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Towards a Comprehensive Analysis of the Impact Biomass Burning Aerosols have on the West African Monsoon


This dissertation is motivated by the lack of research depth on the influence biomass burning produced aerosols impose on West African monsoon dynamics during daily time scales. The impacts of such aerosols on atmospheric thermodynamics and cloud microphysics are investigated using a combination of satellite observations and global climate model simulations.

Using observations, we find (1) suppression of precipitation rates over the Guinea coastline with elevated aerosol transport and (2) an increase in rates with decreased aerosol amounts. Decreases in the amount of shortwave radiation reaching the surface over land and ocean attributed to increases in low-level cloud coverage and the absorptive nature of biomass aerosols observed in nature on dirty conditions are matched with reductions in convective available potential energy (CAPE), vertical air mixing essential for deep-convection cloud cover. Increased aerosol content is also associated with decreases in cloud droplet effective radius, a defining feature of aerosol indirect effects.

Guided by available observations, model experiments are designed for NCAR’s CESM2 with meteorology being nudged to observations using MERRA2 reanalysis to visualize data not available through such observations. Simulations successfully reproduce aerosol transport and corresponding precipitation changes found in observations. However, this model is not in agreement with changes in low-level cloud fraction which leads to increases (decreases) in shortwave radiation reaching the surface (top of atmosphere) during dirty conditions. Our model simulations show that aerosol semidirect and indirect effects interact together to alter cloud formation processes and ultimately control the precipitation response.

Throughout the equatorial Atlantic, smoke aerosols impact the structure of stratocumulus to cumulus transition (SCT) through an alteration of atmospheric stability and moisture availability. Boundary layer deepening and increasing sea surface temperatures put the location of this transition within the Gulf of Guinea. Increased low-level clouds occur over the Atlantic cold tongue where aerosol layers reside above low cloud tops, reflecting a negative aerosol semidirect effect. Coupled with significant changes in cloud top height and tropospheric stability further South, these aerosol effects combine to extend in space during increased aerosol loading episodes.

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