ED 173 - SDU2E
Proposition de Sujet de thèse 2012
Laboratoire (et n° de l’unité) dans lequel se déroulera la thèse :
CNRM-GAME, URA 1357
Titre du sujet proposé :
Modélisation et assimilation d’observations satellitaires micro-ondes dans les
systèmes dépressionnaires tropicaux
Spécialité : (cocher une seule spécialité)
 Astrophysique, Sciences de l’Espace, Planétologie
√ Climat, Océan, Atmosphère, Surfaces Continentales
 Ecologie Fonctionnelle
 Hydrologie, Hydrochimie, Sol, Environnement
 Sciences de la Terre et des Planètes solides
Nom et statut (PR, DR, MCf, CR, …) du (des) responsable(s) de thèse (préciser si HDR) :
Directeur : Dr Jean-François MAHFOUF (Ingénieur en Chef des Ponts, des Eaux et Forêts,
HDR) is head of « Observation » team in the NWP Research Division of CNRM-GAME (URA
1357)
Co-directeur : Dr Matthieu PLU (Ingénieur des Ponts, Eaux et Forêts) – Matthieu Plu,
researcher at LACy (UMR 8105) is associated with the « Ecole Doctorale » of La Réunion
University.
Coordonnées (téléphone et e-mail) du (des) responsable(s) de thèse :
jean-francois.mahfouf@meteo.fr 05 61 07 84 78
matthieu.plu@meteo.fr 02 62 92 11 86
Résumé du sujet de la thèse (le descriptif ne doit pas dépasser une page recto/verso)
Contexte scientifique général, Compétences souhaitables,…
Tropical cyclone forecasts and research face a major problem, which is the paucity of in-situ
observations. In such a context, the observations from different satellite platforms are essential. The
infrared and visible channels measured onboard geostationary satellites allow to monitor cyclonic
systems continuously. The microwave instruments (in frequencies ranging from 10 to 183 GHz)
onboard orbiting satellites provide some essential information about the internal structure of the
cyclones (water vapour, clouds, precipitations). However the orbit of such satellites generates a
temporally discontinuous signal. The emission and the scattering of the microwave signal by solid and
liquid hydrometeors indicate the regions of strong convection and, depending on the channel, their
microphysical properties. Owing to its inclined orbit, the Megha-Tropiques satellite, that loads the
microwave imager MADRAS and the humidity sounder SAPHIR, will provide an unprecedented data
source for observing tropical cyclone systems in microwave channels, including in real-time.
Observation and forecasting of tropical cyclones are among the main objectives of the MeghaTropiques mission (CNES/ISRO).
In the South-West Indian Ocean, Météo-France operates the numerical weather prediction model
Aladin-Réunion, which horizontal resolution is 8 km. This model has its own variational data
assimilation system 3D-Var. In the coming years, this system will evolve towards an Arome
configuration, whose finer resolution (< 4 km) allows to represent explicitly the main convective
processes inside tropical cyclones. The capability of these models to simulate microwave radiances in
cloudy and rainy situations should be higher than for global models, thus their assimilation in
mesoscale models should be considered. However, it remains a complex research issue.
The aims of the PhD are to:
-
assess the capability of numerical mesoscale models to represent realistic « microwave »
structures in tropical cyclones,
-
prepare the assimilation of the satellite radiances of the MADRAS imager and the SAPHIR
sounder in cloudy and rainy situations in mesoscale models for tropical areas.
ED 173 - SDU2E
The first topic consists in comparing the observed radiances in different channels to the ones
simulated by the numerical weather prediction models Aladin and Arome. The radiative transfer model
RTTOV will be used in a version able to describe the scattering by hydrometeors (RTTOV-SCATT).
The simulated radiances will be compared to the ones observed during the Cindy-Dynamo field
campaign (October 2011-march 2012), during which the airborne dataset obtained from the SAFIRE
Falcon-20 should improve the optical properties of clouds in the RTTOV model for tropical oceanic
convection. If the Megha-Tropiques dataset is not available during the campaign, other sensors may
be used (SSMI/S, AMSU-B). Another important aspect of microwave modelling is the distribution of
the hydrometeor size. The present microphysical parameterizations in the Aladin and Arome models
rely on a one-moment scheme: the prognostic variable of a hydrometeor is merely described by its
mixing ratio. A two-moment microphysical scheme, such as the one available in the Meso-NH
research model, uses also the particle concentration. Since scattering depends highly on the size of
hydrometeors, the sensitivity of the simulated channels to the microphysical scheme (one/two
moments) is expected to be high. This sensitivity will be assessed during this PhD. The second topic
aims at preparing the assimilation of microwave radiances from the MADRAS and SAPHIR
instruments. As a first step, the link between the prognostic variables of the Aladin/Arome models and
the simulated radiances will be analysed, using the Jacobian of RTTOV and the information content of
these radiances (as a function of the channels and the types of hydrometeors). These results will allow
to define the channels that may be used in one-dimensional variational assimilation experiments (1DVar). A particular attention will be paid to the specification of the error matrix of the a-priori information
(for short-term forecasts) in cloudy and rainy situations. The impact on storm simulations at different
stages (developing or mature cyclones) will be assessed by introducing the 1D-Var analysed profiles
into the Aladin/Arome initial states.
Profile of the student
Solid general knowledge in dynamics and physics of the atmosphere are required, as well as some
experience in numerical modelling. Some skill and knowledge in tropical meteorology, the radiative
transfer models and satellite imagery will be appreciated.
Some references
Bauer P, E Moreau, F Chevallier, U. O'Keeffe, 2006: Multiple-scattering microwave radiative transfer for data
assimilation applications. Q. J. R. Meteorol. Soc., 132, 1259-1281.
Montroty R, F Rabier, S Westrelin, G Faure N Viltard, 2008: Impact of wind bogus and cloud and rain affected
SSM/I data on tropical cyclones analyses and forecasts. Q.. J. R. Meteorol. Soc. 134.1673-1699.
Wiedner M, C Prigent, JR Pardo, O Nuissier, JP Chaboureau, JP Pinty, P Mascart, 2004: Modeling of passive
microwave responses in convective situations using outputs from mesoscale models: Comparison with
TRMM/TMI satellite observations, J. Geophys. Res., 109, D06214.