Structure of an Atmospheric River Over Australia and the Southern Ocean: II. Microphysical Evolution

Abstract

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.

Southern Ocean cloud and aerosol data: a compilation of measurements from the 2018 Southern Ocean Ross Sea Marine Ecosystems and Environment voyage

“…Observations support the embedded occurrence of a variety of secondary ice formation processes in clouds over the Southern Ocean, which are otherwise dominated by supercooled water. These processes range from as-sociation with seeding of ice crystals from colder cloud levels that appears consistent with a rime splintering process (Finlon et al, 2020) to studies suggesting the vital importance of ice production via breakup following ice-ice collisions (Sotiropoulou et al, 2021;Young et al, 2019).…”

A Detailed, Multi‐Scale Assessment of an Atmospheric River Event and Its Impact on Extreme Glacier Melt in the Southern Alps of New Zealand

“…Furthermore, Finlon et al. (2020) found observational evidence for the seeder‐feeder mechanism operating in an AR over the ocean south‐east of Australia. In accordance with Wratt et al.…”

“…A range of studies have recognized orographic enhancement of frontal precipitation due to either or both of the two named mechanisms (Browning et al, 1974;Carruthers & Choularton, 1983;Rotunno & Houze, 2007;Viale et al, 2013), including for New Zealand (Purdy & Austin, 2003;Purdy et al, 2005;Revell et al, 2002;Wratt et al, 2000). Furthermore, Finlon et al (2020) found observational evidence for the seeder-feeder mechanism operating in an AR over the ocean south-east of Australia. In accordance with Wratt et al (1996), we propose a combination of the seeder-feeder-mechanism and convective dynamics triggered by the release of conditional instability through uplift to be a potential driver of the large observed orographic rainfall enhancement and associated high ablation rates to the west of the Main Divide in the Southern Alps.…”

Characterizing the Occurrence and Spatial Heterogeneity of Liquid, Ice, and Mixed Phase Low‐Level Clouds Over the Southern Ocean Using in Situ Observations Acquired During SOCRATES

“…Such localized regions of heterogeneity, associated with “pockets” of ice and mixed phase within large swaths of supercooled liquid, could be nucleating via heterogeneous nucleation. Recent work has highlighted the relatively sparse but present ice nucleating particles observed over this region (e.g., Finlon et al., 2020; McCluskey et al., 2018).…”