Downward cloud venting of the central African biomass burning plume during the West Africa summer monsoon
<p>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 a...
| Main Authors: | , , , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2020-05-01
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| Series: | Atmospheric Chemistry and Physics |
| Online Access: | https://www.atmos-chem-phys.net/20/5373/2020/acp-20-5373-2020.pdf |
| Summary: | <p>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 (PBL)
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 convection-permitting model simulations with COSMO-ART
to investigate for the first time the contribution of downward mixing
induced by clouds, a process we refer to as downward cloud venting in
contrast to the more common process of upward transport from a polluted PBL.
Based on a monthly climatology, model simulations compare satisfactory with
wind fields from reanalysis data, cloud observations, and satellite-retrieved carbon monoxide (CO) mixing ratio. For a case study on 2 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. Using the CO
dispersion as an indicator for the biomass burning plume, we identify
individual mixing events south of the coast of Côte d'Ivoire due to
midlevel convective clouds injecting parts of the biomass burning plume into
the PBL. Idealized tracer experiments suggest that around 15 % of the CO
mass from the 2–4 km layer is mixed below 1 km within 2 d over the
Gulf of Guinea and that the magnitude of the cloud venting is modulated by
the underlying sea surface temperatures. There is even stronger vertical
mixing when the biomass burning plume reaches land due to daytime heating
and a deeper PBL. 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 downward cloud venting effect over the
Gulf of Guinea and include aspects of aerosol deposition.</p> |
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| ISSN: | 1680-7316 1680-7324 |