Convective clouds can arrange into harmonious patterns from the kilometer-scale to the planetary scale. This results from a complex interplay between the atm

ospheric circulation, convection, and the condensation of water vapor, of which our understanding remains elusive. Despite major advances during the last decade, various theories are still proposed to describe this phenomenon, which remains a key challenge for improving weather forecasts and anticipating both the magnitude and the impacts of future global warming.

In this thesis, we explore this question by focusing on the clear air surrounding the clouds, that is governed by simple and well-established physical laws.

We first show that it is possible to measure the vertical velocity of this clear air, and present an archive of such measurements based on geostationary satellites and infrared sounders. The observations reveal the rich wave activity of the clear-air tropical atmosphere. They also show strong subsidence in the vicinity of deep convective systems.

To explain this subsidence, we propose a conceptual model, the dipole model, in which thermals, shallow, and deep convective clouds are assimilated to hydrodynamic dipoles that transport air upwards in the atmosphere. We study the theoretical implications of this mass transport on the atmospheric circulation in the surrounding clear air. The model is able to explain some characteristic features of tropical convection, such as the formation of moist halos around clouds and the spontaneous clustering of clouds. It also highlights the importance of cloud geometry, and suggests that the depth of clouds controls a range of variables from the average relative humidity profile in the tropics to the characteristic horizontal scale of the cloud patterns.

We also propose a simple model of the dry boundary layer that leads to the propagation of non linear waves. We interpret the intertropical convergence zone (ITCZ) as a stationary shock wave. Conversely, we suggest that the doldrums, ubiquitous areas of weak and variable winds, are rarefaction waves. By coupling the boundary layer with the dipole model, we derive a dimensionless number that controls the transition between a single and a double ITCZ.

Our theoretical analysis is evaluated against space observations, as well as data from various airborne field campaigns. The observations have striking similarities with the theory, but also notable discrepancies, raising new questions and highlighting possible avenues for future work.

Informations to connect remotely:

https://cnrs.zoom.us/j/99218804782?pwd=1b9UqlqhXBmAzVsbvUkw89nWM7fPvk.1

Meeting ID: 992 1880 4782
Secret code: c0nvecti0n

Aymeric Spiga, Sorbonne Université (Président)

George Craig, LMU München (Rapporteur)

Rémy Roca, CNRS (Rapporteur)

Caroline Muller, ISTA (Examinatrice)

Hirohiko Masunaga, University of Nagoya (Examinateur)

Corentin Herbert, CNRS (Examinateur)

Sandrine Bony, CNRS (Directrice de thèse)