Possible roles of ice nucleation mode and ice nuclei depletion in the extended lifetime of Arctic mixed-phase clouds

The sensitivity of Arctic mixed phase clouds to the mode of ice particle nucleation is examined using a 1-D cloud model. It is shown that the lifetime of a simulated low-level Arctic mixed-phase stratus is highly sensitive to the number concentration of deposition/condensation-freezing nuclei, and much less sensitive to the number of contact nuclei. Simulations with prognostic ice nuclei concentration exhibit rapid depletion of deposition/ condensation-freezing nuclei due to nucleation scavenging which significantly extends the mixed-phase cloud lifetime. In contrast, scavenging has little impact on the number of contact nuclei. Thus, contact mode nucleation generally dominates in the cloud layer when both modes are simultaneously considered. The dominance of contact nucleation in Arctic mixed-phase clouds is consistent with a number of in situ observations, remote retrievals, and laboratory experiments. A conceptual model of long-lived Arctic mixed-phase clouds is developed that explains their persistence through the rapid depletion of deposition/ condensation-freezing ice nuclei and a self-regulating drop-contact freezing feedback.