Observational evidence of fire-driven reduction of cloud fraction in tropical Africa

Anthopogenic savanna fires in sub-Saharan Africa emit smoke that affects cloudiness in the region. We measured the cloud response to fire aerosols using aerosol data from the Multi-angle Imaging SpectroRadiometer (MISR) and cloud fraction data from the morning and afternoon overpasses of the Moderate Resolution Imaging Spectroradiometer (MODIS) instrument. Considering the same cloud scene from the morning and afternoon satellite observations allowed us to observe the temporal relationship between clouds and aerosols. Level 2 data from 35 individual scenes during the fire season (December) between 2006 and 2010 were analyzed to quantify changes in MODIS cloud fraction from morning (10:30 A. M. local time) to afternoon (1:30 P. M. local time) in the presence of different morning aerosol burdens (from MISR). We controlled for the local meteorology by analyzing scenes from November, when fire activity and aerosol optical depth were low but cloud fraction and meteorological variables (boundary layer height, pressure, total column water vapor, temperature, and convective available potential energy) were similar to those of the fire season. High-fire-driven aerosol optical depth (AOD) was associated with reduced cloud fraction in both the raw and meteorologically normalized data. Fire aerosols reduced the relative cloud fraction in all sky conditions, but the effects were progressively larger in high-AOD conditions. These results may provide observational evidence of the semidirect cloud decimation effect in tropical regions and suggest a positive feedback loop between anthropogenic burning and cloudiness-where more aerosols lead to decreased clouds, increased surface exposure and drying, more fire, and thus more aerosols-which is consistent with previous studies linking smoke aerosols to reduced cloudiness and vice versa.