ECOLE DOCTORALE DES SCIENCES DE L

ECOLE DOCTORALE DES SCIENCES DE L’ENVIRONNEMENT
D'ILE DE France N° 129
Proposition de sujet de thèse pour la rentrée 2016
Nom du Laboratoire d’accueil :Laboratoire de Météorologie Dynamique
N° UMR :8539
Nom du Directeur du laboratoire :Vincent Cassé
Adresse complète du laboratoire : Laboratoire de Météorologie Dynamique, Ecole Normale Supérieure , 24
rue Lhomond, 75005 PARIS
Nom de l’Equipe d’accueil et adresse si différente de celle du laboratoire : DPAO
Nom du Directeur de thèse HDR: Bernard Legras, DRCE
Téléphone : 01 4432 2228
Mail :
[email protected]
Co-encadrants non HDR : Geneviève Sèze et Pasquale Sellitto
• Titre de la thèse : Impact de la convection pendant la mousson d'Asie sur la tropopause tropicale et la
stratosphère
Impact of the Asian monsoon convection on the tropical tropopause layer and the stratosphere
• Sujet proposé (2 pages maximum) :
The Tropical Tropopause Layer is the region of the atmosphere between 30S and 30N which is flanked by the
convectively dominated tropical troposphere below and the layered motion in the stratosphere regulated by the
Brewer Dobson circulation above. The TTL receives air drawn by convection from the boundary layer with
compositionaffectedbynaturalandanthropogenicemissions.Alargepartofthisairsubsidesbacktolowerlayersin
the tropics but a part is transported to the stratosphere across the tropopause. The TTL is the unique gateway
throughwhichairentersthestratosphereasstratosphericmotionisdescendingeverywhereoutsidethetropicsand
exchanges through vertical turbulent diffusion do not penetrate higher than a thin layer above the tropopause. As
suchtheTTLisakeyregionthatdeterminesthecompositionofthestratosphereandcontrolsacrucialpartofthe
climatesystem.
The convective clouds only very rarely penetrate high enough to reach the tropopause, near 17.5 km. Most of the
cloudtopsarefoundnear12-13kmandtheirdistributiondecreasesveryfastwithaltitude.Animportantlevelinthe
TTListhelevelofzeroradiativeheatingrate(LZRH)thatseparatestheregionofascendingmotionabovefromthe
regionofdescendingmotionbelow.
This mean level is not, however, an impermeable barrier as cloud heating associated with convection or cirrus can
disturb it considerably. It is now well established that the air goes through the TTL by a combination of fast
convectivemotionandslowascent,thesecondeffectbeingmoreandmoreprevalentwithaltitude.Thecrossingof
the TTL is not uniformly distributed around the globe. During half of the year, centred on the winter season, the
crossing occurs over the major convective region of the so-called warm pool than spans Indonesia and the West
Pacificconvectionoverthewarm-poolismostlymaritime.
Duringsummer,themajorcrossingoftheTTLisrelatedtotheAsianmonsoonbutthenthesituationismuchmore
complexbecauseofthedistinctivecharactersoftheconvectionoverAsianlandandtheconvectionoversurrounding
seas(mostlytheBayofBengalandtheSouthChinaSea).OvertheTibetanplateau,theconvectionisveryefficientat
carryingboundarylayercompoundstothestratosphere.Theupperatmosphericcirculationisstronglyinfluencedby
the Asian monsoon anticyclone (AMA) which ventilate the convection beneath and redistribute its product over a
vastareathatextendtoAfricaandtheMediterraneanbasin.TheAMAalsotrapscompoundsinsideandisassociated
with an aerosol layer (ATAL) which has shown a dramatic increase over the last decade, suspected to be
anthropogenic due to the increased pollution in Asia. The convection over other regions like Africa and South and
CentralAmericadoesacontributionbutitislimitedtolessthan20%inaverage.
Althoughthemajorpatternofsourcesandtransportisfairlywellunderstoodasdescribedabove,therearestilllarge
uncertaintiesinthedetaileddistributionofsourceswhichisdependingontherepresentationofhighaltitudeclouds
andoftheanalysedwindsprovidedbyweatherforecastcentres.ForinstancearedistributionbetweentheTibetan
plateauandindustrializedregionsofAsiahasalargeimpactlinkedtotheeffectofpollutiononthestratosphere.The
residence time in the TTL is also highly debated with consequences on the composition of air entering the
stratosphere.Satelliteobservationsandaerosolcountersdetectedthepollutioncontentsofthestratospherebutthe
precisecompositionoftheATALisstillunknown.
Lagrangiantrajectoriesareatoolofchoiceforallstudiesinvolvingtransportissuesintheatmosphere.Theysuppress
thespuriouseffectsassociatedwithnumericaldiffusionandcanconcentratetheresolutioninsometargetregions.
Forinstance,backwardtrajectorieslaunchedwithhighdensityalonganaircraftpathoroveraninterestingstructure
detectedbysatellitecanrelatetheobservationstotheprevioushistoryandprocessesundergonebythesampledair
mass.
Trajectoriesaresensitivetothequalityofwindsandheatingratesprovidedbythereanalysis.Thisisamajorissuein
the Asian monsoon region where significant discrepancies are observed between the data sources. The StratoClim
campaigndataandnewdetailedradiativecalculationswillbeusedtoclarifythisissue.
Thedevelopmentstobemadeduringtheprojectinclude:
(i) Refinement of the partitioning of sources according to relevant geographical regions in Asia which exhibit
different convective signature (Arabian Sea, Indian peninsula, Bay of Bengal, Monsoon Trough region, Tibet
Plateau,IndoChina,Myanmar,etc),(ii)Improvingourunderstandingofradiativeheatingratesbycomparisonsof
reanalysis heating rate with campaign data and radiative calculations using observed distribution of clouds by
activeinstruments(DARDARproductsbasedonCALIPSO+CLOUDSAT)(iii)Usinghightemporalresolution,refined multi-spectral cloud top retrievals from new generation geostationary and polar satellites, (iv) Statistical
representationoftransientovershootsaboveconvectiveanvilsbasedonknowledgeacquiredfromLESmodelsof
atmospheric convection and satellite observations, (v) Parameterization of fast vertical motion and temperature
variationsintheTTLduetogravitywaves,basedonquasi-Lagrangianinsituobservationsbyhighaltitudeballoons
andwind-profilerobservationsfromIndia,(vi)Usinghighresolutionanalysedwindfromstateoftheartmodelfor
case studies related to the campaign, using last generation modern reanalysis for multi-year studies, (vii) Using
windsfrommeso-scalemodelsimulationperformedatLA(Toulouse) andcomparisonwithresultsfromanalysed
winds,(vii)Runningdetailedboxmodelsofaerosolsandicecloudsalongtrajectoriesderivedfrompreviousworkin
theteam.
• Type de Financements prévu autre que ED 129 (CNES, CEA, ADEME etc…) :
Nous demandons un support à l’ED 129 car le projet StratoClim qui oriente notre programme de recherche
dans les prochaines années est malheureusement largement sous financé pour tous les partenaires eu égard
aux objectifs.
• Encadrement :
. Ann’Sophie Tissier, thèse soutenue le 8 janvier 2016, en contrat post-doc jusque fin mai 2016