Conditions favorable for secondary ice production in Arctic mixed-phase clouds

Abstract

Abstract. The Arctic is very susceptible to climate change and thus warming much faster than the rest of the world. Clouds influence terrestrial and solar radiative fluxes, and thereby impact the amplified Arctic warming. The partitioning of thermodynamic phases (i.e. ice crystals and water droplets) within mixed-phase clouds (MPCs) especially influences their radiative properties. However, the processes responsible for ice crystal formation remain only partially characterized. In particular, so-called secondary ice production (SIP) processes, which create supplementary ice crystals from primary ice crystals and the environmental conditions that they occur in, are poorly understood. The microphysical properties of Arctic MPCs were measured during the Ny-Ålesund AeroSol Cloud ExperimENT (NASCENT) campaign to obtain a better understanding of the atmospheric conditions favorable for the occurrence of SIP processes. To this aim, the in-situ cloud microphysical properties retrieved by a holographic cloud imager mounted on a tethered balloon system were complemented by ground-based remote sensing and ice nucleating particle measurements. During six days investigated in this study, SIP occurred during 40 % of the in-cloud measurements and high SIP events with number concentrations larger than 10 L-1 of small pristine ice crystals in 3.5 % of the in-cloud measurements. This demonstrates the role of SIP for Arctic MPCs. The highest concentrations of small pristine ice crystals were produced at temperatures between -3 °C and -5 °C and were related to the occurrence of drizzle drops freezing upon collision with ice crystals. This suggests that a large fraction of ice crystals in Arctic MPCs is produced via the droplet shattering mechanism. From evaluating the ice crystal images, we could identify ice-ice collision as a second SIP mechanism that dominated when fragile ice crystals were observed. Moreover, SIP occurred over a large temperature range and was observed in up to 95 % of the measurements down to -24 °C due to the occurrence of ice-ice collisions. This emphasizes the importance of SIP at temperatures below -8 °C, which are currently not accounted for in most numerical weather models.

Retrieving ice-nucleating particle concentration and ice multiplication factors using active remote sensing validated by in situ observations

“…At temperatures warmer than −15 • C, previous studies consistently confirm this observation and suggest that even IMFs of 10 4 are frequently observed in the atmosphere. The observations of Stith et al (2011) and Pasquier et al (2022) suggest SIP also occurring at temperatures much lower than −15 • C, supporting our findings. Especially at these colder temperatures, the contribution of SIP processes remains uncertain due to the lack of available measurements.…”

“…Thus, we compare our findings only to field observations of selected studies from the past decade in which the effect of shattering was mitigated. Details about the individual studies and the procedure of obtaining IMFs from the ice crystal and INP data are described in Appendix G. Figure 14 comprises the range of observed IMFs in this study along with derived IMFs from previous studies which observed maritime convective systems (Stith et al, 2011), continental clouds (Crawford et al, 2012;Taylor et al, 2016), tropical clouds (Lasher-Trapp et al, 2016;Ladino et al, 2017), orographic MPCs (Mignani et al, 2019;Lauber et al, 2021), and Arctic MPCs (Pasquier et al, 2022). The IMFs derived from the previous studies not only exhibit great variability among each other but also span over a considerable value range in each individual study.…”

“…Thus, the Hallett-Mossop process can play an important role, especially over orographic terrain, given that the orographic-forcing-induced updrafts could sustain the availability of liquid water droplets (Lohmann et al, 2016). Various field studies have observed ICNC exceeding the ambient INP concentration by several orders of magnitude (see e.g., Koenig, 1963;Auer et al, 1969;Hobbs and Rangno, 1985, 1990Gayet et al, 2009;Crosier et al, 2011;Stith et al, 2011;Crawford et al, 2012;Heymsfield and Willis, 2014;Lawson et al, 2015;Lasher-Trapp et al, 2016;Ladino et al, 2017;Mignani et al, 2019;Lauber et al, 2021;Pasquier et al, 2022). However, the underlying SIP processes are weakly constrained (Korolev and Leisner, 2020).…”

“…For measurements close to the melting layer, a temperature of −0.5 • C was used for plotting. IMFs fromPasquier et al (2022) were provided by personal communication Li et al (2021). do not provide correlated data of ICNC, INP concentration, and temperature.…”

Observation of secondary ice production in clouds at low temperatures

“…There is a large number of past and recent in situ observations of SIP within the HM temperature range (e.g., Hallett et al, 1978;Crawford et al, 2012;Keppas et al, 2017;Lasher-Trapp et al, 2016;Lauber et al, 2021;Li et al, 2021;Luke et al, 2021;Ramelli et al, 2021, to name a few). However, there are fewer observations of SIP outside the HM temperature range (e.g., Hobbs, 1969;Costa et al, 2017;Lawson et al, , 2022Mignani et al, 2019;Pasquier et al, 2022). Most of these studies reported observations of enhanced concentration of ice particles which exceeded expected concentration of INPs at the temperature of observation.…”