PhD student (M/F) in Couplings between convective aggregation and tropical circulations on an intraseasonal scale

Job Description

Thesis Subject

The Laboratoire de Météorologie Dynamique is is opening a PhD position about the role of deep convective aggregation on atmospheric circulations associated with tropical modes of variability and large-scale convergence zones. Small storm systems over land can for instance merge in the course of the diurnal cycle, potentially modulating larger-scale atmospheric circulations. Understanding the coupling between deep convective organization, circulation within convergence zones, and intraseasonal modes of variability represents a major theoretical challenge for the connection between weather and climate. This coupling remains poorly represented in traditional climate models and is insufficiently constrained in future climate projections.

Your Work Environment

Deep tropical convective organization remains one of the least understood emergent properties of the atmospheric circulation, often associated with intense rainfall. Organized convection occurs across a wide range of spatial scales, from deep convective systems and mesoscale squall lines (20–2,000 km) to clusters of convective systems at meso-alpha and synoptic scales (500–5,000 km). These convective aggregates exhibit considerable diversity: they may form through the merging of small storm cells generated by the diurnal cycle over land, or persist for several days as part of synoptic-scale disturbances associated with tropical waves, or be modulated by larger-scale convergence-zone circulations. Understanding the interactions between deep convective organization, large-scale convergence, and modes of intraseasonal variability remains a major challenge in weather and climate science. These processes are still inadequately represented in current climate models, contributing to substantial uncertainties in future climate projections.
The aim of this PhD project is to advance our understanding of the fundamental mechanisms linking organized convection and atmospheric circulation in the tropics. In particular, the project seeks to determine how the growth of convective aggregates can amplify or help sustain specific circulation over time. The research will focus on two key tropical environments: convective systems developing within meanders of the Intertropical Convergence Zone (ITCZ) over the tropical Atlantic Ocean, and convective aggregates associated with African Easterly Waves (AEWs). Several characteristics of circulations identified in the literature will be investigated, including the amplitude of ITCZ meanders (Mapes et al. 2018), the amplitude and persistence of African easterly waves (Norquist et al. 1977, Berry & Thorncroft 2012), the average low-level convergence and vertical momentum fluxes (Lafore et al. 1988), and the horizontal extent of the associated circulations. The central hypothesis of the project is that a critical spatial scale exists for organized convective structures. Below this threshold, large-scale circulation would primarily act as a forcing mechanism on convection; above it, convection would actively force or modify key properties of the larger-scale circulation through feedback processes.
The project combines observational analysis with theoretical modelling. The observational component will draw on existing kilometer-scale numerical simulations, geostationary satellite observations, and ERA5 reanalysis data, together with a Lagrangian tracking algorithm designed to identify and follow organized convective aggregates. The student will first characterize the circulation characteristics described above as a function of the life cycle of convective structures and will develop diagnostics to estimate the characteristic spatial scales of the associated atmospheric circulations. In a second stage, simplified theoretical models will be employed to represent both the growth of organized convective structures and the evolution of tropical waves. These models will be used to investigate the nature of their coupling and to identify the forcing and feedback mechanisms that govern their interaction. The objective will be to reproduce the behaviors diagnosed from observations and thereby assess the relative importance of different coupling processes. A range of idealized configurations will be explored, including cases with and without large-scale convergence, steady-state experiments designed to quantify sensitivity to convective scale, transient experiments allowing convective growth, and idealized setups in which specific coupling mechanisms are selectively activated or suppressed.
The outcomes of this research will contribute to improving our understanding of systematic biases in projections of future tropical precipitation. Such biases remain significant because organized convection and several modes of tropical variability are still poorly represented in conventional climate models. If time permits, the student will conduct simulations using the Meso-NH model forced by different types of tropical waves, with the aim of quantifying systematic errors arising from neglecting the influence of convection on the large-scale atmospheric

Constraints and risks

-

Compensation and benefits

Compensation

2300 € gross monthly

Annual leave and RTT

44 jours

Remote Working practice and compensation

Pratique et indemnisation du TT

Transport

Prise en charge à 75% du coût et forfait mobilité durable jusqu’à 300€

About the offer

About the CNRS

The CNRS is a major player in fundamental research on a global scale. The CNRS is the only French organization active in all scientific fields. Its unique position as a multi-specialist allows it to bring together different disciplines to address the most important challenges of the contemporary world, in connection with the actors of change.

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