Observations of Cloud-Aerosol Interaction Over the South Atlantic Ocean

The impact of biomass burning aerosols (BBAs) on cloud microphysical properties is poorly characterised. BBAs are a complex aerosol species, comprised of black carbon mixed with organic and inorganic compounds. As such there are large uncertainties in the role they play in cloud-aerosol interactions, leading to uncertainties in predicting cloud radiative properties, cloud lifetime and precipitation. These uncertainties limit our ability to accurately constrain large scale NWP and climate models to predict weather and future climate scenarios.

Africa is the dominant source of BBA (~50% of global emissions). These are generally lofted high into the free troposphere and then transported offshore over long distances in a layer above the extensive sheets of stratocumulus clouds that form over the eastern Atlantic. The evolving aerosols eventually mix down into the boundary layer and interact with the underlying cloud decks often many hundreds of km away from source. The CLouds and Aerosol Radiative Impacts and Forcing (CLARIFY) project is a major consortium programme that was based at Ascension Island in the mid-Atlantic during an intensive four week period in August-September 2017. The objectives of this project are to understand, evaluate and improve; the physical, chemical, optical and radiative properties of BBAs; the physical properties of stratocumulus clouds; the representation of aerosol-radiation interactions in weather and climate models; and the representation of aerosol-cloud interactions across a range of model scales. The project delivered state of the art cloud microphysics, aerosol, trace gas and radiation measurement capabilities to the South Atlantic from a suite of instrumentation aboard the FAAM (Facility for Airborne Atmospheric Measurements) BAe-146 Atmospheric Research Aircraft (ARA). These together with ground based insitu and remote sensing measurements from sites on Ascension Island (the mobile ARM and Met Office Wideawake facilities) were used to elucidate the processes occurring and potentially reduce the uncertainties in large scale models.

During this month long detachment the conditions in the region mainly fell into three distinct vertical profile regimes featuring; a polluted marine boundary layer (MBL) with a clean airmass above the inversion; a clean MBL with pollution lofted above the inversion; and a scenario where aerosols had mixed throughout the MBL from the layers above. We present analysis from sorties featuring clean and polluted MBL cases to demonstrate the influence of BBA on cloud microphysical properties in this region. Preliminary analysis demonstrates that the aerosol population is typically dominated by organic compounds and that the cloud droplet number concentration (CDNC) was significantly enhanced in polluted MBL cases (cloud base CNDC 206±106 cm^-3) compared to those in a clean MBL (cloud base CNDC 92±2 cm^-3); liquid water content profiles were similar in each case. We use the aerosol and meteorological observations from these cases to constrain the input to a detailed cloud parcel model (Aerosol-Cloud and Precipitation Interactions Model, ACPIM) that we use to better understand the interactions occurring, the results from which we then compare to the cloud microphysics observations. By way of contrast we also present analysis from a Pocket of Open Cells case (Abel et al., this conference) which featured an exceptionally clean MBL, resulting in a very low concentration of large cloud droplets and drizzle.