Cloud-venting induced downward mixing of the Central African biomass burning plume during the West Africa summer monsoon

Abstract

Between June and September large amounts of biomass burning aerosol are released into the atmosphere from agricultural fires in Central and southern Africa. Recent studies have suggested that this plume is carried westward over the Atlantic Ocean at altitudes between 2 and 4 km and then northward with the monsoon flow at low levels to increase the atmospheric aerosol load over coastal cities in southern West Africa (SWA), thereby exacerbating air pollution problems. However, the processes by which these fire emissions are transported into the planetary boundary layer are still unclear. One potential factor is the large-scale subsidence related to the southern branch of the monsoon Hadley cell over the tropical Atlantic. Here we use convectionpermitting model simulations with COSMO-ART to investigate for the first time to what extent mixing related to cloud venting contributes to the downward transport of the biomass burning plume. Based on a monthly climatology, model simulations compare satisfactory with wind fields from reanalysis data, cloud observations, and satellite retrieved CO mixing ratio. For a case study on 02 July 2016, modelled clouds and rainfall show overall good agreement with Spinning Enhanced Visible and InfraRed Imager (SEVIRI) cloud products and Global Precipitation Measurement Integrated Multi-satellite Retrievals (GPM-IMERG) rainfall estimates. However, there is a tendency for the model to produce too much clouds and rainfall over the Gulf of Guinea. Looking into the CO dispersion, used as an indicator for the biomass burning plume, we identified individual mixing events south of the coast of Côte d’Ivoire due to midlevel convective clouds injecting 30 parts of the biomass burning plume into the boundary layer. This cloud venting is modulated by the underlying sea surface temperatures. Idealized tracer experiments suggest that about 20% of the CO mass from the 2–4km layer are mixed below 1km within two days over the Gulf of Guinea. There is even stronger vertical mixing when the biomass burning plume reaches land due to daytime heating. In that case, the long-range transported biomass burning plume is mixed with local anthropogenic emissions. Future work should provide more robust statistics on the cloud venting effect over the Gulf of Guinea and include aspects of aerosol deposition