plan tropical meteorology training course realized in 2005 by florent beucher, enm/egm...

PlanPlanTropical MeteorologyTropical Meteorology

Training course realized in 2005 byTraining course realized in 2005 byFlorent Beucher, ENM/EGMFlorent Beucher, ENM/EGM

[email protected]@meteo.fr office : C153 office : C153 : 94-30 : 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 :

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http://webrp.enm.meteo.fr/cms/view/12/content/meteo_trop/cours_meteotrop_anglais

Tropical MeteorologyTropical MeteorologyBibliography 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

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Tropical MeteorologyTropical 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.

Equator

A

A

Ridge = 30°N

Ridge = 30°S

Location of tropical atmosphere in annual mean or at equinox :

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Tropical MeteorologyTropical MeteorologyTropical MeteorologyTropical Meteorology

Definition :



Equator

A

A

Ridge = 35°N

Ridge = 25°S

Location of tropical atmosphere in august :

Northwardshift of tropicalatmospherefrom februaryto august

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Definition :



Equator

A

A

Ridge = 25°N

Ridge = 35°S

Southwardshift of tropicalatmosphere fromfrom august to february

Location of the tropical atmosphere in february :

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equator

A

A

Ridge = 25°N

Ridge = 35°S

Météorologie Tropicale Météorologie Tropicale

= vertical sounding

- Realize a radiosondage in the summer hemisphere of the tropical atmosphere :

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Acentre de ladorsale = 25°N

• 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 is observed : - in february within a latitud band 10°N-15°S, - in august within a latitud band 30°N-10°S

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equator

A

A

Ridge = 25°N

Ridge = 35°S

Météorologie Tropicale Météorologie Tropicale

= vertical sounding

- Realize a radiosondage in the winter hemisphere of the tropical atmosphere :

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Acentre de ladorsale = 25°N

• Comme l’atmosphère tropicale est quasi-barotrope, le cisaillement vertical de vent est faible :- alizés en basses couches (5 à 10kt)- vent d’est en moyenne tropo (10kt)- vent d’est en haute tropo (20 kt)

• Ce radiosondage typique s’observe : - en février entre 10°N-15°S, - en août entre 30°N-10°S

• On the poleward flanks ofthe tropical atmosphere, the baroclinicity and the vertical wind shear increase :- trades winds in low tropo. (5 à 10kt)- light westerlies in mid-tropo. (10kt)- strong westerlies in upper tropo. (60 kt)= JOST

• For instance, during the winter season in the french tropical islands, the mean state of the tropical atmosphere nearly behaves like in mid-latitudes. The theory ofAnasyg-Presyg could be work ?

• 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

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Radiosondage Analyse ARP1.5 Source : Météo-France

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 meridional radiative desequilibrium between the equator and the poles

⇨ initiates a planetary-scale meridian circulation in atmopshere called ‘Hadley Cell’‘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 verticalvertical 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


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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)

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PlanPlan

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)

Quasi-horizontal

balanced flow :

Geostophic and hydrostatic

equilibrium

Moi

st c

onve

ctiv

e in

stab

lilit

y 3D

Méso

-scale

MCS

R

(km)

11

100

10

10 000

1000

10010 10 0001000L (km) =

Horizontal

scale

Source : Ooyama 1982

L = L

= H

H thickness of

the atmosphere

10 km

Mid-

Latitudes

Tropics

Tropical

Cyclone

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PlanPlan







7. ENSO

200 hPa

Interactions between the convection (little scale) and larger scales :

Chap 2.Energy sources for the initiation and growth of the equatorial waves and tropical disturbances

⇨ 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)

850 hPa

200 hPa

850 hPa

time

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Source : Météo-FranceLafore

Illustration of this process at the end of May after the Indian Monsoon onset :

Chap 2.Energy sources for the initiation and growth of the equatorial waves and tropical disturbances

• All over tropics [30°N-30°S], by thermal forcing we observe high geopotential but with a slack gradient since tropics arenearly barotropic• By release of latent heat over Indian and Asian monsoon occur an increase of geopotential

H H

H

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Géopotentiel at 200 hPa; 22/07/05; Analyse CEP 1.5. Source : Météo-France

PlanPlan







7. ENSO

Meridional desequilibrium energy : Hadley cell in annual mean

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’

+ 60 W/m2

Circulation linked to the Hadley cell ofSouthern hemisphere

km

Circulation linked to the Hadley cell ofNorthern hemisphere

-100 W/m-100 W/m22-100 W/m-100 W/m22

°N°S

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Modèle 2D méridien ; Source : Météo-France

Seasonal variability :Hadley cell in march (spring equinox)

ITCZ located at equator

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ITCZ located at 1°N

Seasonal variability of the Hadley cell from march to july

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Circulation linked to the Hadley cell of summer hemisphere

Circulation linked to theHadley cell of the winterhemisphere, 10 times more developped than in summer

⇨ 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

+ 100 W/m+ 100 W/m22- 180 W/m- 180 W/m22 - 80 W/m- 80 W/m22

°N°S

Seasonal variability : Hadley cell in july

ITCZ

12°N

Retour début animation

STJ STJ

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Vertical desequilibrium energy : Walker and Haldey cells

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 verticalvertical radiative desequilibrium between surface and top of atmosphere

⇨ initiates strong vertical velocities = ascent branch of both Walker cells and Hadley cells

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Source : Météo-France. Florent Beucher

Walker cell :Shematic description

• When vertical- zonalvertical- zonal circulations are averaged over one year and over a latitude band 15°N-15°S, the averaged circulation is nearly vertical-equatorial 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

Annual mean [15°S-15°N] vertical cross-section of circulationSource : Newell, 1979

z

equator

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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

plan tropical meteorology training course realized in 2005 by florent beucher, enm/egm...

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