Marine Stratocumulus Properties from the FPDR—PDI as a Function of Altitude and Aerosol during ORACLES Phase 1 and 2

Aerosol-cloud interactions in marine stratocumulus clouds (MSc) in the southeastern Atlantic (SEA) region were investigated during years 1 and 2 of the ObseRvations of Aerosols above CLouds and their intEractionS (ORACLES) field project in Aug-Sept 2016 and August 2017. This region is of interest due to seasonally persistent MSc decks that are an important component of the climate system due to their radiative and hydrologic impacts. The SEA MSc deck is unique due to the interactions between these clouds and transported biomass burning aerosol during the July-October fire season. These biomass burning aerosol play multiple roles in modifying the cloud deck through interactions with radiation as absorbing aerosol and through modifications to cloud microphysical properties as cloud condensation nuclei. This work uses in situ cloud data obtained with a Flight Probe Dual Range – Phase Doppler Interferometer (FPDR – PDI), standard aerosol instrumentation on board the NASA P-3, and reanalysis data to investigate Aerosol-Cloud Interactions (ACI). Also included are aerosol characteristics from the HiGEAR aerosol instrumentation suite (including an onboard AMS).

The FPDR – PDI provides unique cloud microphysical observations of individual cloud drop arrivals allowing for the computation of a variety of microphysical cloud properties including individual drop size, cloud drop number concentration, cloud drop size distributions, liquid water content, and cloud thickness. The FPDR – PDI measurement technique also provides droplet spacing and drop velocity information which is used to investigate turbulence and entrainment mixing processes.

We first investigate MSc properties as a function of altitude in cloud based on “sawtooth” flight patterns. This provides insight into the vertical structure of the MSc under a variety of conditions. With these “sawtooths” we can determine how drop sizes, drop size distributions, and liquid water content change with altitude in cloud. We can then consider cloud structure as a function of latitude and longitude to determine how distance from the coast, the source of biomass burning, and other factors such as SST are related to cloud properties. Secondly, we use aerosol information such as average background aerosol amount (low, mid, high) and location relative to cloud (above or mixing) to sort FPDR – PDI cloud properties. To control for meteorological co-variances we further sort the data within aerosol categories by lower tropospheric stability, vertical velocity, and surface wind direction. We then determine general marine stratocumulus cloud characteristics under each of the various aerosol categories to investigate ACI in the SEA.