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Abstract:
During wintertime, the main source of energy to the Arctic comes through moist intrusions. The air mass cools radiatively during transport, promoting the formation of low-level mixed-phase clouds (MPCs). Previous studies have shown that the MPCs that form can exist in either a stable (stratus) or a convective (stratocumulus) state. In this study, we examine the processes and conditions that promote a transition between the two states through idealized simulations using a three-dimensional large-eddy simulation model coupled with a one-dimensional multilayer sea ice model. We find that the availability and properties of aerosols as well as the cloud ice content can delay or even prevent stratocumulus formation. If no suitable cloud condensation nuclei are available at the base of the cloud at the time of the transition, then no new droplets form, the buoyancy remains low and the cloud remains in its stable state. Furthermore, a lower cloud ice water content results in a more stably stratified cloud layer and a delay in the transition. The transition from the stable to the convective state has a substantial effect on the surface warming induced by the cloud in the model; simulations with a transition generally show larger surface warming than simulations without a transition. Our results suggest that the low-level mixed-phase cloud evolution and the thermodynamic transition of an airmass during a moist intrusion are closely linked to the aerosol processing by the cloud, i.e. a chemical transformation, and that the two processes should be considered simultaneously.
