The Importance of the Free Troposphere for Maintaining Arctic Low-Level Clouds

In the high Arctic, particularly in the summertime, accumulation mode aerosol concentrations at the surface can fall to very low levels, often less than 20 cm^-3. At such low levels, it is theoretically difficult to maintain mixed-phase low-level clouds that are ubiquitous in this region, yet observations suggest that clouds can persist despite such low concentrations. One idea is that in these cases, supersaturation in the clouds becomes high enough to activate Aitken mode particles which have typically been neglected as potential CCN due to their small size and the high supersaturations required to activate them. Both modeling and observations support this idea. Here we argue that aerosol concentrations in the free troposphere additionally support the existence of long-lived mixed-phase clouds in the high Arctic. Observations from ASCOS and MOSAiC both indicate that aerosol concentrations in summer are frequently an order of magnitude higher just above the boundary layer top. We ran a suite of LES simulations to investigate the importance of these free tropospheric concentrations on the maintenance of the low-level clouds. We found that with insufficient concentrations in the free troposphere, the clouds dissipated. However, with higher concentrations, concentrations consistent with the observations, long-lived clouds could be simulated. Moreover, these simulations were able to maintain very low aerosol concentrations in the boundary layer which suggests that more routine observations of concentrations at both the surface and in the lower troposphere are needed to understand the evolution of these clouds.