Description du sujet de thèse

In equatorial Central Africa, deep convection plays a key role in the energy balance and the water cycle, producing a large proportion of precipitation. The seasonality of precipitation, in particular the length of the dry and wet seasons, and the intensity and quantity of rainfall are key parameters in the composition and structure of Central Africa's forests. On a global scale, this region accounts for around 50% of aerosols from biomass combustion, mainly emitted during the dry season from June to September. These aerosols have a major impact on the hydrological cycle and the radiation balance, both directly and indirectly, through changes in solar radiation, atmospheric stability, convection processes and cloud properties. The aim of this thesis is therefore to study the impact of aerosols emitted by biomass fires on the thermodynamic properties of the troposphere, clouds and convection in Central Africa. These interactions will be studied using high-resolution regional modelling in conjunction with in-situ measurements that will be obtained as part of a field campaign in Central Africa (BACCOPA project). In parallel, satellite data will also be used to evaluate the modelling exercises.

Contexte de travail

The aim of this thesis is to study the impact of aerosols emitted by biomass fires on the thermodynamic properties of the troposphere, clouds and convection in Central Africa.

In equatorial Central Africa, deep convection plays a key role in the energy balance and the water cycle, producing a large proportion of precipitation. The seasonality of rainfall, in particular the length of the dry and wet seasons, and the intensity and quantity of rainfall are key parameters in the composition and structure of Central African forests. On a global scale, this region accounts for around 50% of aerosols from biomass combustion, mainly emitted during the dry season from June to September [Van der Werf et al., 2006]. These aerosols have a major impact on the hydrological cycle and the radiation balance, both directly and indirectly, through changes in solar radiation, atmospheric stability, convection processes and cloud properties. The difficulty associated with studying these impacts is linked to the many complex processes involving aerosols, which operate on different spatial and temporal scales. At present, these interactions between aerosols and convective systems remain one of the main sources of uncertainty in our understanding of the indirect radiative effect of aerosols on climate.

The first important process is directly linked to the capacity of biomass fire aerosols to act as condensation nuclei (CCN) [Petters et al., 2009], and therefore to modulate convection and associated precipitation (indirect radiative effect). In addition, biomass fire plumes strongly absorb solar radiation [Mallet et al., 2021], inducing atmospheric warming that tends to stratify the lower troposphere (direct and semi-direct effects), potentially modifying convection and associated precipitation [Tosca et al., 2015]. At the same time, the reduction in solar radiation received at the surface can also affect latent heat fluxes between the surface and the atmosphere, and therefore moisture sources [Mallet et al., 2020]. To date, very little attention has been paid to the role of aerosols from biomass combustion on radiation, convection, precipitation and the hydrological cycle over equatorial central Africa. In this context, the BACCOPA (Biomass-burning Aerosol, Clouds, Convection and Precipitation in central equatorial Africa) project was developed to improve our understanding of aerosol-cloud-convection interactions in central Africa and their impact on its climate.

The person recruited will be responsible for developing and studying these interactions on seasonal/climatic scales using regional modelling at kilometre resolution (regional climate model AROME, Caillaud et al. 2021). These interactions between aerosols, radiation and convective clouds are very complex, and high-resolution regional models represent original tools for tackling these scientific questions on a seasonal and climatic scale. By taking into account both the direct and semi-direct radiative effects, as well as the indirect effects of aerosols using the TACTIC interactive aerosol scheme (Nabat et al. 2020) coupled with the LIMA two-moment microphysical scheme (Vié et al. 2016), this numerical tool will make it possible to study the main impacts in detail. The simulations carried out using the AROME regional model will be evaluated using observations from the field campaign planned for September 2026, as well as using satellite observations (EarthCare, 3MI or MTG sensors). This thesis will be carried out as part of the ANR BACCOPA project (https://anr.fr/Projet-ANR-23-CE01-0013).

Contraintes et risques

No particular constraints. Screen work.