Soutenance

Small-scale, subgrid processes need to be parametrized in Global Climate Models (GCMs) in order to improve current model projections of climate change. The overall goal of my research activities is to use theoretical and numerical models, from idealized high-resolution simulations to global GCMs in realistic configuration, in order to improve our fundamental understanding of these small-scale processes in the atmosphere and in the ocean. Important examples that I work on are internal waves in the ocean, and cloud processes in the atmosphere. 

More specifically, in this presentation, I will focus on three main topics:

(1) What role does the small-scale seafloor topography play in the generation of internal waves in the abyssal ocean? What is the contribution of those internal waves to ocean mixing, and what impact on the ocean large-scale circulation? 

Internal waves are ubiquitous in the ocean, and contribute significantly to the ocean global energetics. But their scales are too small, both in space and time, to be explicitly resolved in GCMs. Thus their effect on water masses and on the large-scale ocean circulation must be parameterized. The goal of my research is to clarify the physical mechanisms leading to the instability, overturning and breaking of those waves, as well as to quantify the mixing induced by wave breaking events.

(2) What is the response of the hydrological cycle (both mean precipitation and extreme precipitation) to global warming? 

Understanding the response of precipitation extremes to climate change is a crucial question, with important societal impacts. Numerous studies, using both regional high-resolution models and global coarse-resolution models, predict an increase of the intensity of precipitation extremes with warming. The physical origin of this increase is well understood, thanks to numerous theoretical studies, notably its link with a strong thermodynamic constraint (related to water vapor). But if the thermodynamic contribution to precipitation extremes is well understood, a large uncertainty remains regarding the dynamic contribution linked to vertical velocities in updrafts, and the microphysics contribution related to precipitation efficiency. My work aims at quantifying these contributions, in idealized simulations without and with convective organization.

(3) What are the physical processes responsible for the organization of tropical clouds? 

Arguably, the most spectacular example of organized convection is the tropical cyclone, with its eye devoid of deep convection, surrounded by a cloudy eyewall where extreme winds are found. There are other types of organized convection, (convection refers to the overturning of air within which clouds are embedded). In fact, organized convection is ubiquitous in the tropics. But it is still poorly understood and typically not accounted for in global climate models, despite strong societal and climatic impacts. One of the goals of my research is to clarify the physical processes responsible for organizing convection, and to investigate their response to climate change.

In this HDR presentation, I will describe the major scientific outcomes of my research activities in those three topics. I will also discuss my perspectives and future research directions, as well as outstanding open questions.