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 (Td) 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 Td 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. Plain Language Summary: Clouds strongly influence the energy budget of the central Arctic and tend to warm the surface, in particular during winter. We have used a detailed numerical model to study low‐altitude clouds that form when warm, moist air is transported over the Arctic pack ice. The clouds can exist as two types, either as stratus (thin clouds with little turbulent motion) or stratocumulus (thicker clouds with substantial turbulence). The thicker stratocumulus clouds warm the surface more than the thinner stratus clouds and it is therefore important to understand why some clouds transition into stratocumulus while others stay as stratus. We find that the vertical profile of atmospheric moisture plays a key role for the transition; if the air is moist at the surface but relatively dry aloft, then the transition occurs faster than if the air is relatively moist throughout the atmospheric column. Aerosol particles also play an important role for the transition; in the clean central Arctic, a lack of aerosol particles can prevent cloud droplet formation and thereby also the formation of stratocumulus clouds. Aerosol particles can also influence the formation of ice crystals in the cloud, which we show can affect if and when a stratocumulus cloud forms. Key Points: Mixed‐phase clouds that form during Arctic moist intrusions can transform from a stable (stratus) to a convective state (stratocumulus)The initial moisture profile fundamentally influences whether the transition between the two states occurs or notThe presence or absence of aerosols acting as cloud condensation or ice nuclei can delay or even prevent the transition [ABSTRACT FROM AUTHOR]