25 July 2018

Biomass Smoke from Southern Africa Can Significantly Enhance the Brightness of Stratocumulus Over the Southeastern Atlantic Ocean

Science

Aerosol and cloud interactions remain one of the largest uncertainties in model projections of future climate change. During the fire season (July-September) of Southern Africa, a large portion of biomass burning (BB) aerosols is transported over the southeastern Atlanta, where semi-permanent stratocumulus deck resides. The colocation of BB aerosols and stratocumulus provides an ideal venue for studying aerosol-cloud interactions. In our study, using high-resolution model simulations in conjunction with satellite observations, we find that at ~50% of the times BB smoke is mixed with the semi-permanent stratocumulus decks underneath. This mixing causes a substantial cooling (daily mean −8.05 W m−2) over S.E. Atlantic during the fire season (July-September), primarily as a result of clouds brightening by reducing the cloud droplet size (the Twomey effect) and secondarily through modulating the diurnal cycle of cloud liquid water path and coverage (the cloud lifetime effect). 

Impact

Biomass smoke from southern Africa can significantly cool the regional climate by enhancing the brightness of stratocumulus over S.E. Atlantic. This study helps to improve our understanding of aerosol effects on clouds and regional climate. 

Summary

Using a state-of-the-art model in combination with satellite observations, we found that BB aerosols that are entrained into the clouds function as cloud condensation nuclei, and increase the brightness of stratocumulus clouds (indirect effect). This indirect effect dominates the direct and semi-direct effects of BB aerosols over this region. 

Contact
Xiaohong Liu
University of Wyoming
Publications
Lu, Z, X Liu, Z Zhang, C Zhao, K Meyer, C Rajapakshe, C Wu, Z Yang, and J Penner.  2018.  "Biomass Smoke from Southern Africa Can Significantly Enhance the Brightness of Stratocumulus Over the Southeastern Atlantic Ocean."  Proceedings of the National Academy of Sciences 115(12): 2924-2929, doi:10.1073/pnas.1713703115.