Spatial heterogeneity of liquid, ice and mixed phase low-level clouds over the Southern Ocean derived using in situ observations acquired during SOCRATES

Mixed-phase clouds (i.e. clouds containing both liquid and ice particles in the same volume) and supercooled liquid clouds over the Southern Ocean are poorly represented in global climate models and numerical weather prediction models as the phase fraction detrained from convection may be set as a simple function of temperature. However, liquid water often exists at much lower temperatures than the threshold values used in these parameterizations and the processes controlling its formation and dissipation are poorly known. Thus, observational datasets are required to improve knowledge of the conditions under which supercooled water occurs as needed to improve model representations of clouds over the Southern Ocean.

This study characterizes the frequency and spatial distribution of different cloud phases (liquid, ice and mixed) using in-situ observations acquired during the Southern Ocean Clouds, Radiation, Aerosol Transport Experiment Study (SOCRATES). In-situ observations during SOCRATES were acquired using probes installed on the NSF/NCAR G-V research aircraft as it sampled low-level clouds over the Southern Ocean. Results show a large frequency of supercooled liquid water from -20°−0°C.

Cloud phase spatial heterogeneity is determined by relating the total number of samples from a given cloud to the number of segments whose neighboring samples are the same phase. Liquid and mixed phase conditions are associated with greater in-cloud spatial heterogeneity compared to ice. The partitioning of ice and liquid within mixed-phase cloud regimes is also examined as a function of the scale over which the partitioning is calculated. For example, liquid droplet mean diameters are found to be greatest between liquid and total condensate mass ratios of 0.6−0.8, possibly suggesting preferential evaporation of smaller drops during the Wegener-Bergeron-Findeison process.