Remote Sensing Study of the Relationships between Biomass Burning Aerosols and Marine Stratocumulus during ORACLES Campaign

Relationships between biomass burning aerosols and marine stratocumulus over the South-East Atlantic Ocean were studied using data collected from the NASA ORACLES (ObseRvations of Aerosols above CLouds and their intEractionS) campaign. This region is of particular interest due to its semi-permanent stratocumulus cloud deck that is an important component in the climate system, and the periodic biomass burning smoke plumes that are transported over the extensive Sc cloud decks. Before mixing with a cloud, elevated smoke plumes are often separated by smoke-free gaps for a period. The smoke aloft can influence the underneath clouds by changing radiative fluxes and local instability (semi-direct effect). The smoke entrained into a cloud can modify cloud microphysical properties by serving as cloud condensation nuclei (indirect effect). These effects may cause either positive or negative cloud radiative forcing depending on cloud type, smoke layer position, smoke optical properties, meteorological conditions, etc. But, the governing physical processes of these interactions and their representations in climate models remain largely uncertain. ORACLES provides a great opportunity to study these interactions with comprehensive in-situ and airborne remote sensing data of aerosol optical depth (AOD), cloud liquid water path (LWP), cloud top effective radius (R_e), particle concentration, and thermodynamic profiles. In this study, we focus on remote sensing data from the High Spectral Resolution Lidar 2 (HSRL-2), Research Scanning Polarimeter and Enhanced MODIS Airborne Simulator. Two typical aerosol/cloud vertical distributions were identified using HSRL-2 measurements. The cloud properties and environmental conditions were examined under different aerosol loadings and different aerosol/cloud vertical distributions. Results show contrasting responses of cloud LWP and R_e to above cloud AOD between cases where smoke and cloud are in contact and cases where they are separated, indicating microphysical and radiative responses to the presence of absorbing aerosols. The observed difference between contact and separated cases are primarily caused by the variations of cloud-top entrainment and evaporation rate due to aerosols, and the meteorological conditions within and above the boundary layer.