In-Situ Observations of the Boundary Layer Structure, Aerosol, Clouds and Precipitation in Pockets of Open Cells in the Remote Southeast Atlantic

Aircraft and ground based observations made across the transition between overcast closed cellular stratocumulus and a pocket of open cells (POC) feature in the remote south-east Atlantic are presented. The airborne measurements were made on the FAAM BAe-146 research aircraft during the Cloud Aerosol Radiation Interactions and Forcing (CLARIFY) field campaign. The aircraft was based on Ascension Island in August and September 2017, alongside the ARM Mobile Facility suite of ground based instruments deployed during the Layered Atlantic Smoke Interactions with Clouds (LASIC) campaign. We focus on a case-study where the aircraft measured the same airmass containing the POC feature on consecutive days (approximately 21 hr apart) up and downwind of Ascension Island. This feature advected over the ARM site in between the two aircraft flights. We link the various measurements with model trajectories and by tracking the POC using SEVERI satellite imagery.

Throughout the observing period, the aircraft observations show that a significant biomass burning aerosol (BBA) layer that had been transported from southern Africa sat immediately above the inversion at the top of the marine boundary layer (MBL), in both the closed and open cell regions. Aerosol number concentrations, Naer, measured with a PCASP instrument exceeded 1000 cm-3 in this elevated layer. Cleaner conditions in the MBL were observed outside of the POC (Naer ~ 150 cm-3), although the aerosol black carbon fraction was similar in both the MBL and free-troposphere, suggesting that entrainment of BBA into the MBL had occurred previously. In contrast, the POC feature showed extremely clean aerosol conditions in the MBL, with Naer ~ 10 to 20 cm-3 between the sea surface and the lifting condensation level (LCL), and dropping to ultra-clean values (Naer ~ 1 cm-3) between the LCL and MBL inversion. The cleaner MBL in the POC region was concomitant with lower black carbon aerosol fractions, lower cloud drop number concentrations and significant enhancements in precipitation compared to those found in the surrounding closed cell stratocumulus regions. The ARM data also show both marked reductions in the aerosol concentration measured at the surface and significant increases in rainfall from the ground based radars as the POC feature advected over Ascension Island. These observations suggest that i) collision-coalescence processes in the POC are very efficient at removing MBL aerosol and that ii) entrainment of overlying biomass burning aerosol into the POC region is insufficient to replenish the loss of MBL aerosol via precipitation.