ABSTRACT

Clouds in the marine boundary layer (say the kilometer of the atmosphere just above the ocean surface) have a global cooling effect as they reflect sunlight back to space but are too low to effectively trap outgoing infrared radiation. The climatic effect of these clouds has become an important issue in climate change research, as warmer temperatures may alter either the reflectivity of the clouds or their typical lifetimes, the factors which determine the strength of their cooling effect. The reflectivity of marine boundary layer clouds is determined in large part by the abundance of cloud condensation nuclei (CCN), the tiny aerosol particles which absorb moisture from the atmosphere to seed the growth of cloud droplets. When CCN are more abundant the available cloud water is spread over a larger number of smaller droplets, leading to a more reflective cloud than would occur with fewer CCN and hence a smaller number of larger droplets. Clouds made of smaller droplets also last longer, as the process of collision and coalescence that combines cloud droplets (perhaps a million or so) to form a raindrop takes longer when the droplets are smaller and thus more are needed to make a raindrop, meaning a drop big enough to fall from the cloud.

Previous work by the PIs developed a simplified budget equation for CCN relating changes in CCN abundance to sources, including the generation of aerosols by biological processes in the surface ocean, and sinks, including precipitation. Precipitation is quite effective in removing CCN from liquid clouds as each one of the million or so droplets in a raindrop contains a CCN. The PIs used their budget analysis to show that CCN removal by precipitation, referred to as precipitation scavenging or coalescence scavenging, is the dominant mechanism for CCN removal and accounts for much of the geographic variability of CCN abundance over the oceans. They also used their budget to derive a formula for the droplet number concentration (Nd) in marine boundary layer clouds under the assumption that the CCN sources and sinks are balanced and all the CCN in a cloud have seeded droplets.

Work performed here applies the budget equation and the Nd formula to the boundary layer clouds in the storm track over the Southern Ocean using observations collected during the 2018 Southern Ocean Clouds, Radiation, Aerosol, Transport Experimental Study (SOCRATES), a field campaign that used a research aircraft to sample clouds on flights over the ocean south of Tasmania (see AGS-1660609). The aircraft measured CCN concentrations in, above, and below the clouds, and used radar and lidar to observe cloud droplets and raindrops, information which can be combined with satellite data and meteorological analysis to determine CCN concentrations and measure CCN sources and sinks. Results of the budget study are compared with output from a Large Eddy Simulation (LES) model which generates detailed clouds, CCN, and precipitation simulations based on large-scale meteorological inputs and measurements of above-cloud CCN concentrations. The project also processes and analyzes data from the aircraft radar and lidar to examine the precipitation produced by the SO clouds, looking at the amount of precipitation produced, the size distribution of raindrops, and the extent to which ice phase precipitation is also produced.

The work is of societal as well as scientific interest as change in the extent of cooling provided by low clouds is among the largest uncertainties in estimates of the sensitivity of global temperature to greenhouse gas increases. The SOCRATES campaign was largely motivated by concern that the SO clouds are poorly represented in climate models used to make projections of future climate change. The project also produces datasets on cloud, precipitation, and CCN properties that can be used by the worldwide community of climate researchers. In addition, the award provides support and training for a graduate student, thereby promoting the future workforce in this research area.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.

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