Nevertheless, the occurrence and efficiency of secondary ice processes is still an area of open research.Īnother and larger scale approach to assess the influence of INPs on MPCs has been through the so-called supercooled liquid fraction (ratio of supercooled liquid to ice). The main explanation for this discrepancy is secondary ice production (SIP Hallett & Mossop, 1974 Korolev & Leisner, 2020), which has been shown to rapidly increase the concentration of ice crystals in MPCs through what has been described as a cascading process (Lawson et al., 2015). This would suggest that INPs are not as important for ice crystal formation in naturally occurring MPCs. However, when in situ ice crystal number concentrations are compared with INP concentrations, they seldomly agree (Mignani et al., 2019) and ice crystal concentrations often exceed INP concentrations by several orders of magnitude (Ladino et al., 2017 Rangno & Hobbs, 2001 Ramelli et al., 2021). When implemented into Earth System Models, different INP parametrizations can have profound effects on both MPC optical properties and lifetimes. Meanwhile, in more remote regions and at higher temperatures, biological sources such as sea spray aerosol are believed to be the most important source of INPs (Burrows et al., 2013 McCluskey et al., 2018 Schnell & Vali, 1975 Wilson et al., 2015 Irish et al., 2019).īased on the fundamental importance of INPs for ice crystal formation in MPCs, INP parametrizations have been developed to account for different aerosol species and the observed variability from field and laboratory studies. Close to the Earth's major deserts, dust is the primary source of INPs, especially at temperatures below −15☌ (Atkinson et al., 2013 Boose et al., 2016 Murray et al., 2012). This variability is partially explained by the location and type of aerosol acting as INPs (Kanji et al., 2017). These studies have found that INP concentrations can vary by several orders of magnitude at a given temperature. Therefore, field measurements including precipitation sampling (Petters & Wright, 2015 Vali, 1971), airborne (Borys, 1989 DeMott et al., 2010 Pratt et al., 2009 Rogers et al., 2001), ship (Welti et al., 2020 Wilson et al., 2015), and mountaintop measurements (Lacher et al., 2017) have been conducted to investigate the abundance of INPs, globally. Laboratory experiments show that pure water does not freeze without the presence of an INP until it is supercooled to around −36☌. The importance of ice-nucleating particles (INPs) for forming ice in MPCs is undisputed. But what controls the formation of ice and the thermodynamic phase composition in MPCs? This cloud phase feedback makes the accurate representation of ice crystal concentrations in MPCs in Earth System Models essential for correctly predicting the future climate (Forster et al., 2021 Tan et al., 2016). In a warming climate, MPCs are believed to transition toward a state with more liquid water and a higher albedo, which limits future warming (Bjordal et al., 2020 Zelinka et al., 2020). Simultaneously, the thermodynamic phase composition controls the radiative properties of MPCs due to the different scattering properties between liquid water and ice. The amount of liquid and ice within mixed-phase clouds (MPCs) influences precipitation formation, cloud lifetime, and electrification (Cantrell & Heymsfield, 2005). In a warming world with retreating sea ice and snow cover, our results suggest that clouds in these regions will contain ice crystals at warmer temperatures than previously estimated and, thus, have potential implications for future warming predictions.

This indicates that sea ice and snow cover act as a lid that inhibits the emission of INPs from the ocean.

Using satellite observations, we determine the transition temperature (T*) where MPCs become more frequent than liquid clouds and find that T* is strongly dependent on snow and sea ice cover over the Arctic and Southern Ocean.

However, a large-scale relationship between INPs and ice formation in clouds has yet to be observed. INPs are required for liquid cloud droplets to freeze at temperatures warmer than −36☌. The formation of ice crystals in MPCs requires a special subset of aerosol particles called ice-nucleating particles (INPs). For example, the number of ice crystals in MPCs determines how much sunlight is reflected by the cloud and how efficiently the cloud can form precipitation. Mixed-phase clouds (MPCs), which consist of both liquid droplets and ice crystals, play an important role for the Earth's climate system.

Cool math gravity guy