : theory 5.6.4 Forecasting of hurricane Sommaire chap.5 sommaire."— Presentation transcript:

1 Chap. 5.6 Hurricanes Hurricane : introduction Hurricane structure Hurricane : theory Forecasting of hurricane Sommaire chap.5 sommaire

2 5.6.2 Hurricane structure Cloudy rolling of deep organized convection, with nearly circular shape, which can extent to 1000 km of diameter, generally associated with an eye, without frontal system, and occurred in the summer hemisphere Cyclone with hot core Balanced fluid (Rossby radius of 20 km) with - cyclostrophic equilibrium < 40 km - gradient wind balanced > 40 km Source : Cyclone Edwina, le 28/01/93 à 0146 UTC; Satellite NOAA 10

3 In low troposphere : The airstream is spiraling inwards the cyclone centre with : - a tangential flow cyclonic : > 0 - a radial inflow whence convergence : < 0) Convection is organized into, long, narrow rainbands (5 to 50 km large), known as spiral bands (visible through RADAR) Speed velocity, rain, Hu, increase as and when the airstream approaches the eyewall of the hurricane Spiral bands Vr < 0 Eyewall D Hurricane structure Northern Hemisphere Source : from Stormfury, 1970

4 5.6.2 Hurricane structure H From a certain distance of the cyclone centre (from 10 to 40 km), the airstream is suddenly rejected upwards = initiation of the eyewall In upper troposphere one part of the airstream is spiraling outward the eyewall with : - a tangential flow anticyclonic : < 0 - a radial outflow whence divergence : > 0 the other part of the airstream is subsiding inward the eyewall originating the eye Northern Hemisphere Source : from Stormfury, 1970

5 Eyewall : maximum intensity of hurricane with circle of CB with top around 12 to 15 km and torrential rain maximum of horizontal and vertical wind : radial and vertical section : center hurricane Eyewall structure Source : Cyclone Edwina, le 28/01/93 à 0146 UTC; Satellite NOAA 10

6 Horizontal scale of the eyewall ~ 10 to 20 km large The eyewall tilts outwards with altitude : the max. of horizontal wind is tilted outwards the max. of vertical wind is tilted outwards (-7 to –9 m/s) the precipitations fall outwards the cone Bigger the eye is, more tilted the cone is Eyewall structure Source : from Jorgensen, 1984

7 Eye Eye structure Source : Cyclone Edwina, le 28/01/93 à 0146 UTC; Satellite NOAA 10

8 Diameter of the eye ~ 30 to 60 km (mini 8 km, max. 200 km) strong subsidence (~ + 3 m/s) especially along the inward side of the eyewall and between PBL - upper tropo subsidence explains that eye temperature is warmer than the surrounding environment (10°C at 12km, 0 to 2°C at surface) Eye structure Source : from Jorgensen, 1984

9 Weather associated : minimum of mean-sea level pressure observed at center no rain and light wind variable cloudiness : sky clear with cirrus but sometimes overcast with low clouds (Sc/St) when the low-level is moist with a thermal inversion above Shape of the eye : circular or elliptic : no relation or very light correlation established between size of the eye and intensity of hurricane Speed phase of the eye ~ 20 km/h D Eye structure Chap Source : from Jorgensen, 1984

10 eyewall eye Spiral bands Hurricane structure : radar Source : from Burpee and Marks, 1984

11 Radial windTangential wind Horiz. wind Bell-shaped Cyclonic circulation Anticyclonic circulation Radial inflow Radial outflow som. chap Structure du cyclone tropical Source : from Burpee and Marks, 84. Source : from Gray, 79

12 For a rotating fluid, the wind can be divided into For hurricanes, equations are written in cylindrical co- ordinates (r, λ,z) around the cyclone centre (axis-symmetric flow). r λ z Radial equation of motion (above PBL) Outward Acceleration null Coriolis Force Pressure force Centrifugal force : relative tangential wind component : relative radial wind component Outer eyewall (R>40 km) = gradient-wind balanced Inner eyewall (R<40 km) = cyclostrophic balanced Hurricane structure : a balanced fluid

13 r λ z Outer eyewall (R>40 km) = gradient-wind balanced Hurricane structure : a balanced fluid Centrifugal Force Coriolis Force Pressure force Northern Hemisphere Chap

14 r λ z Inner eyewall (R<40 km) = cyclostrophic balanced Hurricane structure : a balanced fluid Pressure force Northern Hemisphere Centrifugal Force

15 Spiral bands Hurricane structure : spirals bands Source : image from satellite NOAA10 of 28/01/93 at 0146TU. Cyclone Edwina over Indian Océan

16 5.6.2 Hurricane structure : upper outflow Source : image issue du satellite à défilement NOAA 10

17 Circular eye Elliptic eye Hurricane structure : radar Source : daprès Burpee et Marks, 84

18 Vertical cross-section of temperature anomaly for hurricane Inez +3° at surf. +15° at 250 Hpa Hurricane structure : hot core Source : from Hawkins and Rubsam, 68.

19 References - Burpee, R. W., and F. D. Marks, Jr., 1984 : Analyses of digital radar data obtained from coastal radars during Hurricanes David (1979), Frederic (1979), and Alicia (1983). Preprints, 10th Conference on Weather Forecasting and Analysis, Clearwater beach, Fla., American Meteorological Society, Boston, Gray, W. M., 1979 : Hurricanes, their formation, structure, and likely role in the tropical circulation. Pp in : Shaw, O. B., ed., Meteorology over the tropical oceans, Conference, August 1978, Royal Meteorological Society, Bracknell, 278 pp. - Hawkins, H. F., and D. T. Rubsam, 1968 : Hurricane Hilda, 1964, II : Structure and budgets of the hurricane on October 1, Mon. Wea. Rev., 96, pp Jorgensen David P., 1984 : Mesoscale ans convective-scale characterictics of mature hurricane, Pt.2, Inner core structure of hurricane Allen (1980). J. of Atm. Sci., vol. 41, n°8, pp 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 - STORMFURY, 1970 : Projet international sur les cyclones tropicaux. NOAA Technical Memorandum ERL NHRL N° 95, 57 pages : Experiments which provide theorical guidance for project STORMFURY are summarized.


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