Secondary ice production in summer clouds over the Antarctic coast: an underappreciated process in atmospheric models

Abstract

Abstract. The correct representation of Antarctic clouds in atmospheric models is crucial for accurate projections of the future Antarctic climate. This is particularly true for summer clouds which play a critical role in the surface melting of the ice-shelf in the vicinity of Weddell Sea. However these clouds are often poorly represented, as ice crystal number concentrations (ICNCs) are undepredicted by atmospheric models, even when primary ice formation is constrained with aerosol measurements. Rime-splintering, thought to be the dominant secondary ice production (SIP) mechanism at temperatures between −8 and −3 °C, is also very weak in summer Antarctic conditions. Including a parameterization for SIP due to break-up (BR) from collisions between ice particles in the Weather and Research Forecasting model bridges the gap between observations and simulations, suggesting that BR could account for the enhanced ICNCs in the pristine Antarctic atmosphere. These results are insensitive to uncertainties in primary ice production. The BR mechanism is currently not represented in most weather prediction and climate models; including this process can have a significant impact on the Antarctic radiation budget and thus in projections of the future regional climate.

Challenging and Improving the Simulation of Mid‐Level Mixed‐Phase Clouds Over the High‐Latitude Southern Ocean

“…As previously mentioned, Sotiropoulou et al. (2020) suggest that secondary ice production through ice particle collisional break‐up might be an important process in coastal Antarctic clouds. We have assessed the model sensitivity to this process on our study case (details in supporting information S4).…”

“…Sotiropoulou et al. (2020) suggest that it may be due to the absence of parametrization for the secondary ice production through ice particle break‐up after hydrometeor collision. By default, in our simulations we do not activate a parameterization of collisional break‐up but complementary sensitivity experiments have been carried out.…”

“…They can potentially lead to substantial underestimation of SLW droplets in clouds and hence major radiative biases in models (Vergara‐Temprado et al., 2018). In addition, previously underappreciated processes like secondary ice production through ice particle break‐up also seem particularly critical to explain the concentration of ice crystals in clouds over the Antarctic coast (Sotiropoulou et al., 2020; Young et al., 2019).…”

“…This is highly problematic for reproducing the net cloud radiative forcing at the ice sheet surface and for predicting melting events associated with oceanic intrusions of warm, moist and cloudy air masses (Gilbert et al., 2020; Nicolas et al., 2017; Silber, Verlinde, Cadeddu, et al., 2019; Wille et al., 2019). Along the Antarctic edge, SLW is also a key ingredient for cloud (Silber, Fridlind, et al., 2019; Lubin et al., 2020; Zhang et al., 2019) and precipitation formation and growth, in particular through secondary ice production processes (Sotiropoulou et al., 2020; Young et al., 2019) and the riming of snowflakes (Grazioli et al., 2017; Vignon, Besic, et al., 2019).…”

On the drivers of droplet variability in Alpine mixed-phase clouds

“…This mechanism counters the glaciation indirect effect, where increases in INP concentrations elevate ice crystal number concentration (ICNC) and promotes the conversion of liquid water to ice -therefore the amount of ice-phase precipitation (Lohmann, 2002). Increases in CCN can also decrease cloud droplet radius, and impede cloud glaciation, owing to reductions in secondary ice production (SIP), which includes rime splintering, collisional break-up and droplet shattering (Field et al, 2017;Sotiropoulou et al, 2020aSotiropoulou et al, , 2020b).…”