The Role of Supersaturation in Tropical Atlantic Marine Boundary Layer Aerosol-cloud Observations

JERAMY DEDRICK, Christian Pelayo, Lynn M. Russell, Dan Lubin, Mark Miller, Johannes Mülmenstädt, Scripps Institution of Oceanography

     Abstract Number: 96
     Working Group: Aerosols, Clouds and Climate

Abstract
Low-level cloud microphysical properties are very sensitive to changes in the properties of aerosol, particularly in clean marine regions where small relative number concentration changes are amplified in low baseline conditions. In this work, we evaluated the Twomey effect using aerosol, cloud, and radiative observations below low-level marine clouds on a remote tropical South Atlantic island during the DOE ARM Layered Atlantic Smoke Interactions with Clouds (LASIC) campaign. Measured aerosol size distributions were fit with three modes representing Aitken, accumulation, and sea spray aerosol and combined with submicron composition to estimate hygroscopicity. To investigate cloud supersaturation (S), we retrieved the Hoppel minimum diameter from modal fits and hygroscopicity estimated from submicron composition. Accumulation-mode aerosol contributed 75% of the cloud condensation nuclei (CCN) number at S<0.3%. Three ensemble-based and two parcel-based approaches were employed to further estimate S using measured aerosol and droplet concentrations and local cloud base updrafts across clean and smoky conditions. S agreed well with a parameterization constrained by the observed accumulation-mode aerosol and updraft, but not for the quasi-steady state approximation. Increases in accumulation-mode number increased cloud drop number, showing the activation response to aerosol, and reduced supersaturation most strongly at low updraft, illustrating the supersaturation response to aerosol. Droplet size sensitivities to aerosol number were included in a modified decomposition of the cloud albedo susceptibility into Twomey and meteorological components. These results show a strong sensitivity of the LASIC cloud baseline to aerosol perturbations.