Characterization of Marine Stratocumulus Clouds and Aerosol-Cloud Interactions During Oracles.

Abstract

ORACLES (ObseRvations of Aerosols above CLouds and their intEractionS) is a 3-year field campaign taking place during the months of August, September, and October in the years 2017, 2016, and 2018, respectively, off the coasts of Namibia and São Tomé in the southeastern Atlantic (SEA). The purpose of this campaign is to study the effects of biomass burning aerosols (BBA) on climatologically important stratocumulus clouds. For this project, we focus on data collected during the 2016 field deployment and specifically focus on in-cloud data collected with the Flight Probe Dual Range - Phase Doppler Interferometer (FPDR-PDI) aboard the NASA P-3 aircraft. The FPDR-PDI has the ability to measure microphysical cloud properties such as instantaneous cloud drop size, cloud drop concentration, drop size distributions and liquid water content. In addition, we used black carbon (BC) measurements from the Single Particle Soot Photometer (SP2) to characterize aerosol and cloud properties during the flight. We found that for the high BC level leg segments, cloud properties exhibited characteristics of the 1st Indirect Effect. The median diameter for all of the level legs combined was 14.6 μm. The median diameter was found to be 12.81 μm for the high BC cases, and 15.39 μm for the low BC cases, which indicates a shift of the size distribution to smaller droplet sizes with more “polluted” clouds and to larger droplet sizes for the “clean” clouds. The median effective radius for the high BC cloud legs was 9.02 μm and 11.43 μm for the low BC cloud legs. Overall, total number concentration (TNC) increased with BC, with the mean shifting downwards toward lower TNC with the low BC cases, and upwards towards higher TNC with the high BC cases.