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PhD Defense

Florentin Lemonnier (LMD)

Title : Observation and modelling of precipitation in Antarctica using radar techniques and the LMDz model

Date and time : The 03-12-2019 at 14h00

Type : thèse

Université qui délivre le diplôme : Sorbonne Université

Location : Sorbonne Université, Campus Pierre et Marie Curie, Amphithéâtre 55B (Tour 55, niveau dalle, 4 place Jussieu 75005)
Members of jury :

Brice BOUDEVILLAIN, Physicien adjoint, Institut des Géosciences de l’Environnement, Rapporteur
Xavier FETTWEIS, Chargé de Recherches F.R.S.-FNRS, Laboratoire de Climatologie et de Topoclimatologie, Rapporteur
Francis CODRON, Professeur, Laboratoire d’Océanographie et du Climat, Examinateur
Masa KAGEYAMA, Directrice de Recherches, Laboratoire des Sciences du Climat et de l’Environnement, Examinatrice

John KING, Senior scientist, British Antarctic Survey, Examinateur
Chantal CLAUD, Directrice de Recherches, Laboratoire de Météorologie Dynamique, Directrice de thèse

Jean-Baptiste MADELEINE, Maître de conférence, Laboratoire de Météorologie Dynamique, Co-directeur de thèse

Summary :

Antarctica is an immense continent made of ice. This region remains to this day one of the most unknown regions of our planet Earth. It contains nearly 90% of the world's fresh water, and in the current situation of global warming, this frozen reservoir is under serious threat. In the absence of reliable observations or efficient climate models, it is so far difficult to verify what is happening in Antarctica, and how it may evolve in the coming decades. Indeed, some studies predict significant ice melt in the west, other studies predict snow accumulation at the surface of its eastern region, mostly by precipitation events. It is, however, difficult to know if one of these two processes takes priority over the other, and which one.

The CloudSat satellite is the only precipitation survey tool available on the continental scale. However, there are significant uncertainties about its measurements and the confidence that can be given to it is inevitably very low. Using precisely calibrated radar instruments deployed on the Dumont d'Urville coast station and the Princess Elisabeth continental station, we compared observations of precipitation from both surface and space. Comparison of these observations between the two datasets allowed to validate the satellite's precipitation measurements by re-evaluating an uncertainty on the measurement -- initially between 150 and 250% -- to less than 24%.

This result gives more confidence to the measurements of this satellite, and from four years of continuous precipitation observation, we have developed the first three-dimensional and model-independent climatology of snowfall in Antarctica. Comparison of this dataset with theoretical precipitation rates calculated from the forced uplift of an air mass along a topographic slope shows that snowfall appears to be controlled at the first order by large-scale advection and forced uplift of wet flows against topography.

We compared this dataset to the global climate model LMDz to assess its capability to represent precipitation over this polar region. Using several simulation configurations, we identified dynamic, warm and moist biases as well as biases in the moisture advections of the model. We have explored ways to reduce these biases, such as the use of atmospheric forcings (winds, temperature and specific humidity) or the sensitivity of microphysical precipitation parameters. In addition, a combination of forcing and numerical adjustment reducing the dissipation of the LMDz model on diabetic perturbations allows precipitation to be simulated in accordance with the available observations.

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