The bulk microphysical properties and number distribution functions (N(D)) of supercooled liquid water (SLW) and ice inside and between ubiquitous generating cells (GCs) observed over the Southern Ocean (SO) during the Southern Ocean Clouds Radiation Aerosol Transport Experimental Study (SOCRATES) measured by in situ cloud probes onboard the NCAR/NSF G‐V aircraft are compared. SLW was detected inside all GCs with an average liquid water content of 0.31 ± 0.19 g m−3, 11% larger than values between GCs. The N(D) of droplets (maximum dimension D < 50 μm) inside and between GCs had only slight differences. For ice particles, on the other hand, the mean concentration (median mass diameter) with D > 200 μm inside GCs was 2.0 ± 3.3 L−1 (323 ± 263 μm), 65% (37%) larger than values outside GCs. As D increases, the percentage differences became larger (up to ~500%). The more and larger ice particles inside GCs suggest the GC updrafts provide a favorable environment for particle growth by deposition and riming and that mixing processes are less efficient at redistributing larger particles. The horizontal scale of observed GCs ranged from 200 to 600 m with a mean of 395 ± 162 m, smaller than GC widths observed in previous studies. This study expands knowledge of the microphysical properties and processes acting in GCs over a wider range of conditions than previously available.

Abstract. A major challenge for in situ observations in mixed-phase clouds remains the phase discrimination and sizing of cloud hydrometeors. In this work, we present a new method for determining the phase of individual cloud hydrometeors based on their angular-light-scattering behavior employed by the PHIPS (Particle Habit Imaging and Polar Scattering) airborne cloud probe. The phase discrimination algorithm is based on the difference of distinct features in the angular-scattering function of spherical and aspherical particles. The algorithm is calibrated and evaluated using a large data set gathered during two in situ aircraft campaigns in the Arctic and Southern Ocean. Comparison of the algorithm with manually classified particles showed that we can confidently discriminate between spherical and aspherical particles with a 98 % accuracy. Furthermore, we present a method for deriving particle size distributions based on single-particle angular-scattering data for particles in a size range from 100 µm ≤ D ≤ 700 µm and 20 µm ≤ D ≤ 700 µm for droplets and ice particles, respectively. The functionality of these methods is demonstrated in three representative case studies.

An atmospheric river affecting Australia and the Southern Ocean on 28-29 January 2018 during the Southern Ocean Clouds, Radiation, Aerosol Transport Experimental Study (SOCRATES) is analyzed using nadir-pointing W-band cloud radar measurements and in situ microphysical measurements from a Gulfstream-V aircraft. The AR had a two-band structure, with the westernmost band associated with a cold frontal boundary. The bands were primarily stratiform with distinct radar bright banding. The microphysical evolution of precipitation is described in the context of the tropical-and midlatitude-sourced moisture zones above and below the 0°C isotherm, respectively, identified in Part I. In the tropical-sourced moisture zone, ice particles at temperatures less than −8°C had concentrations on the order of 10 L −1 , with habits characteristic of lower temperatures, while between −8°C and −4°C, an order of magnitude increase in ice particle concentrations was observed, with columnar habits consistent with Hallett-Mossop secondary ice formation. Ice particles falling though the 0°C level into the midlatitude-sourced moisture region and melting provided "seed" droplets from which subsequent growth by collision-coalescence occurred. In this region, raindrops grew to sizes of 3 mm and precipitation rates averaged 16 mm hr −1. Plain Language Summary Atmospheric rivers (ARs) are long, narrow zones of enhanced horizontal poleward water vapor transport that have profound effects on the global hydrological cycle. This and a companion study are the first to analyze an AR affecting Australia and the Southern Ocean through an observational and modeling approach, respectively. There were two precipitation bands associated with the AR, which were primarily stratiform in character with an enhancement of radar reflectivity near the melting level. Part I showed that moisture in the upper part of the AR (above the 0°C level) was primarily sourced from the tropics, while moisture in the lower part (below the 0°C level) was primarily sourced from the middle latitudes. The microphysical evolution of precipitation through these two zones from cloud top to the ground within this AR is investigated. Ice particles forming and growing at altitudes above the 0°C level served as the "seeds" that allowed for further growth below the 0°C level as they melted and collided with droplets. The concentrations, shapes, and size distributions of the ice particles falling through each zone are presented and related to microphysical growth processes occurring within the bands.

Maritime boundary layer clouds in the SO have a significant shortwave (SW) cloud radiative effect associated with them (Haynes et al., 2011), and consequently they have an important role when estimating climate sensitivity to cloud feedbacks

Recent studies have suggested a correct representation of cloud phase in the Southern Ocean region is important in climate models for an accurate representation of the energy balance. Satellite retrievals indicate many of the clouds are predominantly liquid, despite their low temperatures. However, clouds containing high numbers of ice crystals have sometimes been observed in this region and implicated the secondary ice production process called rime splintering. This study re‐examines rime splintering in Southern Ocean cumuli using both a new data set and high‐resolution numerical modeling. Measurements acquired during the Southern Ocean Clouds Radiation Aerosol Transport Experimental Study (SOCRATES) provide an evaluation of the amount of ice in shallow cumuli sampled over two days in this region. The measurements sometimes exhibit seven orders of magnitude or more ice particles compared to amounts expected from measurements of ice‐nucleating particles (INP) on the same days. Cumuli containing multiple updrafts had the greatest tendency to contain high ice concentrations and meet the expected conditions for rime splintering. Idealized numerical modeling, constrained by the observations, suggests that the multiple updrafts produce more frozen raindrops/graupel, and allow them to travel through the rime‐splintering zone over an extended period of time, increasing the number of ice particles by many orders of magnitude. The extremely low number of INP in the Southern Ocean thus appears to require special conditions like multiple updrafts to help glaciate the cumuli in this region, potentially explaining the predominance of supercooled cumuli observed there.