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Maritime boundary‐layer clouds over the Southern Ocean (SO) have a large shortwave radiative effect. Yet, climate models have difficulties in representing these clouds and, especially, their phase in this observationally sparse region. This study aims to increase the knowledge of SO cloud phase by presenting in‐situ cloud microphysical observations from the Southern Ocean Clouds, Radiation, Aerosol, Transport Experimental Study (SOCRATES). We investigate the occurrence of ice in summertime marine stratocumulus and cumulus clouds in the temperature range between 6 and −25°C. Our observations show that in ice‐containing clouds, maximum ice number concentrations of up to several hundreds per liter were found. The observed ice crystal concentrations were on average one to two orders of magnitude higher than the simultaneously measured ice nucleating particle (INP) concentrations in the temperature range below −10°C and up to five orders of magnitude higher than estimated INP concentrations in the temperature range above −10°C. These results highlight the importance of secondary ice production (SIP) in SO summertime marine boundary‐layer clouds. Evidence for rime splintering was found in the Hallett‐Mossop (HM) temperature range but the exact SIP mechanism active at lower temperatures remains unclear. Finally, instrument simulators were used to assess simulated co‐located cloud ice concentrations and the role of modeled HM rime‐splintering. We found that CAM6 is deficient in simulating number concentrations across the HM temperature range with little sensitivity to the model HM process, which is inconsistent with the aforementioned observational evidence of highly active SIP processes in SO low‐level clouds. Plain Language Summary: Clouds in the Southern Ocean are important for climate but not well represented in climate models. Observations in this remote region have been rare. This study presents results from a recent airborne campaign that took place in the Southern Ocean where low‐ and mid‐level clouds were investigated by detecting individual cloud particles within the clouds. Although large fraction of the observed clouds did not contain ice crystals, occasionally high amounts of ice crystals were observed that cannot be explained by ice formation on aerosol particles but were result of multiplication of existing ice crystals. We tested the capability of a commonly used climate model to represent the observed ice concentrations and their sensitivity to one ice multiplication process parameterized in the model. These investigations revealed that the in the model the ice multiplication process was not responsible for generation of ice, which is in contradiction with the observations. Key Points: Ice concentrations several orders of magnitude higher than ice nucleating particle concentrations were observedSecondary ice production was believed to be responsible for the observed high ice number concentrationsComparison with climate model indicated that secondary ice processes are still inadequately represented in the model [ABSTRACT FROM AUTHOR]

For a given cloud, whether the cloud top is predominately made up of ice crystals or supercooled liquid droplets plays a large role in the clouds overall radiative effects. This study uses collocated airborne radar, lidar, and thermodynamic data from 12 high‐altitude flight legs during the Southern Ocean Clouds, Radiation, Aerosol Transport Experimental Study (SOCRATES) to characterize Southern Ocean (SO) cold sector cloud top phase (i.e., within 96 m of top) as a function of cloud top temperature (CTT). A training data set was developed to create probabilistic phase classifications based on High Spectral Resolution Lidar data and Cloud Radar data. These classifications were then used to identify dominant cloud top phase. Case studies are presented illustrating examples of supercooled liquid water at cloud top at different CTT ranges over the SO (−3°C < CTTs < −28°C). During SOCRATES, 67.4% of sampled cloud top had CTTs less than 0°C. Of the subfreezing cloud tops sampled, 91.7% had supercooled liquid water present in the top 96 m and 74.9% were classified entirely as liquid‐bearing. Liquid‐bearing cloud tops were found at CTTs as cold as −30°C. Horizontal cloud extent was also determined as a function of median cloud top height. Plain Language Summary: Low‐level clouds over the Southern Ocean have a large effect on the region's radiation budget. The radiation budget is strongly influenced by the phase (liquid or ice) of cloud tops, which is where most solar radiation is reflected, and most infrared radiation is radiated to space. For this reason, identifying the phase of cloud tops is important. In this study, airborne radar, lidar, and temperature data from 12 high‐altitude flight legs during the Southern Ocean Clouds, Radiation, Aerosol Transport Experimental Study (SOCRATES) are used to characterize Southern Ocean cloud top phase as a function of cloud top temperature. The results show that liquid is the dominant phase present in clouds over the Southern Ocean, with liquid present at cloud top temperatures as cold as −30°C. Key Points: Airborne radar, lidar, and thermodynamic data are used to determine Southern Ocean cloud top phase as a function of cloud top temperatureConsidering only clouds with subfreezing cloud top temperatures, the dominant phase at cloud top in 74.9% of clouds was liquidLiquid‐bearing cloud tops were even found at temperatures as cold as −30°C [ABSTRACT FROM AUTHOR]

An atmospheric river (AR) impacting Tasmania, Australia, and the Southern Ocean during the austral summer on 28–29 January 2018 during the Southern Ocean Clouds, Radiation, Aerosol Transport Experimental Study campaign is analyzed using a modeling and observational approach. Gulfstream‐V dropsonde measurements and Global Precipitation Measurement radar analyses were used in conjunction with Weather Research and Forecasting model simulations with water vapor tracers to investigate the relative contributions of tropical and midlatitude moisture sources to the AR. Moisture associated with a monsoonal tropical depression became entrained into a midlatitude frontal system that extended to 60°S, reaching the associated low‐pressure system 850 km off the coast of Antarctica—effectively connecting the tropics and the polar region. Tropical moisture contributed to about 50% of the precipitable water within the AR as the flow moved over the Southern Ocean near Tasmania. The tropical contribution to precipitation decreased with latitude, from >70% over Australia, to ~50% off the Australian coast, to less than 5% poleward of 55°S. The integrated vapor transport (IVT) through the core of the AR reached above 500 kg m−1 s−1 during 1200 UTC 28 January to 0600 UTC 29 January, 1.29 times the average amount of water carried by the world's largest terrestrial river, the Amazon. The high IVT strength might be attributed to the higher water vapor content associated with the warmer temperatures across Australia and the Southern Ocean in austral summer. Plain Language Summary: Atmospheric rivers (ARs) are long, narrow corridors of strong horizontal water vapor transport, normally associated with a low‐level jet stream ahead of a cold front. They play a critical role in the global hydrologic cycle and have been studied extensively over the northeast Pacific. This and a companion paper are the first to analyze an AR event impacting Tasmania, Australia, and the Southern Ocean using a modeling and observational approach. The AR extended from the tropics over northern Australia southeastward to the southern Australian coast and over the Southern Ocean to 60°S. The flow at higher altitudes in the AR was primarily composed of moisture originating in the tropics, while at lower altitudes and warmer temperatures, the moisture was primarily sourced from middle latitudes as a result of the circulation associated with a cold front. The tropical contribution to precipitation decreased with latitude, from >70% over Australia, to ~50% off the Australian coast, to less than 5% poleward of 55°S. Water vapor transported through the core of the AR was more than the average amount of water carried by the world's largest river, the Amazon, during the same time interval. Key Points: The contribution of moisture from the tropics to precipitation within an Australian summer atmospheric river is documentedOver 50% of the moisture arriving at the Southern Ocean at altitudes above the 0°C isotherm was sourced from tropical latitudesThe tropical contribution to precipitation decreased with latitude, >70% over Australia, ~50% off Australia, and <10% poleward of 60°S [ABSTRACT FROM AUTHOR]

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. Key Points: The physical structure and microphysical properties of a summer austral atmospheric river are describedIce particle concentrations increased to >100 L−1 in the zone from −8°C to −4°C, consistent with secondary ice formationIce particles falling from the tropical‐sourced moisture stream aloft trigger coalescence below the 0°C isotherm [ABSTRACT FROM AUTHOR]

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. Key Points: Supercooled liquid water was detected in all generating cells (GCs) characterized by small horizontal widths over the Southern OceanThe liquid water content and number concentration of droplets inside GCs were slightly larger than values between the GCsThe ice particle sizes, number concentrations, and dispersions inside GCs were larger than those between GCs [ABSTRACT FROM AUTHOR]

The link between stratiform precipitation microphysics and multifrequency radar observables is thoroughly investigated by exploiting simultaneous airborne radar and in situ observations collected from two aircraft during the OLYMPEX/RADEX (Olympic Mountain Experiment/Radar Definition Experiment 2015) field campaign. Above the melting level, in situ images and triple‐frequency radar signatures both indicate the presence of moderately rimed aggregates. Various mass‐size relationships of ice particles and snow scattering databases are used to compute the radar reflectivity from the in situ particle size distribution. At Ku and Ka band, the best agreement with radar observations is found when using the self‐similar Rayleigh‐Gans approximation for moderately rimed aggregates. At W band, a direct comparison is challenging because of the non‐Rayleigh effects and of the probable attenuation due to ice aggregates and supercooled liquid water between the two aircraft. A variational method enables the retrieval of the full precipitation profile above and below the melting layer, by combining the observations from the three radars. Even with three radar frequencies, the retrieval of rain properties is challenging over land, where the integrated attenuation is not available. Otherwise, retrieved mean volume diameters and water contents of both solid and liquid precipitation are in agreement with in situ observations and indicate local changes of the degree of riming of ice aggregates, on the scale of 5 km. Finally, retrieval results are analyzed to explore the validity of using continuity constraints on the water mass flux and diameter within the melting layer in order to improve retrievals of ice properties. Key Points: Signature of moderately rimed aggregates in triple‐frequency radar reflectivity measurements is confirmed by in situ microphysicsSelf‐similar ice crystal models coupled with Rayleigh‐Gans approximation ensure consistency between in situ and radar observationsProfiles of solid and liquid precipitation microphysics are retrieved by combining triple‐frequency radars [ABSTRACT FROM AUTHOR]

Abstract: Scattering models of precipitation‐size ice particles have shown that aggregates and spheroidal particles occupy distinct regions of the Ku‐Ka‐W‐band dual‐frequency ratio (DFR) plane. Furthermore, past ground‐based observations suggest that particle bulk density and characteristic size can be retrieved from the DFR plane. This study, for the first time, evaluates airborne DFR observations with coincident airborne microphysical measurements. Over 2 hr of microphysical data collected aboard the University of North Dakota Citation from the Olympic Mountains Experiment are matched with Airborne Precipitation and cloud Radar Third Generation triple‐frequency radar observations. Across all flights, 31% (63%) of collocated data points show nonspheroidal (spheroidal) particle scattering characteristics. DFR observations compared with in situ observations of effective density and particle characteristic size reveal relationships that could potentially be used to develop quantitative dual‐ and triple‐frequency DFR ice property retrievals. [ABSTRACT FROM AUTHOR]