Turbulence as a Key Driver of Ice Aggregation and Riming in Arctic Low-Level Mixed-Phase Clouds, Revealed by Long-Term Cloud Radar Observations

Turbulence in clouds is known to enhance particle collision rates, as widely demonstrated for warm rain formation. A similar impact on ice growth processes is expected but a solid observational basis is missing. A statistical analysis of a 15-month data set of cloud radar observations allows for the first time to quantify the impact of turbulence on ice aggregation and riming in Arctic low-level mixed-phase clouds. Increasing eddy dissipation rate (EDR), from below 10-4 to above 10-3 m2 s-3, yields larger ice aggregates, and higher particle concentration, likely caused by increasing fragmentation. In conditions more favorable to riming, higher EDR is associated with dramatically higher particle fall velocities (by up to 125%), under similar liquid water paths, indicative of markedly higher degrees of riming. Our findings thus reveal the key role of turbulence for cold precipitation formation, and highlight the need for an improved understanding of turbulence-hydrometeor interactions in cold clouds. Liquid and frozen precipitation mainly forms by collision and subsequent aggregation of small particles. Collisions between cloud particles, such as droplets and ice crystals, are thought to be increased by turbulence. While this effect has been intensively studied for liquid-only clouds, the impact of turbulence on ice-ice collisional growth (aggregation) and ice-liquid collisional growth (riming) is expected but has so far been poorly quantified. We study the effect of turbulence on aggregation and riming based on a long-term remote-sensing data set of low-level clouds containing both ice and liquid particles, recorded at the Arctic site of Ny-angstrom lesund, Svalbard. Cloud radar observations are used to retrieve the dissipation rate of turbulent kinetic energy (i.e., the eddy dissipation rate; EDR), which is the relevant quantity driving increases in collision rates, and to characterize ice particle properties. We find evidence that higher EDR regimes enhance the aggregation of particles, and are associated with signatures of increased ice particle concentration, possibly caused by the production of particle fragments upon collision. In temperature regimes more favorable to riming, turbulence dramatically enhances the particles' fall velocity, denoting higher degrees of riming. Our findings thus highlight a key role of turbulence for the formation of precipitable ice. Relation between turbulence and ice growth investigated based on long-term remote sensing data set of Arctic low-level mixed-phase clouds Higher eddy dissipation rate (EDR) correlates with larger ice aggregates, and possibly higher degrees of fragmentation High EDR is an essential component needed for the formation of rimed particles