Biomass Burning Aerosols from the African Continent Cause Microphysical Changes in Transition Cumulus Clouds in the South Atlantic which are Modulated by Thermodynamic Differences between Oceanic and Continental Airmasses

Abstract:

We present observations of aerosol cloud interactions that occur when biomass burning aerosols (BBA) from the African continent descend from the free troposphere to mix with the transition cumulus clouds located in the cloudy boundary layers of the South Atlantic Ocean. Data come from two field campaigns that took place on Ascension Island during 2017. LASIC (Layered Atlantic Smoke Interactions with Clouds) was a deployment of the ARM (Atmospheric Radiation Monitoring) Mobile Facility while CLARIFY-2017 (CLoud-Aerosol-Radiation Interaction and Forcing: Year-2017) operated the FAAM BAe146 large research aircraft from there during August and September 2017.

Covariance of the thermodynamic properties of the boundary layer and the cloud and aerosol microphysics was typical during the observation period as the biomass burning plumes advected westwards towards Ascension Island within airmasses of continental origin, displacing the more pristine oceanic conditions with a source region to the south east towards St Helena. Compositing of in situ and remote sensing observations by pollution loading allows us to show that BBA causes changes to the microphysical structure of the clouds resulting in a strong albedo effect.

Thermodynamic changes associated with the arrival of the BBA laden continental airmasses cause bulk cloud responses in liquid water path and cloud fraction. Using single column radiative transfer modelling of idealised cases we attempt to partition the impact of aerosols and thermodynamics on the top of atmosphere radiation balance in order to ascertain the magnitude of the anthropogenic contribution to the changes in cloud properties. The sporadic nature of precipitation events in transition cumulus clouds and limited in situ observations make determination of lifetime effects challenging although ground based remote sensing observations offer some insights to the response of the macro structure of the clouds.