Observational Evidence of Cloud Fraction Reduction and Convective Inhibition Attributed to Fire-Emitted Aerosols in Tropical Africa

Abstract:

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 permitted us to observe the temporal relationship between clouds and aerosols. Level 2 data from thirty-five individual scenes during the fire season (December) between 2006 and 2010 were analyzed to quantify changes in MODIS cloud fraction from morning (10:30am local time) to afternoon (1:30pm local time) in the presence of different morning aerosol burdens (from MISR). We controlled for the local meteorology by analyzing thirty-five scenes from November, when fire activity and aerosol optical depth were low, but cloud fraction and meteorological conditions were similar to 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. We also analyzed temporal changes in several meteorological variables from the ECMWF ERA-Interim dataset, in an effort to understand mechanisms for cloud decimation. Initial results suggest a direct negative impact of fire aerosols on atmospheric stability and convectivity. Our results provide observational evidence of the semi-direct cloud-aerosol effect in tropical regions, and suggest a positive feedback loop between anthropogenic burning and cloudiness.