Evolution of Cloud Droplet Number Concentrations Along Parcel Trajectories in the High Latitude Southern Ocean Summer

Recent work has demonstrated a latitudinal gradient of cloud droplet number concentrations (Nd) in the summertime Southern Ocean along East Antarctica with a significant increase in Nd in the region adjacent to the East Antarctic Continent as compared to the open ocean to the north. This gradient in Nd has been shown from ship-borne observations to be in association with high concentrations of cloud condensation nuclei (CCN) that are predominantly composed of biogenic sulfate as compared to CCN of predominantly sea salt in the storm track latitudes to the north. The Nd gradient is sufficiently large to influence the albedo of the region and, therefore, impacts both the solar energy flux into the ocean and the top of atmosphere energy budget of the region. While the presence of these gradients in Nd and CCN appear robust, the processes that produce and maintain them are poorly understood. While the storminess in the summer is much reduced compared to other times of the year, air masses are still in motion and the processes that cause evolution of particle and droplet microphysics are active. Here we follow air mass trajectories as they move within the Southern Ocean and, along these trajectories, using satellite and ship-based remote sensing, we examine how the microphysical properties of low-level clouds change with time. Typically, air masses entering the high latitude Southern Ocean from farther north have Nd (re) increasing (decreasing) over multiple days as the air masses move over the highly productive waters. Of more interest, however, is the finding that the highest cloud Nd air masses are those that have recently spent time over the high-altitude ice sheets of the Antarctic continent. These air masses descend to the ocean surface in katabatic flows. Following these air masses over time, we find that cloud microphysics evolve to have lower Nd and higher effective radii as the air masses spend time over the Southern Ocean. This finding is summarized in the attached figure that shows, using MODIS data from 5 summer seasons in the East Antarctic region, that low-level liquid-dominant clouds that have the highest Nd are much more likely to have recently passed over the ice sheet than compared to low Nd clouds that are much more likely not not have had recent interaction with the continent. In this paper, we will illustrate these time dependent changes with case studies and statistics derived over multiple seasons.