Oral Presentation AMOS Annual Meeting and International Conference on Tropical Meteorology and Oceanography

Wintertime In-situ Cloud Microphysical Properties in the Mixed-phase Temperature Regime over the Mid-latitude Southern Ocean (#78)

Yi Huang 1 , Steven Siems 1 2 , Michael Manton 1 , Eunmi Ahn 1 , Mark DeHoog 3
  1. School of Earth, Atmosphere and Environment, Monash University, CLAYTON, Victoria, Australia
  2. Australian Research Council (ARC) Centre of Excellence for Climate Extremes, Monash University, CLAYTON, Victoria, Australia
  3. Hydro Tasmania, Hobart, Tasmania, Australia

The best constructions of the Earth's climate continue to suffer from large biases in the radiation budget over the Southern Ocean (SO), which trace to errors in the representation of clouds, aerosols, precipitation and their interactions over this region. A poor understanding of these processes is due, to a significant degree, to sparse field observations in this remote region.

In this study, unique in-situ observations made by 20 flights with a lightly instrumented aircraft are analysed to investigate the microphysical properties of the mid-latitude SO clouds for three winters (June-October, 2013-2015). We focus on the mixed-phase temperature regime (−15 – 0°C), where liquid droplets and ice crystals can coexist. When only pristine conditions (observations over the upwind ocean) are considered, 61% of the cloud samples fall within these constraints. These samples are further categorised into three classes: liquid (37%), mixed-phase (60%), and glaciated clouds (3%).

In-situ microphysical properties including liquid / ice mass and number concentrations, particle size distribution, and asphericity properties are analysed. Evidence of secondary ice production was commonly observed in the Hallett–Mossop temperature zone (−3 to −8°C), with high number concentrations recorded up to 54 L−1. These mixed-phase clouds were also commonly observed to be precipitating. Using the aerosol particle size measurements and number concentrations, ice nucleating particle (INP) number concentrations are estimated with a recognised ice nuclei parameterisation scheme. It is found that the observed ice number concentrations are typically a few orders of magnitude higher than the estimated INP concentrations. The observed high ice number concentrations are largely consistent with the theoretical values when ice crystals are produced via a splinter production. Our analysis suggests that secondary ice processes (likely the Hallett–Mossop ice multiplication mechanism) may be playing a key role in ice (and precipitation) production in shallow convective clouds during winter over the SO.