Statistical Distributions of Macrophysical Cloud and Aerosol Properties over the Southern Ocean during MARCUS: Impacts of Environmental Conditions

The fundamental characteristics of clouds and aerosols and their interactions over the Southern Ocean(SO) are poorly known due to limited measurements, leading to large uncertainties and biases in the simulation of the global energy budget and cloud radiative forcing. It is essential to quantify the distributions of cloud and aerosol properties in various environmental regimes to provide a reference for cloud-aerosol process studies, to develop model and retrieval parameterization schemes and to evaluate models and retrievals. In this study, measurements collected with the Department of Energy Atmospheric Radiation Measurement Program’s Mobile Facility installed on the icebreaker Aurora Australis as it made routine transits from Hobart to the Australian Antarctic stations Mawson, Casey and Davis, and Macquarie Island from October 2017 to April 2018 during the Measurements of Aerosols, Radiation and CloUds over the Southern Oceans (MARCUS) are utilized to investigate the seasonal, latitudinal, and meteorological variability of cloud and aerosol properties, and their covariability with dynamics and thermodynamics. Measurements of aerosols included cloud condensation nuclei (CCN), aerosol concentration and size distribution measured by the Condensation Particle Counter and Ultra-High Sensitivity Aerosol Spectrometer. Macrophysical cloud properties of liquid water path (LWP), cloud base height (CBH), boundary layer height (BLH) and cloud top height (CTH) were retrieved from microwave radiometer, ceilometer, and Marine W-band ARM Cloud Radar respectively. And, the lower tropospheric stability (LTS) was calculated from soundings that were released every six hours.

Statistical distributions of these cloud and aerosol properties were determined as a function of multiple environmental conditions, including the location of the measurements relative to the oceanic polar front (PF), the sea surface temperature (SST), the chlorophyll concentration, whether or not precipitation was occurring, aerosol source of origin as determined by back trajectory modeling from the Hybrid Single Particle Lagrangian Integrated Trajectory Model, and wind speed and direction. Notable differences of the distributions were determined as a function of SST and precipitation occurrence. For example, the CBH and BLH were less for lower SSTs in a statistically significant manner, which is consistent with reports from satellite products in previous studies, while the CTH has a greater probability to be both higher than 7 km and lower than 2km for SST > 4℃. Precipitating clouds overall have higher CTH, lower CBH, and greater LWP. The LWP is a stronger function of SST in precipitating clouds than in non-precipitating clouds for SST > 4℃. Different from previous studies that found CTH increases as SST decreases, here CTH only decreased as SST decreased when SSTs were between 3~10℃ in precipitating clouds. The BLH was positively correlated with SST for SSTs between 3~10℃, but there was more variability for higher SSTs in precipitating clouds. Aerosol properties exhibited only weak correlations with SST and precipitation occurrence, with higher aerosol concentrations over warmer water and larger CCN concentrations (>500/cm3) in the vicinity of non-precipitating clouds. The significance of these findings for understanding processes occurring in Southern Ocean clouds and for improving parameterizations of cloud and aerosol properties in models will be discussed.