Moisture and Aerosols as Key Drivers of the Turbulent State of Arctic Mixed-Phase Clouds During Idealized Moist Intrusions

Previous studies have shown that low-level mixed-phase clouds that form during idealized moist intrusions into the Arctic can exist in either a stable (stratus) or a convective (stratocumulus) state. Here, we examine the conditions that promote a transition from the stable to the convective state through idealized simulations using a three-dimensional large-eddy simulation model coupled with a one-dimensional multilayer sea ice model. We find that the vertical distribution of the initial dew point temperature (T_d) profile fundamentally influences whether a transition between the two states occurs or not. If the initial moisture content of the advected airmass decreases rapidly with height, then a turbulent transition is likely to occur and a stratocumulus cloud can form. However, the availability and properties of aerosols as well as the cloud ice content can delay or even prevent stratocumulus formation, regardless if the conditions in terms of the initial T_d profile are favorable. A low cloud ice water content promotes a stably stratified cloud layer and delays the transition. Furthermore, if no cloud condensation nuclei are available at the base of the cloud when a cloud-layer instability forms, then there is no new droplet formation, the buoyancy remains low and the cloud remains as a stratus. Our results suggest that the low-level mixed-phase cloud evolution and the thermodynamic transition of an airmass during a moist intrusion into the Arctic are closely linked to the aerosol processing by the cloud, that is, a chemical transformation, and that the two processes should be considered simultaneously.