Effects of Aerosol Concentration on the Dissipation of Arctic Mixed-Phase Clouds

Abstract:

Arctic mixed-phase boundary layer clouds are able to persist for days and then dissipate in a matter of hours. It is sometimes unknown what causes this sudden dissipation, but aerosol-cloud interactions may sometimes be at play. Specifically, CCN concentrations in the Arctic can be low enough to affect cloud formation and structure, and it has been hypothesized that some instances of cloud dissipation are caused by a decrease in CCN concentration below some critical value.

In this study, we use observations taken from the Department of Energy ARM sites on the north slope of Alaska to identify potential cases where Arctic boundary-layer clouds dissipate coincidentally with a drop in CCN concentrations. Data taken from these cases are used to initialize large eddy simulations in which aerosol concentrations are forced either to decrease at the observed rate or to be held constant above and below the cloud. Ice nuclei concentrations are also varied independently. Results from these simulations are examined to identify the microphysical and cloud-aerosol processes affecting cloud dissipation in a CCN-limited situation. When concentrations decrease both above and below cloud, the cloud eventually dissipates, and the rate of dissipation is compared to the observed rate of dissipation. When the concentrations decrease only below cloud but are held constant above, the dissipation is slowed. This result suggests that the entrainment of aerosol particles above cloud top is important for the evolution and properties of Arctic boundary layer clouds. The ice nuclei concentration modulates the dissipation rate as anticipated, but does not fundamentally change the qualitative response of the cloud to the decreasing CCN concentrations.