The Evolution of the Southeast Atlantic Boundary Layer Determined from Meteosat-10 Cloud Retrievals, GEOS-5 Model Outputs, and Observations during ORACLES 2016

The evolution of the marine boundary layer and the stratocumulus-to-cumulus transition over the southeast Atlantic is described by means of satellite cloud retrievals from Meteosat-10 matched with 5-day trajectories obtained from dedicated GEOS-5 simulations and observations during ORACLES 2016 (August-September). The downwind transition of the cloud droplet number concentration, height, and liquid water path is analyzed as a function of the parcel distance from the coast, with trajectories initialized at locations between 0-12˚E at the 25th parallel south. The equatorial-ward boundary layer deepening and the cloud microphysical interaction with the overlying smoke plume (first indirect effect) is investigated with the use of vertically-resolved aerosol retrievals from the airborne NASA Langley High Spectral Resolution Lidar (HSRL), which is in turn applied for discerning the clouds more prone to be affected by entrained biomass burning aerosols. The satellite-based diurnal cycle of cloud height is utilized for quantifying the entrainment rate in a mixed-layer framework, whereas the potential effect of the free tropospheric stabilization due to absorbing aerosols (semi-direct effect) is analyzed for different aerosol loadings by means of the liquid water path and cloud fraction diurnal cycle. Lastly, satellite retrievals are complemented and evaluated with a suite of in-situ ORACLES observations. A companion large-eddy simulations study (Fridlind et al.) aims to interpret the statistical relationships derived from this investigation.