Secondary Ice Production – No Evidence of Efficient Rime-Splintering Mechanism

Abstract

Abstract. Mixed-phase clouds are essential for Earth’s weather and climate system. Ice multiplication via secondary ice production (SIP) is thought to be responsible for the observed strong increase of ice particle number concentration in mixed-phase clouds. In this study, we focus on the rime-splintering also known as the Hallett-Mossop (HM) process, which still lacks physical and quantitative understanding. We report on an experimental study of rime-splintering conducted in a newly developed setup under conditions representing convective mixed-phase clouds in the temperature range of −4 °C to −10 °C. The riming process was observed with high-speed video microscopy and infrared thermography, while potential secondary ice particles (SI) in the super-micron size range were detected by a custom-build ice counter. Contrary to earlier HM experiments, where up to several hundreds of SI particles per mg rime were found at −5 °C, we found no evidence of productive SIP, which fundamentally questions the importance of rime-splintering. Further, we could exclude two potential mechanisms suggested as explanation for rime-splintering: freezing of droplets upon glancing contact with the rimer and fragmentation of spherically freezing droplets on the rimer surface. The break-off of sublimating fragile rime spires was observed to produce very few SI particles, insufficient to explain the large numbers of ice particles reported in earlier studies. In the transition regime between wet and dry growth, in analogy to phenomena of deformation of drizzle droplets upon freezing, we also observed formation of spikes on the rimer surface, which might be a source of SIP.

Secondary Ice Production Improves Simulations of Freezing Rain

“…While the model results presented in this study with the HM process included are encouraging and show a systematic improvement in hindcast simulations of FR, one must keep in mind that laboratory studies of rime splintering and other SIP processes have several limitations and the reproducibility of the original experiments reported in Hallett and Mossop (1974) have been recently questioned (Seidel et al, 2023). Thus, model parameterizations of SIP based on past studies are uncertain.…”

Effect of Secondary Ice Production Processes on the Simulation of ice pellets using the Predicted Particle Properties microphysics scheme

“…HM is the most studied SIP process (Field et al, 2017). Although some doubts remain concerning its physics (Seidel et al, 2023), it is thought to occur when a fast-falling solid particle collects small cloud droplets at temperatures between -8°C and -3°C. FFD occurs through the deformation of relatively large liquid drops (e.g.…”

“…Keinert et al, 2020;Phillips et al 2018). In general, the efficiency of FFD and HM processes remains difficult to establish and requires more research in the field and in the lab (Korolev et al, 2020;Korolev and Leisner, 2020;Lawson et al, 2023;Seidel et al, 2023).…”