Tropical Meteorology
Plan
Training course realized in 2005 by
Florent Beucher, ENM/EGM
florent.beucher@meteo.fr
office : C153  : 94-30
Objectives :
Descriptive knowledge of the medium state
and the variability of the tropical atmosphere
for a 15 to 20 hours training course
This course is available on this web site :
http://webrp.enm.meteo.fr/cms/view/12/content/meteo_t
rop/cours_meteotrop_anglais
general contents
Tropical Meteorology
Bibliography of this course :
• S.Hastenrath : Climate and circulations of the tropics, 1985
• G.C Asnani : Tropical Meterology (Vol.1 et vol.2), 1982
• Adrian E. Gill : Atmosphere-Ocean Dynamics (chap.11 on
tropics)
• Ding Yihui : China Monsoon,1994
• Herbert Riehl : Climate and Weather in the tropics, 1979
• Robert A. Houze : Clouds dynamics
• In French : OMM n° 305 : guide su systeme mondial du
traitement des données, chap.5 , written in 1993
• And various paper from JAS, MWR etc…
More details about tropical meteorology
on UFR Web-site :
http://intra-ufr.enm.meteo.fr/pages/UFR/ufr_index.htm
general contents
Tropical Meteorology
Definition :
• Tropics are located between the both belt of subtropical
high pressure = about 50% of the sphere (30°N/30°S)
• These belts as the tropic regions move with the season.
Location of tropical atmosphere in annual mean or at equinox :
Ridge = 30°N
A
Equator
Ridge = 30°S
general contents
A
Tropical Meteorology
Definition :
• Tropics are located between the both belt of subtropical
high pressure = about 50% of the sphere (30°N/30°S)
• These belts as the tropic regions move with the season.
Location of tropical atmosphere in august :
Ridge = 35°N
A
Northward
shift of tropical
atmosphere
from february
to august
Equator
Ridge = 25°S
general contents
A
Tropical Meteorology
Definition :
• Tropics are located between the both belt of subtropical
high pressure = about 50% of the sphere (30°N/30°S)
• These belts as the tropic regions move with the season.
Location of the tropical atmosphere in february :
Ridge = 25°N
A
Equator
Ridge = 35°S
general contents
A
Southward
shift of tropical
atmosphere from
from august to
february
Météorologie Tropicale
Location of the tropical atmosphere in february :
- Realize a radiosondage in the summer hemisphere of the
tropical atmosphere :
= vertical sounding
Ridge = 25°N
A
Southward
shift of tropical
atmosphere from
from august to
february
equator
Ridge = 35°S
general contents
A
• As the tropical atmosphere is
nearly barotropic, the vertical shear
is light :
- trades winds in low tropo. (5 à 10kt)
- easterlies in mid-tropo. (10kt)
- easterlies in upper tropo. (20 kt)
• This representative vertical souding
centre
de la
is observed
dorsale = 25°N
:
A
- in february within a latitud band 10°N-15°S,
- in august within a latitud band 30°N-10°S
general contents
Météorologie Tropicale
Location of the tropical atmosphere in february :
- Realize a radiosondage in the winter hemisphere of the
tropical atmosphere :
= vertical sounding
Ridge = 25°N
A
Southward
shift of tropical
atmosphere from
from august to
february
equator
Ridge = 35°S
general contents
A
Radiosondage Analyse ARP1.5
Source : Météo-France
• On the poleward flanks of
the tropical atmosphere, the
baroclinicity and the vertical wind
shear
increase
:
• Comme
l’atmosphère
tropicale
- trades winds in low tropo. (5 à 10kt)
est quasi-barotrope, le cisaillement
- light westerlies in mid-tropo. (10kt)
vertical
de vent est
faible tropo.
:
- strong westerlies
in upper
(60 kt)=
- alizés
JOST en basses couches (5 à 10kt)
- vent d’est en moyenne tropo (10kt)
d’est enduring
hautethe
tropo
(20season
kt) in
•- vent
For instance,
winter
the french tropical islands, the mean state
of
theradiosondage
tropical atmosphere
nearly
behaves:
• Ce
typique
s’observe
centre
defévrier
la
like
in
mid-latitudes.
The theory of
en
entre 10°N-15°S,
A
dorsale = 25°N
Anasyg-Presyg
could
be
work
?
- en août entre 30°N-10°S
• This representative vertical sounding
is observed : :
-in february northward of 15°N and
southward of 25°S,
-in august northward of 35°N and
southward of 15°S
general contents
Tropical Meteorology
5 main characteristics of the tropics :
The first three ones because of radiative considerations:
① Between 30°S/30°N, radiative energy >0 at the top of
atmosphere
Outward tropics, radiative energy <0 at the top of
atmosphere :
⇨ we observe a strong meridional radiative
desequilibrium between the equator and the poles
⇨ initiates a planetary-scale meridian circulation in
atmopshere called ‘Hadley Cell’
⇨ initiates a planetary-scale ocean circulation directed
northward (Gulf Stream, Kuroshio etc..)
② Close to the equator, the radiative energy is much higher
at surface than at the top of atmosphere :
⇨ we observe a strong vertical radiative desequilibrium
between surface and top of atmosphere
⇨ initiates strong vertical velocities = ascent branch of
both Walker cells and Hadley cells
③ Diurnal variability is higher than annual variability
⇨ the diurnal cycle is very important under tropics
general contents
Tropical Meteorology
5 main characteristics of the tropics (the continuation) :
The last two ones because of light Coriolis parameter f
(10-5 s-1 at 10° of latitude) :
④ Horizontal gradient of geopotential slack compared with
mid-latitude
⇨ the tropics are nearly ‘barotropic’
⑤ The flow is essentially divergent in tropics, i.e. the
rotational part of the flow is insignificant except two cases :
- at planetary scale as equatorial waves (geostrophic
balance)
- cyclones (cyclostrophic balance)
general contents
Plan
1. Different scales of the atmosphere
2. Energy sources for the initiation and the
growth of the equatorial waves and tropical
disturbances
3. Regional climates in tropics
4. Equatorial trapped waves and planetary
waves oscillations (MJO,QBO)
5. Conceptual models of synoptic tropical
disturbances in summer
6. Interactions between the mid-latitudes and
the tropics
7. ENSO
Chap.1
Different scales of the atmosphere
Meso-scale
or convective scale
= ‘little scale’
Synoptic or
Planetary scale
= ‘large scale’
R ~ NH/(f+ ζr)
Atmospheric deformation radius
(depends on f, stability of the
atmosphere and the relative vorticity)
R
(km)
Tropics 1000
Mid100
Latitudes
Tropical
10
Cyclone
Moist convective
instablility 3D
10 000
H thickness of
the atmosphere
Quasi-horizontal
balanced flow :
Geostophic and hydrostatic
equilibrium
 10 km
1
1
10
100
1000 10 000
Source : Ooyama 1982
general contents
L (km) =
Horizontal
scale
Plan
1. Different scales of the atmosphere
2. Energy sources for the initiation and the
growth of the equatorial waves and tropical
disturbances
3. Regional climates in tropics
4. Equatorial trapped waves and planetary
waves oscillations (MJO,QBO)
5. Conceptual models of synoptic tropical
disturbances in summer
6. Interactions between the mid-latitudes and
the tropics
7. ENSO
Chap 2.
Energy sources for the initiation and growth of
the equatorial waves and tropical disturbances
Interactions between the convection (little scale)
and larger scales :
200 hPa
200 hPa
850 hPa
850 hPa
Source : Météo-France
Lafore
time
⇨ The convection produces synoptic disturbances over a
horizontal-scale λR (about 1000 km under tropics) after
1/f time-scale (about 1 day under tropics)
⇨ Interactions between convection and larger scale are
realized through inertial-gravity waves (IG)
⇨ But this interaction is efficient only if release of latent
heat is important (big population of cumulonimbus)
general contents
Chap 2.
Energy sources for the initiation and growth of
the equatorial waves and tropical disturbances
Illustration of this process at the end of May
after the Indian Monsoon onset :
H
H
H
Géopotentiel at 200 hPa; 22/07/05; Analyse CEP 1.5. Source : Météo-France
• All over tropics [30°N-30°S], by thermal forcing we observe
high geopotential but with a slack gradient since tropics are
nearly barotropic
• By release of latent heat over Indian and Asian monsoon
occur an increase of geopotential
general contents
Plan
1. Different scales of the atmosphere
2. Energy sources for the initiation and the
growth of the equatorial waves and tropical
disturbances
3. Regional climates in tropics
4. Equatorial trapped waves and planetary
waves oscillations (MJO,QBO)
5. Conceptual models of synoptic tropical
disturbances in summer
6. Interactions between the mid-latitudes and
the tropics
7. ENSO
Plan
1. Different scales of the atmosphere
2. Energy sources for the initiation and the
growth of the equatorial waves and tropical
disturbances
3. Regional climates in tropics
4. Equatorial trapped waves and planetary
waves oscillations (MJO,QBO)
5. Conceptual models of synoptic tropical
disturbances in summer
6. Interactions between the mid-latitudes and
the tropics
7. ENSO
Plan
1. Different scales of the atmosphere
2. Energy sources for the initiation and the
growth of the equatorial waves and tropical
disturbances
3. Regional climates in tropics
4. Equatorial trapped waves and planetary
waves oscillations (MJO,QBO)
5. Conceptual models of synoptic tropical
disturbances in summer
6. Interactions between the mid-latitudes and
the tropics
7. ENSO
Plan
1. Different scales of the atmosphere
2. Energy sources for the initiation and the
growth of the equatorial waves and tropical
disturbances
3. Regional climates in tropics
4. Equatorial trapped waves and planetary
waves oscillations (MJO,QBO)
5. Conceptual models of synoptic tropical
disturbances in summer
6. Interactions between the mid-latitudes and
the tropics
7. ENSO
Plan
1. Different scales of the atmosphere
2. Energy sources for the initiation and the
growth of the equatorial waves and tropical
disturbances
3. Regional climates in tropics
4. Equatorial trapped waves and planetary
waves oscillations (MJO,QBO)
5. Conceptual models of synoptic tropical
disturbances in summer
6. Interactions between the mid-latitudes and
the tropics
7. ENSO
Meridional desequilibrium energy :
Hadley cell in annual mean
km
Modèle 2D méridien ; Source : Météo-France
Circulation linked to
the Hadley cell of
Southern hemisphere
-100 W/m2
Circulation linked to
the Hadley cell of
Northern hemisphere
+ 60 W/m2
°S
-100 W/m2
°N
Between 30°S/30°N, the radiative energy is positive at the top
of atmosphere (# 60 W/m2 at equator):
⃗ Planetary-scale meridian circulation called ‘Hadley Cell’
general contents
Seasonal variability :
Hadley cell in march (spring equinox)
Modèle 2D méridien ; Source : Météo-France
ITCZ located at equator
general contents
Seasonal variability of the
Hadley cell from march to july
Modèle 2D méridien ; Source : Météo-France
ITCZ located at 1°N
general contents
Seasonal variability of the
Hadley cell from march to july
Modèle 2D méridien ; Source : Météo-France
ITCZ located at 2°N
general contents
Seasonal variability of the
Hadley cell from march to july
Modèle 2D méridien ; Source : Météo-France
ITCZ located at 3°N
general contents
Seasonal variability of the
Hadley cell from march to july
Modèle 2D méridien ; Source : Météo-France
ITCZ located at 4°N
general contents
Seasonal variability of the
Hadley cell from march to july
Modèle 2D méridien ; Source : Météo-France
ITCZ located at 5°N
general contents
Seasonal variability of the
Hadley cell from march to july
Modèle 2D méridien ; Source : Météo-France
ITCZ located at 6°N
general contents
Seasonal variability of the
Hadley cell from march to july
Modèle 2D méridien ; Source : Météo-France
ITCZ located at 7°N
general contents
Seasonal variability of the
Hadley cell from march to july
Modèle 2D méridien ; Source : Météo-France
ITCZ located at 8°N
general contents
Seasonal variability of the
Hadley cell from march to july
Modèle 2D méridien ; Source : Météo-France
ITCZ located at 9°N
general contents
Seasonal variability of the
Hadley cell from march to july
Modèle 2D méridien ; Source : Météo-France
ITCZ located at 10°N
general contents
Seasonal variability of the
Hadley cell from march to july
Modèle 2D méridien ; Source : Météo-France
ITCZ located at 11°N
general contents
Seasonal variability of the
Hadley cell from march to july
Modèle 2D méridien ; Source : Météo-France
ITCZ located at 12°N
general contents
Seasonal variability :
Hadley cell in july
Circulation linked to the
Hadley cell of the winter
hemisphere, 10 times more
developped than in summer
Circulation linked to the
Hadley cell of summer
hemisphere
+ 100 W/m2
- 180 W/m2
STJ
STJ
Modèle 2D méridien ;
Source : Météo-France
°S
- 80 W/m2
ITCZ
°N
12°N
Retour début animation
⇨ Consequently, the upper troposheric jet at 30° of latitude,
called ‘subtropical Jet or STJ’ is much more developped in the
winter hemisphere than in the summer hemisphere
general contents
Vertical desequilibrium energy :
Walker and Haldey cells
Source : Météo-France.
Florent Beucher
Close to the equator, the radiative energy is much higher at
surface (+140 W/m2) than at the top of atmosphere (+60 W/m2) :
⇨ we observe a strong vertical radiative desequilibrium
between surface and top of atmosphere
⇨ initiates strong vertical velocities = ascent branch of
both Walker cells and Hadley cells
general contents
Walker cell :
Shematic description
z
equator
Annual mean [15°S-15°N] vertical cross-section of circulation
Source : Newell, 1979
• When vertical- zonal circulations are averaged over one
year and over a latitude band 15°N-15°S, the averaged
circulation is nearly vertical-equatorial (see the figure above)
and called ‘Walker cell’
• The 3 ascending branches explains the 3 deep convection
pole : Africa, Indonesia in january then India in july,
Central America
• The 3 descending branches explains subsidence over
Eastern Pacific, Eastern Atlantic and Western Ocean
Indian
general contents
References
- De Moor G. et P. Veyre, 1991 : ‘Les bases de la météorologie dynamique’
Cours et Manuel n°6 - p.193
- Lafore : Support de cours ‘Convection’, Partie 2 écrite par J. P. Lafore
CNRM/GMME.
- Morel P. éditeur (1973) : ‘Dynamic Meteorology’ –D. Reidel Publishing
Company – 622 p.
- Newell, R. E., 1979 : ‘Climate and the Ocean’ . Amer. Sci., 67, pp. 405-416
- Ooyama, 1982 : ‘Conceptual evolution of the theory and modeling of the
tropical cyclone. J. Meteor. Soc. Japan,, 60, pp. 369-380