Secondary ice production processes in wintertime alpine mixed-phase clouds

Abstract

Abstract. Observations of orographic mixed-phase clouds (MPCs) have long shown that measured ice crystal number concentrations (ICNCs) can exceed the concentration of ice nucleating particles by orders of magnitude. Additionally, model simulations of alpine clouds are frequently found to underestimate the amount of ice compared with observations. Surface-based blowing snow, hoar frost, and secondary ice production processes have been suggested as potential causes, but their relative importance and persistence remains highly uncertain. Here we study ice production mechanisms in wintertime orographic MPCs observed during the Cloud and Aerosol Characterization Experiment (CLACE) 2014 campaign at the Jungfraujoch site in the Swiss Alps with the Weather Research and Forecasting model (WRF). Simulations suggest that droplet shattering is not a significant source of ice crystals at this specific location, but breakups upon collisions between ice particles are quite active, elevating the predicted ICNCs by up to 3 orders of magnitude, which is consistent with observations. The initiation of the ice–ice collisional breakup mechanism is primarily associated with the occurrence of seeder–feeder events from higher precipitating cloud layers. The enhanced aggregation of snowflakes is found to drive secondary ice formation in the simulated clouds, the role of which is strengthened when the large hydrometeors interact with the primary ice crystals formed in the feeder cloud. Including a constant source of cloud ice crystals from blowing snow, through the action of the breakup mechanism, can episodically enhance ICNCs. Increases in secondary ice fragment generation can be counterbalanced by enhanced orographic precipitation, which seems to prevent explosive multiplication and cloud dissipation. These findings highlight the importance of secondary ice and seeding mechanisms – primarily falling ice from above and, to a lesser degree, blowing ice from the surface – which frequently enhance primary ice and determine the phase state and properties of MPCs.

Ice microphysical processes in the dendritic growth layer: a statistical analysis combining multi-frequency and polarimetric Doppler cloud radar observations

“…Griggs and Choularton (1986) also reports that dendrite crystals can fragment without collisions simply due to air drag. Recent modelling studies suggest that ice collisional fragmentation can elevate the ice number concentration by three orders of magnitude (Georgakaki et al, 2022).…”

“…This secondary ice process (SIP) gains increasing attention by the scientific community (e.g. Georgakaki et al, 2022;Phillips et al, 2018) as an important process which could explain the discrepancy between number of ice nucleating particles (INPs) and ice particle number concentration (IPNC) (Kanji et al, 2017). Unlike for example the Hallett-Mossop rime splintering process (Field et al, 2017;Hallett and Mossop, 1974), ice collisional fragmentation could provide new secondary ice particles over a wide temperature range.…”

Role of K-feldspar and quartz in global ice nucleation by mineral dust in mixed-phase clouds

“…This entails a shorter cloud lifetime, lower albedo, less reflected radiation, and thus less cooling of the atmosphere than by liquid clouds (DeMott et al, 2010). Secondary processes (ice-ice collisions, droplet riming and droplet shattering) can modulate further these phenomena (e.g., Georgakaki et al, 2022;Sotiropoulou et al, 2021) so that ice production is promoted even at extremely low INP concentrations (e.g., Morales Betancourt et al, 2012;Sullivan et al, 2018). High concentrations of CCN can further modulate these processes by reducing the efficiency of riming and droplet shattering (e.g., Lance, 2012).…”

Heavy snowfall event over the Swiss Alps: Did wind shear impact secondary ice production?

“…Rime splintering has been used extensively in models but has been shown to be inadequate, having ice number concentrations orders of magnitude less than observed, in SIP in wintertime orographic MPCs (Henneberg et al, 2017;Dedekind et al, 2021;Georgakaki et al, 2022). Ice-ice collisions have been more widely used in models in the last decade (Yano and Phillips, 2011;Phillips et al, 2017;Sullivan et al, 2018;Hoarau et al, 2018;Sotiropoulou et al, 2020;Zhao et al, 2021) since they were first studied in laboratory conditions about four decades ago (Vardiman, 1978;Takahashi et al, 1995).…”

“…Ice-ice collisions have been more widely used in models in the last decade (Yano and Phillips, 2011;Phillips et al, 2017;Sullivan et al, 2018;Hoarau et al, 2018;Sotiropoulou et al, 2020;Zhao et al, 2021) since they were first studied in laboratory conditions about four decades ago (Vardiman, 1978;Takahashi et al, 1995). SIP as a result of ice-ice collisions was shown to contribute significantly to the ice crystal number concentrations and thereby explain the discrepancy between models and observations in the Arctic (Sotiropoulou et al, 2020;Zhao et al, 2021), Antarctic (Sotiropoulou et al, 2021b) and mid-latitudes (Sullivan et al, 2018;Dedekind et al, 2021;Georgakaki et al, 2022). The enhancement of smaller ice particles cause an increase in the combined growth rates (riming and deposition) of up to 33% resulting in larger latent heat release and stronger updraft velocities (Dedekind et al, 2021).…”