The Transition from Supercooled Liquid Water to Ice Crystals in Mixed-phase Clouds based on Airborne In-situ Observations

The on-set of ice nucleation in mixed-phase clouds determines cloud lifetime and their microphysical properties. In this work, we develop a novel method that differentiates the early and later transition phases of mixed-phase clouds, i.e., ice crystals are initially surrounded by supercooled liquid water droplets, then as they grow, pure ice segments are formed. Using this method, we examine the relationship between the macrophysical and microphysical properties of mixed-phase clouds. The results show that evolution of cloud macrophysical properties, represented by the increasing spatial ratio of regions containing 10 ice crystals relative to the total in-cloud region (defined as ice spatial ratio), is positively correlated with the evolution of microphysical properties, represented by the increasing ice water content and decreasing liquid water content. The mass partition transition from liquid to ice becomes more significant during the later transition phase (i.e., transition phase 3) when pure ice cloud regions (ICRs) start to appear. Occurrence frequencies of cloud thermodynamic phases show significant transition from liquid to ice at a similar temperature (i.e., -17.5°C) among three types of definitions of mixed-phase clouds 15 based on ice mass fraction, ice number fraction, or ice spatial ratio. Aerosol indirect effects are quantified for different transition phases using number concentrations of aerosols greater than 100 nm or 500 nm (N >100 and N >500 , respectively). N >500 shows stronger positive correlations with ice spatial ratios compared with N >100 . This result indicates that larger aerosols potentially contain ice nucleating particles, which facilitate the formation of ice crystals in mixed-phase clouds. The impact of N >500 is also more significant on the earlier transition phase when ice crystals just start to appear compared with the later 20 transition phase. The thermodynamic and dynamic conditions are quantified for each transition phase. The results show in-cloud turbulence as a main mechanism for both the initiation of ice nucleation and the maintenance of supercooled liquid water, while updrafts are important for the latter but not the former. Overall, these results illustrate the varying effects of aerosols, thermodynamics, and dynamics throughout cloud evolution based on this new method that categorizes cloud transition phases. the from to in-cloud in the the dynamical coexistence of and several dynamic mechanisms were proposed in the study and the critical thresholds of vertical motion for sustaining supercooled liquid water. both and in-cloud turbulence when liquid phase becomes more dominant in terms of spatial extent (i.e., low ice spatial ratio). The updrafts were observed in transition phase 3 in segments containing both ice and liquid (i.e., MCR), indicating that updrafts are needed to supercooled liquid water when they are surrounded by ice crystals. This observation-based method can be used to assess the contribution from different dynamic mechanisms for maintaining different evolution stages of mixed-phase clouds in various field