Chap. 5 Conceptual models of synoptic Tropical disturbances in summer Definition of tropical...
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![Page 1: Chap. 5 Conceptual models of synoptic Tropical disturbances in summer Definition of tropical disturbances : A discrete tropical weather system of apparently.](https://reader035.fdocuments.net/reader035/viewer/2022062421/56649cec5503460f949b896c/html5/thumbnails/1.jpg)
Chap. 5 Conceptual models of synoptic
Tropical disturbances in summer
Definition of tropical disturbances :
• A discrete tropical weather system of apparently organized convection generally 200 to 600 km in diameter, originatingin the tropics or subtropics, having a nonfrontal migratorycharacter, and maintening its identity for 24 hours or more.
• It may or may not be associated with a detectable perturbation of the wind field.
• Disturbances associated with perturbations in the wind field and progressing through the tropics from east to west are also known as ‘easterly waves’.
![Page 2: Chap. 5 Conceptual models of synoptic Tropical disturbances in summer Definition of tropical disturbances : A discrete tropical weather system of apparently.](https://reader035.fdocuments.net/reader035/viewer/2022062421/56649cec5503460f949b896c/html5/thumbnails/2.jpg)
An example of evolution of tropical disturbances over Atlantic in summer:
Chap. 5 Conceptual models of synoptic
Tropical disturbances in summer
![Page 3: Chap. 5 Conceptual models of synoptic Tropical disturbances in summer Definition of tropical disturbances : A discrete tropical weather system of apparently.](https://reader035.fdocuments.net/reader035/viewer/2022062421/56649cec5503460f949b896c/html5/thumbnails/3.jpg)
5.1 Easterly waves (West Africa, Caribbean Sea, Pacific)
5.2 Troughs of Upper Tropical Troposphere (TUTT)
5.3 Monsoon Depression
5.4 Mid-tropospheric cyclones
5.5 Tropical storms
5.6 Hurricanes
sommaire général
Chap. 5 Conceptual models of synoptic
Tropical disturbances in summer
![Page 4: Chap. 5 Conceptual models of synoptic Tropical disturbances in summer Definition of tropical disturbances : A discrete tropical weather system of apparently.](https://reader035.fdocuments.net/reader035/viewer/2022062421/56649cec5503460f949b896c/html5/thumbnails/4.jpg)
5.1 African Easterly waves
• Located at the southern flank (12°N) of AEJ (15°N) from June to October
• Initiate by barotropic instability (inflexion point of PV on the isentrope surface 315°K)
• Growth by release of latent heat
• Speed phase = -8 m/s
• Period = 3 to 5 days
• Scale lenght = about 2500 km
• Bad weather in general on the western side of the trough
![Page 5: Chap. 5 Conceptual models of synoptic Tropical disturbances in summer Definition of tropical disturbances : A discrete tropical weather system of apparently.](https://reader035.fdocuments.net/reader035/viewer/2022062421/56649cec5503460f949b896c/html5/thumbnails/5.jpg)
Streamline and wind at 700 hPa : we observe 2 easterly waves at the southern flank of AEJ
5°N
10°N
15°N
Troughs of easterly waves are visibles through the sharp shift of the wind
5.1 African Easterly waves
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On NIMBUS software, look after the streamline at 850 hPa(closed circulation) or at 700 hPa (sinusoide circulation)
but easterly waves are not visible in upper troposphere.
5.1 African Easterly waves
![Page 7: Chap. 5 Conceptual models of synoptic Tropical disturbances in summer Definition of tropical disturbances : A discrete tropical weather system of apparently.](https://reader035.fdocuments.net/reader035/viewer/2022062421/56649cec5503460f949b896c/html5/thumbnails/7.jpg)
Composite structure : rain, clouds on the western side of the trough : divergence at 200 hPa and convergence at 850 hPa on the western side of the trough
T= Trough R= Ridge
5.1 African Easterly waves
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• Origin : African easterlies wave which cross Atlantic• Occurrence/ frequency over Carribean : from may to november once every 3 to 4 days, either about 60 over the season• Speed phase = -5 to -8 m/s• Scale lenght = about 3000 km • Bad weather, generally, on the eastern side of the trough
5.1 Carribean Easterly waves
![Page 9: Chap. 5 Conceptual models of synoptic Tropical disturbances in summer Definition of tropical disturbances : A discrete tropical weather system of apparently.](https://reader035.fdocuments.net/reader035/viewer/2022062421/56649cec5503460f949b896c/html5/thumbnails/9.jpg)
• Maximum signal of the wave = between 500 and 700 hPa
• Vertical tilt of the trough : below 700 hPa, westward tilt with altitude(not shown on the above figure)
above 700 hPa, eastward tilt with altitude
5.1 Carribean Easterly waves
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5.1 Pacific Easterly waves
• Eastern Pacific (80°E to 170°E) : - scale lenght = 3000 to 3500 km - period = 4 to 6 days - phase speed = -5 to -7 m/s - in surface, we observe a fluctuation of wind as large as 4 m/s - in upper troposphere, lighter fluctuation of wind
• Western Pacific (170°E to 120°E) : - scale lenght = 3500 to 4000 km - period = 6 days - phase speed = -9 m/s - in low troposphere (800 hPa), first peak of fluctuation of wind as large as 3 to 4 m/s and strong convergence - in upper troposphere (175 hPa), second peak of fluctuation of wind as large as 2 to 3 m/s and strong divergence - in mid- troposphere (500 to 300 hPa) : no fluctuation of wind
• The vertical tilt of the trough:
It varies from eastern Pacific (eastward vertical tilt) to western Pacific (westward vertical tilt); it probably depends of the environment, either the vertical shear wind which is the opposite between Eastern and Western Pacific
Chap 5.2
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Chap. 5Differents types of
tropical disturbances in summer
5.1 Easterly waves (West Africa, Caribbean Sea, Pacific)
5.2 Troughs of Upper Tropical Troposphere (TUTT)
5.3 Monsoon Depression
5.4 Mid-tropospheric cyclones
5.5 Tropical storms
5.6 Hurricanes
sommaire général
![Page 12: Chap. 5 Conceptual models of synoptic Tropical disturbances in summer Definition of tropical disturbances : A discrete tropical weather system of apparently.](https://reader035.fdocuments.net/reader035/viewer/2022062421/56649cec5503460f949b896c/html5/thumbnails/12.jpg)
5.2 Trough of Upper Tropical Troposphere (TUTT)
• In summer, over N. Atlantic, an upper through quasi-zonal, lying WSW/ENE, is visible on monthly maps (not necessarily daily !)
• Hypothesis of initiation This upper trough is a typical example of the interactions between tropics and mid-latitudes :
‣ we observe low ratio of water vapour along the TUTT linked to (?) the successive Rossby Wave Breakings (RWB) which occur in summer over mid-oceans (called ‘surf zone’). RWB result in stratospheric mid-latitude intrusion (strong PV and light humidity) into the tropical troposphere
‣ along the axis of TUTT, as the atmosphere is very dry, we observe a very strong radiative cooling (-6°K/day) which implies a strong synoptic-scale subsidence (- 2 cm/s) along the axis (cf. thermo
equation in tropics)
200 hPa streamline in august :
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5.2 TUTT ‘cells’ or TUTT ‘lows’Definition of a TUTT ‘cell’: Cold vortex observed in upper troposphere along the climatological TUTT disturbing the tropical atmosphere, sometimes, down to the surface (should the occasion arise, a subtropical disturbance occur and rarely tropical storm).
• Vertical velocity : in vortex, subsidence max. at 300 hPa whence sky clear (or isolated cumulus) and radiational cooling
• Cold core max. at 300 hPa,
• Cyclonic vorticity max. at 200 hPa
• Deep convection : located upstream the vortex in the warm and divergent upper tropo,either at SE of the vortex in northen hemisphere (resp. NE in southern hemisphere)
Anomaly of VV, in microbars/s
hPa
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5.2 TUTT ‘cells’ or TUTT ‘lows’
Other features of a TUTT ‘cell’:
• Move : Stationnary or westward move (in both hemisphere) from 0 to 6° of longitude per day
• Life span : 2 to 31 days !!
• MSLP : the fall of MSLP depends on the interaction between the upper tropo (intrusion of high PV) and the low tropo (line of convergence coupled with high tp’w ..)
• Future study : a TUTT event seems to be linked with an intrusion of stratospheric air into tropical troposphere (linked with the Rossby wave Breaking ?) So, high values of PV (>2 PVU) on 350°K should be a good proxy to detect ‘TUTT cell’ ?
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5.2 TUTT ‘cells’ or TUTT ‘lows’
Relation between ‘TUTT cell’ and tropical cyclone: *
⇨ The ‘TUTT cell’ vanish the tropical cyclone when the vertical shear (induced by the TUTT ‘cell’) is greater than 12m/s between the surface and 200 hPa .
⇨ Under this threshold vertical shear, a tropical cyclone could be enhanced when it is located on the eastern side on the TUTT ‘cell’, that is to say in the Upper warm and ascent outflow
Chap 5.3
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Chap. 5Differents types of
tropical disturbances in summer
5.1 Easterly waves (West Africa, Caribbean Sea, Pacific)
5.2 Troughs of Upper Tropical Troposphere (TUTT)
5.3 Monsoon Depression
5.4 Mid-tropospheric cyclones
5.5 Tropical storms
5.6 Hurricanes
sommaire général
![Page 17: Chap. 5 Conceptual models of synoptic Tropical disturbances in summer Definition of tropical disturbances : A discrete tropical weather system of apparently.](https://reader035.fdocuments.net/reader035/viewer/2022062421/56649cec5503460f949b896c/html5/thumbnails/17.jpg)
5.3 Indian monsoon : monsoon dépression
‣ mid-tropo :moderate signalof convergence
Shaded area=wind > 40 kt
‣ low tropo :Maximum signal of wind and convergence from 600 to 800 hPa
‣ upper tropo : light anticyclonic circulation and light
signal of divergence
200 hPa : Streamline and isotach
500 hPa: Streamline and isotach
850 hPa: Streamline and isotach
H
C
C
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5.3 Indian monsoon : Monsoon depression
• Synoptic-scale : about 2000 km of diameter
• Location of initiation ‣ 80 % in tha Bay of Bengale, ‣ 10% in Arabian Sea ‣ 10 % over land (Bangladesh)• MSLP ~ as low as 990 hPa
• Livespan : from 3 to 5 days
• Frequency : twice a month over Bay of Bengale
• Move : westward or northwestward at 2 or 3 m/s in direction of heat low at least as far as Central India before decaying
• Closed circulation between surface-300 hPa max. intensity (wind, convergence) from 600 to 800 hPa
• Temperature ‣ cold core between surface-600 hPa ‣ above, between 500-200 hPa, hot core ‣ but, some of them no cold core observed
• Evolution : no risk of development of tropical cyclone because of the strong vertical shear (TEJ in upper tropo and SW monsoon flow in low tropo)
• Origin hypothesis : baroclinic instability (eastward vertical tilt coupled with westward vertical shear )
+ CISK (convergence linked to the surface trough)
Main Key features :
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5.3 Indian monsoon : Monsoon depression
• Deep convection and vertical velocity max. in the SW quadrant
• Rain : in the SW quadrant between 100 mm to 300 mm per day
Location of rain and vertical velocity :
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Chap. 5Differents types of
tropical disturbances in summer
5.1 Easterly waves (West Africa, Caribbean Sea, Pacific)
5.2 Troughs of Upper Tropical Troposphere (TUTT)
5.3 Monsoon Depression
5.4 Mid-tropospheric cyclones
5.5 Tropical storms
5.6 Hurricanes
sommaire général
![Page 21: Chap. 5 Conceptual models of synoptic Tropical disturbances in summer Definition of tropical disturbances : A discrete tropical weather system of apparently.](https://reader035.fdocuments.net/reader035/viewer/2022062421/56649cec5503460f949b896c/html5/thumbnails/21.jpg)
5.4 Indian monsoon Mid-tropospheric cyclone
Streamline and isotach at (left) 925 hPa, (right) 600 hPa
• Closed circulation between 700-300 hPa
• Maximum of intensity (wind, convergence) in mid-troposphere from 500 to 600 hPa
• At low and upper troposphere : absence or light signature in wind (manifested as a trough in the streamline) whence risk of ‘messing- up’ for forecasters
925 hPa 600 hPa
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• Location of initiation : Mid-tropospheric cyclone occur in NE of Arabian Sea, South Vietnam, South China Sea from
• Period : from may to october
• Synoptic scale : ~ 3000 km
• Livespan : from 3 to 7 days, even 10 days !
• Frequency : less than monsoon depression
• Move : stationnary or westward
• Temperature ‣ cold core between surface-600 hPa ‣ above between 500-200 hPa, hot core
• Heavy rains : in western quadrant (location of ascending motion) up to 200 mm per day
• Origin hypothesis ‣ for initiation, barotropic instability of the
monsoon flow at 700 hPa ‣ for growth, release of latent heat
Main Key features :
5.4 Indian monsoon Mid-tropospheric cyclone
![Page 23: Chap. 5 Conceptual models of synoptic Tropical disturbances in summer Definition of tropical disturbances : A discrete tropical weather system of apparently.](https://reader035.fdocuments.net/reader035/viewer/2022062421/56649cec5503460f949b896c/html5/thumbnails/23.jpg)
Chap. 5Differents types of
tropical disturbances in summer
5.1 Easterly waves (West Africa, Caribbean Sea, Pacific)
5.2 Troughs of Upper Tropical Troposphere (TUTT)
5.3 Monsoon Depression
5.4 Mid-tropospheric cyclones
5.5 Tropical storms
5.6 Hurricanes
sommaire général
![Page 24: Chap. 5 Conceptual models of synoptic Tropical disturbances in summer Definition of tropical disturbances : A discrete tropical weather system of apparently.](https://reader035.fdocuments.net/reader035/viewer/2022062421/56649cec5503460f949b896c/html5/thumbnails/24.jpg)
Chap. 5Differents types of
tropical disturbances in summer
5.1 Easterly waves (West Africa, Caribbean Sea, Pacific)
5.2 Troughs of Upper Tropical Troposphere (TUTT)
5.3 Monsoon Depression
5.4 Mid-tropospheric cyclones
5.5 Tropical storms
5.6 Hurricanes
sommaire général
![Page 25: Chap. 5 Conceptual models of synoptic Tropical disturbances in summer Definition of tropical disturbances : A discrete tropical weather system of apparently.](https://reader035.fdocuments.net/reader035/viewer/2022062421/56649cec5503460f949b896c/html5/thumbnails/25.jpg)
5.2 Rossby Wave Breaking (RWB)
Event of RWB from 27/05 06TU to 29/05/98 18TU Source : ERA15 CEP : PVU on the surface 350°K
high PVhigh PV
high PVhigh PV
high PVhigh PV
RWB RWB
Cutt-off ofhigh PV
Cutt-off ofhigh PV
![Page 26: Chap. 5 Conceptual models of synoptic Tropical disturbances in summer Definition of tropical disturbances : A discrete tropical weather system of apparently.](https://reader035.fdocuments.net/reader035/viewer/2022062421/56649cec5503460f949b896c/html5/thumbnails/26.jpg)
5.2 Rossby Wave Breaking
Event of RWB : zoom on 27/05/98 18TU Source : ERA15 CEP : PVU on the surface 350°K
• Deep convection occur upstream the RWB, northeastward the ‘TUTT cell’, in the upper warm and divergent flow
Cut-off ofhigh PV : Stratospheric air
Cut-off of low PV :tropospheric air
![Page 27: Chap. 5 Conceptual models of synoptic Tropical disturbances in summer Definition of tropical disturbances : A discrete tropical weather system of apparently.](https://reader035.fdocuments.net/reader035/viewer/2022062421/56649cec5503460f949b896c/html5/thumbnails/27.jpg)
5.2 Example of a RWB over the Southern Atlantic
04/02/04 : Geopotential and wind on the surface 2 PVU :
• Deep convection occur upstream the RWB, northeastward the ‘TUTT cell’, in the upper warm and divergent flow