Impact of Biomass Burning Aerosols on Marine Stratocumulus during ORACLES—Evidence of the Semidirect Effect?

Aerosol-cloud interactions in the southeastern Atlantic (SEA) region were investigated during three filed deployments (September 2016, August 2017, and October 2018) during the ObseRvations of Aerosols above CLouds and their intEractionS (ORACLES) EV-2 campaign. This region is of interest due to seasonally persistent marine stratocumulus (MSc) cloud decks that are an important component of the climate system due to their radiative and hydrologic impacts. The SEA deck is unique due to the interactions between these clouds and transported biomass burning aerosol during the July-October fire season. These biomass burning aerosol play multiple roles in modifying the cloud deck through interactions with radiation as absorbing aerosol and through modifications to cloud microphysical properties as cloud condensation nuclei. This work uses in situ cloud data obtained with a Flight Probe Dual Range – Phase Doppler Interferometer (FPDR – PDI) and aerosol data to evaluate the impact of Biomass Burning Aerosol on MSc clouds.

The combination of the FPDR – PDI and the suite of aerosol instrumentation on board the NASA P-3 allows for a thorough analysis of aerosol-cloud interactions in the SEA. The FPDR – PDI provides unique cloud microphysical observations of individual cloud drop arrivals allowing for the computation of a variety of microphysical cloud properties including individual drop size, cloud drop number concentration, cloud drop size distributions, liquid water content, and cloud thickness. Observations made during level legs were analyzed as a function of Black Carbon (BC) content during the 2016-2018 campaigns. Preliminary analysis shows evidence for the semi-direct effect, where microphysical properties vary as a function of BC amount in a manner consistent with theoretical predictions of microphysical and radiative responses to the presence of absorbing aerosols.