Motivation & Introduction

The Congo Basin and its north-western surroundings contain nearly 20 % of the world’s tropical rainforest and constitute one of the planet’s most active convective regions. Two rainy seasons dominate the region—March to May (MAM) and September to November (SON)—with SON typically being the wetter period. During the boreal-summer dry season (June–August), the western sector is characterised by persistent low-level clouds, whereas the central basin experiences comparatively sunnier conditions. This dry period is also marked by extensive biomass-burning fires that emit roughly 50 % of global biomass-burning aerosol (BBA).

Convection in the basin is highly stochastic: mesoscale convective systems (MCSs) are more numerous, shorter-lived and propagate more slowly than in the Sahel. This behaviour is linked to low convective inhibition (CIN), weak low-tropospheric shear, and a moist lower troposphere that rapidly rebuilds convective available potential energy (CAPE) after storm passage. Despite the basin’s importance for regional hydrology and its substantial contribution to the global aerosol burden, the mechanisms by which BBA influence the diurnal evolution of the planetary boundary layer (PBL), as well as the timing, size, and organisation of convection, remain poorly understood.

Project aims

The overarching aim is to understand the role of biomass‑burning aerosols during the transition from the boreal‑summer dry season with extensive fires to the subsequent rainy season, with a specific focus on the diurnal cycle of the PBL and convection.

Research objectives

Direct effects – Quantify how BBA delays daytime convective initiation and changes storm size and number by analysing long‑term aerosol, cloud, radiation, and rainfall observations.
This will be achieved by statistically contrasting clean and polluted days under comparable meteorological conditions, using satellite-based tracking of convective systems, aerosol optical depth, and surface solar radiation data across homogeneous subregions.

Semi‑direct effects – Quantify how BBA over PBL clouds delays daytime convective initiation and alters storm size and number, using new satellite retrievals and reanalysis data for aerosol‑over‑cloud.
Cases will be distinguished by the vertical position of aerosol layers relative to the PBL and low clouds using CAMS reanalysis and independent satellite products, and their impact on cloud dissipation and the development of convection will be evaluated statistically. Convective responses will be analyzed using the framework established for direct effects.

Indirect effects – Explore possible microphysical impacts of BBA on individual storms and contrast them with overall radiative effects on storm statistics, combining BACCOPA aircraft measurements with targeted modelling experiments.
Selected case studies will be simulated with high-resolution cloud-resolving models, with aerosol conditions constrained by aircraft observations and complemented by targeted sensitivity experiments. Analyses will address both properties of individual storms and regional convective statistics to assess the roles of aerosol-related microphysical processes and radiative effects.

Partners & Integration in BACCOPA

The project constitutes the German (IAC3) contribution to BACCOPA (Biomass‑burning Aerosol, Clouds, Convection and Precipitation in Central Equatorial Africa), an international campaign coordinated by the French institutes CNRM, LATMOS, LISA and LOA. In September 2026 a French ATR‑42 aircraft will be based in Brazzaville (Republic of Congo) and equipped with lidar, cloud radars, a sun‑photometer and aerosol‑sampling units. Through this close integration we will obtain the first collocated and near-simultaneous measurements of BBA, hydrometeors, vertical motion, and radiation in the Congo Basin, enabling a rigorous test of the three research objectives.