Characterization of Southern Ocean Boundary Layer Clouds Using Airborne Radar, Lidar Data, and In-situ Cloud Measurements: Results from SOCRATES

Climate models have historically struggled with modeling the radiative properties of Southern Ocean (SO) clouds. Without an accurate representation of current SO cloud structure, it will not be possible to make high-confidence projections of future climate feedbacks. In response to this issue, this study demonstrates the use of a novel particle identification (PID) scheme based on data obtained by the HIAPER Cloud Radar and High Spectral Resolution Lidar on the National Science Foundation (NSF) G-V aircraft during the Southern Ocean Clouds, Aerosol, Radiation Transport Experimental Study (SOCRATES) to characterize the vertical structure of SO boundary layer clouds. Phase determined from the PID scheme was compared with that identified from coincident in-situ measurements on the G-V using a scheme based on data from multiple cloud probes to assess the PID schemes accuracy. The scheme was found to correctly identify liquid phase clouds 79% of the time and mixed-phase clouds 58% of the time, although ice clouds were only identified correctly 16% of the time. Additionally, clouds containing supercooled large droplets (SLD) were correctly identified 62% of the time. PID and in-situ data were subsequently used in tandem to identify the vertical structure of cloud phase for a few case studies measured during SOCRATES. Some common features on the vertical structure of clouds over the Southern Ocean were noted as follows: a) Almost all boundary layer clouds contain a small layer of liquid droplets at cloud top, making fully ice clouds very rare. b) Within fully liquid clouds, larger drizzle droplets tend to appear within a couple hundred meters below cloud top. c) Mixed-phase clouds show increasing glaciation with decreasing altitude. d) Drizzle frequently exists concurrently with ice crystals in the middle part of mixed-phase clouds. Implications for the radiative transfer through clouds are discussed.