Chap. 5.6 Hurricanes

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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
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5.6.4 Forecasting of hurricanes
Move of hurricane Storm surge Favorable
synoptic conditions Unfavorable synoptic
conditions Interannual variability Technical
forecasting
Sommaire hurricane
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5.6.4 Forecasting of hurricanes Move of
hurricane

• The vorticity equation, written under the
  hypothesis of
• barotropic atmosphere and above boudary layer,
• indicates the move of hurricane

(1)
Eulerian evolution of f is equal to 0
0
ßv
(2)
(1) (2) ? (3)
? The eulerian evolution of ?r give the move of
the hurricane
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5.6.4 Forecasting of hurricanes Move of
hurricane

effect of the basic current
Budget of

• Firstly, the move of hurricane is given by the
  basic
• current, which advects the relative vorticity ?r
  
• Some studies have shown that this budget
  explains
• 30 to 80 of the variance of the move of
  hurricane under
• 24 to 72 h forecasting (depends on latitude, size
  of the
• hurricane)
• under climatological winds, we can explain
  climatological
• track of hurricanes (generally, westwards move
  since
• easterlies occur throughout atmosphere under
  tropics)

Source Neumann, 93
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5.6.4 Forecasting of hurricanes Move of
hurricane

Budget of
the Rossbys ß - effect
Analyse the evolution of ?r with this only
budget
North Pole
y
1. Northen Hemisphere

• ß ? f/ ? y gt0
• westward to the cyclone,
• vlt0 , whence ßv gt0
• ? cyclonic circulation produces cyclonic
• vorticity westwards and anticyclonic
• vorticity eastwards

Eq.
x
hurricane
y
x
Eq.
2. Southern Hemisphere

• ß ? f/ ? y lt0
• westward to the cyclone,
• vgt0 , whence ßv gt0
• ? idem northern hemisphere

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5.6.4 Forecasting of hurricanes Move of
hurricane

Budget of
the Rossbys ß - effect

? The Rossbys ß effect, without basic current,
move hurricanes westwards for both hemisphere
? This effect accelerate the generally
westwards move of hurricane because of
easterlies basic current
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5.6.4 Forecasting of hurricanes Move of
hurricane

• The Rossbys drift
• The linear ß effect provides a secondary
  circulation in
• the environment which drifts the cyclonic vortex
  in the
• polewards direction

Northern Hemisphere
North
?r lt0
?rgt 0
x
? In northern hemisphere, without basic current,
the combined effect of the Rossbys ß - effect
and the Rossbys drift move the hurricanes
Northwestwards ? In Southern hemisphere,
hurricane move Southwestwards
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5.6.4 Forecasting of hurricanes Move of
hurricane
Influence of the Rossbys drift on the track

• The speed of the Rossbys drift is dependent on
  the intensity and the extension of the vortex.
  More precisely, the poleward drift increase with
  the angular momentum rV?.
• This process explains why at the mature and
  powerful stage, the hurricane is more rapidly
  poleward drifted than at its early stage.
• In addition, most of hurricane end their track
  into mid- latitudes and the hot core is little by
  little modified into extra-tropical cyclones
  (associated with violent and complicated
  phenomenon)
• To sum up, for a good tracking of the hurricane,
  first of
• all, its very important to forecast its
  intensity

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5.6.4 Forecasting of hurricanes Move of
hurricane

Influence of the synoptic environment on the
track

• When a hurricane approaches a through, its very
  
• difficult to forecast the inflexion point of
  the track
• Effet Fujiwhara when 2 vortex are very close
  (less than
• 1500 km), they are attracted to each other and
  turn one
• around the other one

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5.6.4 Forecasting of hurricanes
Move of hurricane Storm surge Favorable
synoptic conditions Unfavorable synoptic
conditions Interannual variability Technical
forecasting
Sommaire hurricane
sommaire
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5.6.4 Hurricanes Storm surges

• When a tropical cyclone hits the coast
  (phenomenon called
• Landfall), there occurs a rise of sea-level and
  rush of sea
• water inland. This inland-moving water is
  sometimes the
• cause of devastating damage to coastal property
  and also
• Loss of life along the coast.
• The following factors may operate together
• Inverted barometer effect, called pressure surge
  
• the sea level rises by about 1 m for such
  pressure
• defect of 100 hPa

Source from Chris Landsea and NOAA website
http//www.aoml.noaa.gov/hrd/tcfaq/C1.html
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5.6.4 Hurricanes Storm surges

• 2. Wind-driven surge cumulative effects of
  fetch of water
• and
  general coastal shelf
• The sustained wind stress due to sustained
  strong winds
• pushes the sea water . This water moves towards
  the coast in that
• sector in which the winds are on-shore and away
  from the coast in
• that sector in which winds are off-shore
• In the northern (southern) hemisphere, the
  on-shore movement
• of the sea water is in the right (left) hand
  sector of the cyclone.
• Due to the coastal shelf, the on-rushing water
  encouters diminishing
• depth (gravity wave in shallow water) as it moves
  from the open sea
• to the coast line.
• ? This leads to tremendous rise of sea level by
  about 6 to 10 m
• in the right hand sector also call dangerous
  mid-circle

Northern hemisphere
Source Mayençon R., 1982
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5.6.4 Hurricanes Storm surges
the dangerous mid-circle another effect is
important ! An example to understand

observed wind
speed of the vortex 20kt
wind in the relative reference of the hurricane
70 kt
70 20 90 Kt
Northern hemisphere
E
W
70 - 20 50 Kt
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5.6.4 Forecasting of hurricanes
Move of hurricane Storm surge Favorable
synoptic conditions Unfavorable synoptic
conditions Interannual variability Technical
forecasting
Sommaire hurricane
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5.6.4 Forecasting of hurricanes Favorable
synoptic conditions
Reminder most of tropical cyclones are initated
near the ITCZ
Case 1 Wind surge over North Indian Ocean

• This favorable synoptic condition occur in
  december-
• january-february when the winter monsoon over
• Iran/Arabia generates over North Indian Ocean
  increase
• of pressure in lower layer (1 to 2 hPa)
• The wind surge increase the gradient pressure
  and the flow
• on the northen flank of the ITCZ and generates
  horizontal
• shear and finally produces relative vorticity
• This seed of relative vorticity can generate a
  tropical cyclone
• several days after in the winter hemisphere
  under
• favorable SST

December to february Surface condition
a
Source F. Beucher, Météo-France
ITCZ
A
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5.6.4 Forecasting of hurricanes Favorable
synoptic conditions
Case 2 Increase of trades over South Indian
Ocean

• A northwards shift or intensification of the
  South Indian
• Ocean high increases in turn trades on the
  southern
• flank of the ITCZ
• Following an increase of horizontal shear and
  production
• of relative vorticity which is favorable to
  initiate a tropical
• cyclone over South Indian Ocean

December to february Surface condition
ITCZ
increased trades
A
Source F. Beucher, Météo-France
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5.6.4 Forecasting of hurricanes Favorable
synoptic conditions
Case 3 Outflow in upper troposphere

• The upper outflow is a convenient way to thrust
  away the
• mass and to fall down the pressure inward a
  hurricane.
• Without this upper outflow, the strong upward
  motion along
• the eyewall would vanished and the hurricane
  would die
• Generally, the upper circulation is organized
  into 1 or 2
• powerful outflow branches
• Example 1 sometimes, the proximity of the
  subtropical jet
• (STJ) is a favorable synoptic
  condition to organize the
• outflow.
• Example 2 in the southern hemisphere, the 2
  outflow
• branches are favourably located Northwestwards
  and
• Southeastward the cyclone in connection with
  STJ and TEJ

December to february 200 hPa
TEJ
H
Source F. Beucher, Météo-France
STJ
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5.6.4 Forecasting of hurricanes
Move of hurricane Storm surge Favorable
synoptic conditions Unfavorable synoptic
conditions Interannual variability Technical
forecasting
Sommaire hurricane
sommaire
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5.6.4 Forecasting of hurricanes Unfavorable
synoptic conditions

• Which consequences of the landing of hurricanes
  ?
• Increased value of surface roughness parameter
  (friction forces) may cause even intensification
  of the low-level inflow and intensity of the
  hurricane.
• At long-term, the eyewall is destroyed
  because the upper part of the cyclone go much
  faster than the lower part.
• Cuts-off of moisture supply (enthalpy energy)
  results at long-term of the weakening of the
  hurricane.

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5.6.4 Forecasting of hurricanes Unfavorable
synoptic conditions

• Over seas
• Except over Kuroshio current or Gulf Stream,
  poleward of 20 latitude, the SST are too cold to
  supply enough enthalpy energy (latent and
  sensible heat) to maintain the intensity of a
  hurricane
• - When the vertical shear is too large ( gt12 m/s
  between surface and upper tropo), the eyewall is
  too much tilted and the interactions between the
  surface and upper troposphere are destroyed.
• - In addition, when the vertical shear is
  large, the mass ventilation is too fast to allow
  concentration and accumulation of heat and is
  thus not leading to a drop
• in surface pressure .
• - Besides, a large vertical shear is favorable
  to dry intrusion which kills or prevents
  initiation of hurricanes.

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5.6.4 Forecasting of hurricanes Unfavorable
synoptic conditions

• What is the Saharian Air Layer (SAL) ?
• How does it affect hurricane ?
• The SAL is a massive sandstorm blowing off NW
  Africa
• desert during late spring, summer, and early
  fall.
• The SAL can cross the Atlantic, and many times
  as far as
• Carribean
• . The SAL extends between 1500-6000m, is
  associated with
• dry air (50 less moisture than a typical
  tropical sounding),
• and strong wind (20-50 kt).
• . The SAL have a negative impact on tropical
  cyclone
• frequency and intensity vertical shear and
  dry air have
• negative impact while the impact of the dust
  is not yet
• clearly established (a priori stop the
  intensification ?)
• . The increase of frequency of SAL are also
  linked with
• problem of pollution and decline of coral
  reef over
• Carribean

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5.6.4 Forecasting of hurricanes Unfavorable
synoptic conditions

• Detection of the SAL (GOES 12)
• IR satellite
• Visible satellite

3.4.2 monsoon
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5.6.4 Forecasting of hurricanes Unfavorable
synoptic conditions
Impact of the SAL (yellow to red color) on Erin
Source Dunion, 2004


reduced intensity
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5.6.4 Forecasting of hurricanes
Move of hurricane Storm surge Favorable
synoptic conditions Unfavorable synoptic
conditions Interannual variability Technical
forecasting
Sommaire hurricane
sommaire
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5.6.4 Forecasting of hurricanes Interannual
variability El Nino

• Reminder EL Nino (EN) is characterized by
  significant
• changes in SST anomalies through alterations in
  sea level
• pressure anomalies, trades winds, and convection.

• During EN event, strong upper level westerly
  shear occur
• Over Atlantic, and as a result development of
  hurricanes are capped.
• But overall, during EN, we observe far less
  hurricanes, especially at low latitudes

Impact of El nino on observed tropical storm
frequency Sources daprès Gray 84a, Chan 85,
Dong 88, Lander 94
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5.6.4 Forecasting of hurricanes Interannual
variability El Nino

• Over Atlantic, EN event favors more troughs in
  the subtropics whence increased occurrence of
  hurricanes at high latitudes and decreased
  occurrence at low latitudes.

Impact of El nino on observed tropical storm
location Sources daprès Gray 84a, Chan 85,
Dong 88, Lander 94

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5.6.4 Forecasting of hurricanes Interannual
variability QBO (west)
Reminder the Quasi Biennal Oscillation (QBO)
is a periodic variation in the direction of
stratospheric wind (west phase and east phase)
across the deep tropics. The QBO causes
alterations in upper level vertical wind shear.
It influences frequency and intensity of
hurricanes.


Impact of westerly phase of QBO on observed
tropical storm frequency Sources daprès
Hastenrath et Wendland 79, Shapiro 82, Gray 84a
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5.6.4 Forecasting of hurricanes Interannual
variability QBO (west)

• During the westerly phase of QBO, increase
  intensity of
• hurricanes overall

Impact of westerly phase of QBO on observed
tropical storm intensity Sources daprès
Hastenrath et Wendland 79, Shapiro 82, Gray 84a

retour 4.2.2 QBO
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5.6.4 Forecasting of hurricanes
Move of hurricane Storm surge Favorable
synoptic conditions Unfavorable synoptic
conditions Interannual variability Technical
forecasting
Sommaire hurricane
sommaire
30
5.6.4 Technical forecasting

• Until the advent of satellites, to judge the
  intensity of the vortex out in the sea, the
  analyse relied only on ships reports and the
  analysts experience with regional climatology of
  storms and hurricanes
• Even at present, aircraft reconnaissance is the
  most dependable technique of knowing the
  intensity of the vortex but this system is very
  expensive and not many meteorological services
  can afford it
• Coastal radar are also useful but they are
  generally effective within a range of about 300
  km but this tool dont leave enough time for
  local authorities to take precautionary measures

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5.6.4 Technical forecasting Satellite pictures

• Dvorak technique
• Finally, forecaster fall back upon visible and IR
  satellite pictures as a practical source of
  information to judge the intensity of a tropical
  cyclonic vortex.
• But, first of all, the satellite pictures must
  be calibrated by a dependable reference system
  which is the aircraft reconnaissance because of
  the cost only USA and Caribbean use this
  technique, called Dvorak technique (1975).
• The Dvorak technique use the difference between
  the temperature of the warm eye and the
  surrounding cold cloud tops. The larger the
  difference, the more intense the tropical cyclone
  is estimated to be.
• This technique furnishes a reliable forecast for
  the intensity of the cyclonic vortex until 24 h.
• But aware ! ! maximum wind speed based upon
  Dvorak technique alone are prone to some error
  since wind is not solely a function of pressure
  gradient (in gradient wind balance, the radius
  also contributes to the speeds wind)

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5.6.4 Technical forecasting Models
A variety of hurricane track forecast models are
run operationally for Atlantic and Northeastern
Pacific. Example of result of these models
track_Ivan

1. Climatological and persistence models
the basic model, called CLIPER
(CLImatology and PERsitence), is a multiple
regression statistical model that best utilizes
the persistence of the current motion and also
incorporates climatological track
information (Alberson, 1998). Surprisingly,
CLIPER was difficult to beat with numerical
model forecast until the 1980s. 2.
Statistical-dynamical models A
statistico-dynamical model, issued by the
national Hurricane Center of Miami, the NHC90
(McAdie 1991) uses geopotential height
predictors from the Aviation model to
produce a track forecast 4 times per day.
The primary synoptic time NHC90 forecasts (00 and
12 UTC) are based upon 12h old Aviation
runs. An update to this model was implemented in
1998 (NHC98)

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5.6.4 Technical forecasting Models
3. Numerical models i. The Beta and
Advection Model (BAM) follows a trajectory
with vertically-averaged horizontal winds
and a correction that accounts for the beta
effect (marks 1992). Three versions (shallow
layer, middle, deep) has been run 4 times
per day since 1990. ii. A nested barotropic
hurricane track forecast model (VICBAR) has
been run 4 times per day since 1990. It uses
run from current NCEP analyses (Alberson and
Demaria 1994) iii. The NCEP Aviation and MRF
models (Lord 1993) used for track forecasting
since 1992. These are global models. iv. A
triply-nested movable mesh primitive equation
model developped at Geophysical Fluid
Dynamics Laboratory (Bender et al 1993) ,
known as GFDL model has provided
forecasts since 1992. v. The UKMETs global
model utilized for track of tropical
cylones around the world (Radford 1994).



Sommaire hurricane
Chap.6
34
References

• Chan, J. C. L., 1985 Tropical cyclone
  activity in the Northwest Pacific in relation to
  El Nino/Southern Oscillation phenomenon. Mon.
  Wea. Rev., Vol.113, p.599-606
• Christopher Landsea, NOAA AOML/ Hurricane
  Research Division, 4301, Rickenbacker causeway,
  Miami, Florida 33149. email landsea_at_aoml.noaa.go
  v
• Dong K., 1988 El Nino and tropical cyclone
  frequency in the Australain region and the
  Northwest Pacific. Aust. Met. Mag., Vol.36,
  p.219-225
• Dunion, J.P., and C.S. Velden, 2004 The
  impact of the Saharan Air Layer on Atlantic
  tropical cyclone activity. Bull. Amer. Meteor.
  Soc., Vol.85, n3, p. 353-365
• - Gray, W. M., 1984a Atlantic seasonal
  hurricane frequency Part I. El Nino and 30 mb
  quasi-biennial oscillation influences. Mon. Wea.
  Rev., Vol.112, p. 1649-1668
• Hastenrath, S. and W. Wendland, 1979 On the
  secular variation of storms in the tropical North
  Atlantic and Eastern Pacific. Tellus, Vol.31,
  p.28-38
• Lander, M., 1994 An exploratory analysis of
  the relationship between tropical storm formation
  in the western North Pacific and ENSO. Mon Wea.
  Rev., Vol.122, p. 636-651
• - Mayençon R., 1982,1992 Météorologie Marine,
  Editions maritimes et dOutre-mer, Rennes,
• 335p.
• Neumann, C. J., 1993 Global Overview. Chapter
  1, Global Guide to Tropical Cyclone Forecasting,
  WMOTC-N560, Report N0 TCP-31, World
  Meteorological Organization, Geneva.