Measurements from the US Department of Energy Atmospheric Radiation Measurement Program's 2004 Mixed-Phase Arctic Cloud Experiment (M-PACE) provide a unique opportunity to study poorly understood ice formation processes in mixed-phase stratocumulus. Using meteorological, aerosol, and ice nucleus measurements to initialize large-eddy simulations with size-resolved microphysics, we compare predicted liquid and ice mass, number, and size distribution with observations from a typical flight. We find that ambient ice nuclei appear insufficient by a few orders of magnitude to explain observed ice, consistent with past literature. We also find that two processes previously hypothesized to explain the discrepancy, shatter of freezing drops and fragmentation during ice-ice collisions, were not significant sources of ice based on parameterizations from existing studies. After surveying other mechanisms that have been hypothesized to explain ice formation in mixed-phase clouds generally, we find two that may be strong enough: (1) formation of ice nuclei from drop evaporation residuals, a process suggested by sparse and limited measurements to date, and (2) drop freezing during evaporation, a process suggested only by inference at this time. The first mechanism can better explain the persistence of mixed-phase conditions in simulations of less vigorous stratus observed during the Beaufort Arctic Storms Experiment (BASE). We consider conditions under which emission of nuclei from the ocean surface or activation through cloud-phase chemistry could provide alternative explanations for M-PACE observations. Additional process-oriented measurements are suggested to distinguish among ice formation mechanisms in future field studies.