Observations and Modeling of Rime Splintering in Southern Ocean Cumuli

Abstract

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.

Characterization of Southern Ocean Boundary Layer Clouds Using Airborne Radar, Lidar, and In Situ Cloud Data: Results From SOCRATES

“…Further, the PID scheme suggests ice growth occurs with decreasing altitude in convective clouds, with P[large ice] ∼10% 150 m below cloud top and P[large ice] ∼50% 700 m below cloud top. This is consistent with previous studies that have shown higher updraft velocities provide a preferential environment for the growth of ice in supersaturated regimes (McFarquhar et al., 2011; Rosenow et al., 2014; Wang et al., 2020), and that SIP during SOCRATES was enhanced in convective regions exhibiting multiple updrafts (including embedded convective elements within stratocumulus clouds), with thin stratiform clouds with a lack of significant updraft activity exhibiting little SIP and remaining mostly composed of SLW (Järvinen et al., 2022; Lasher‐Trapp et al., 2021; Mace et al., 2021). The peak P[any ice] for any part of the cloud averaged among both methods is ∼65% for convective clouds and ∼20% for stratiform clouds, suggesting that ice is about 3 times more likely in convective clouds than in stratiform clouds.…”

Evidence for Secondary Ice Production in Southern Ocean Maritime Boundary Layer Clouds

“…Another factor potentially enhancing production of ice in the stratocumulus clouds is related to the multi‐cell structure of the clouds (see Section 5.1) (Lasher‐Trapp et al., 2021; Mossop et al., 1970). Ice crystals from older cells can be incorporated into newly forming rising cells, where they can initiate ice multiplication processes.…”

“…The minimum temperatures during the segments varied between −0.5 and −9.3°C. For a more detailed discussion of cumulus systems in SOCRATES we refer to Lasher-Trapp et al (2021).…”

“…For a more detailed discussion of cumulus systems in SOCRATES we refer to Lasher‐Trapp et al. (2021).…”

Important Ice Processes Are Missed by the Community Earth System Model in Southern Ocean Mixed‐Phase Clouds: Bridging SOCRATES Observations to Model Developments

“…Using measurements from SOCRATES in combination with high‐resolution numerical modeling, Lasher‐Trapp et al. (2021) re‐examined SIP in the Southern Ocean cumuli, and found that the measured Ni exhibits 7 orders of magnitude or more higher than INP number concentrations, which is also reproduced by their numerical modeling. Based on the global cloud‐resolving simulations, Atlas et al.…”