Research Interests

My research interests lie mainly in Geophysical Fluid Dynamics. I try to combine theoretical approaches, numerical simulations and observations to make progress on questions relating to:

Internal Gravity Waves

Satellite image of a gravity wave made visible in a deck of clouds. Internal gravity waves are waves occuring in the interior of a stratified fluid, the restoring force being due to buoyancy. These waves are typically small-scale features of the flow (10-1000km in the horizontal). A lot can be understood of atmospheric and oceanic dynamics without taking them into account (i.e. using balanced approximations such as the quasi-geostrophic on). Yet, there are aspects of both atmospheric and oceanic circulations which can not be understood without considering gravity waves (e.g. the circulation of the middle atmosphere and in particuar the mesosphere, the energy budget in the ocean). At present, one essential challenge is to understand and quantify the sources of these waves, in order to be able to parameterize them in global models.

The above satellite picture comes from Nasa's Planetary Photojournal. As in this example, gravity waves are sometimes visible thanks to clouds, when water vapor saturates in the regions where air is ascending due to the gravity wave present.



Observations of gravity waves

In observations, gravity waves can often be detected as small-scale variations, in the horizontal wind and/or in the temperature field. There are of course severe limitations to how much can be deduced from these small-scale features, for several reasons, two of which have retained my interest: 1- the separation between the background field and the small-scale part is unavoidably arbitrary; 2- the information we have is always incomplete; linear theory (e.g. the hodograph method to retrieve the intrinsic frequency of a wave from the wind perturbations) is used to infer characteristics of the waves from the available information. Yet linear theory for waves in a fluid at rest is sometimes far from applicable.

Limitations of balance

Much has been understood in atmospheric and oceanic dynamics from models which filter out gravity waves and assume that the flow follows a certain balance (e.g. geostrophic and hydrostatic). Understanding how far these models can be trusted is tied to understanding how much balanced motions and gravity waves interact, and in particular how much balanced motions excite gravity waves.


Nonlinear waves

Gravity waves are most often considered within the framework of linear theory. For internal graviity, it turns out that the linear solution is also solution of the full nonlinear equations. Yet for other systems, e.g. for gravity waves in a two-layer rotating fluid, one can find exact periodic solutions that are nonlinear and exhibit interesting behaviors with amplitude, as suggested in the illustration below.


Baroclinic Instability

In order to study how inertia-gravity waves are generated in different life cycles of baroclinic instability, I have carried out idealized simulations of baroclinic instability with the the Weather Research and Forecast Model. Below are two figures showing, for two very different life cycles, the potential temperature at the surface (left) and on the dynamical tropopause (right).

Symmetric and inertial instability

In the study of the geostrophic adjustment of jets, we have underlined the key role of anticyclonic regions for trapping inertia-gravity waves with frequencies lower than the inertial frequency. Inertial instability appears as the unstable counterpart of these subinertial waves when the anticyconic vorticity becomes lower than -f. This instability, its selection of a vertical scale, its nonlinear saturation and its impacts for mixing are not yet fully understood.

Cheese fondue

The proper combination of cheeses to use for a cheese fondue remains a matter of heatedd debates: emmenthal, comte, beaufort, appenzeller, gruyere, abondance are of course all on the list, but are they all necessary? In which proportions? The choice of the wine and the addition of Kirsch are also a subject of numerous experiments, and for which theory is still lacking. Finally, the status of alternative fondues, involving cheeses like goat, roquefort or munster, or using milk instead of white wine (for pregnant and breastfeeding mothers) are further subjets of investigation.