Characterization of Marine Stratocumulus Clouds and Aerosol–Cloud Interactions during ORACLES

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 2016-2018, 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 clouds. Here, there is a persistent stratocumulus cloud deck that has several climate impacts. First, due to a high albedo of these clouds, there is an overall cooling effect at the surface of the earth. In addition, they play a role in the hydrological cycle due to their drizzle production. In this region, BBA are the most dominant aerosol type and further contribute several impacts on the climate. BBA have the ability to absorb solar radiation, warming the surrounding atmosphere, which in turn can affect cloud development depending on where these aerosols are located with respect to the cloud. Furthermore, these BBA serve as cloud condensation nuclei (CCN), which in differing amounts may alter the cloud properties. These aerosol-cloud interactions continue to be poorly understood and demand more in-depth research. For this project, the Flight Probe Dual Range- Phase Doppler Interferometer (FPDR-PDI) was used aboard the NASA P-3 aircraft, which 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 use aerosol measurements from the HiGEAR suite of aerosol instrumentation and the Georgia Tech CCN instrument to characterize aerosol properties during the flight.

By using data from the above named instruments we can characterize when the plane descended into cloud top, and detect if there were aerosol present immediately above and focus on such cases where these aerosol were present. Then, they are classified into cases of low and high aerosol to characterize cloud properties such as cloud drop size distributions, spectral width, effective radii, and median diameter to see how the properties of clouds differ when exposed to low or high concentrations of aerosol. It is thought that when clouds that are developing in regions with higher concentrations of aerosols, including BBA, they tend to have a higher number of smaller cloud droplets, which could prolong the lifetime of the cloud by suppressing precipitation. In addition, clouds that form with a higher concentration of aerosols have a higher albedo, becoming more reflective, and therefore lowering the amount of solar radiation reaching the surface of the earth, which is known as the indirect effect or cloud-albedo effect. After these characterizations have been made, the latitude and longitudes of the flight paths will be used to indicate if there is a relationship in the location of the occurrences of high or low aerosol amounts with the corresponding clouds and the climatologic location of the BBA plume and marine stratocumulus cloud deck.