Ice formation in Arctic mixed-phase clouds: Insights from a 3-D cloud-resolving model with size-resolved aerosol and cloud microphysics

The single-layer mixed-phase clouds observed during the Atmospheric Radiation Measurement (ARM) program's Mixed-Phase Arctic Cloud Experiment (MPACE) are simulated with a three-dimensional cloud-resolving model, the System for Atmospheric Modeling (SAM), coupled with an explicit bin microphysics scheme and a radar simulator. By implementing an aerosol-dependent and a temperature- and supersaturation-dependent ice nucleation scheme and treating IN size distribution prognostically, the link between ice crystal and aerosol properties is established to study aerosol indirect effects. Two possible ice enhancement mechanisms, activation of droplet evaporation residues by condensation followed by freezing and droplet evaporation freezing by contact freezing inside out, are scrutinized by extensive comparisons with the in situ and remote sensing measurements. Simulations with either mechanism agree well with the in situ and remote sensing measurements of ice microphysical properties but liquid water content is slightly underpredicted. These two mechanisms give similar cloud properties, although ice nucleation occurs at very different rates and locations. Ice nucleation from activation of evaporation nuclei occurs mostly near cloud top areas, while ice nucleation from the drop freezing during evaporation has no significant location preference. Both ice enhancement mechanisms contribute dramatically to ice formation with ice particle concentration of 10-15 times higher relative to the simulation without either of them. Ice nuclei (IN) recycling from ice sublimation contributes significantly to maintaining concentrations of IN and ice particles in this case, implying an important role to maintain the observed long-term existence of mixed-phase clouds. Cloud can be very sensitive to IN initially but become much less sensitive as cloud evolves to a steady mixed-phase condition.