The influence of parameterized ice habit on the glaciation of Arctic clouds

Low level, stratiform clouds containing mixes of liquid and ice (mixed-phase clouds) persist over extensive areas of the Arctic throughout much of the year. The partitioning between liquid and ice in these clouds exerts a strong control over the surface radiative budget of the ocean and sea-ice. This is important because a number of recent studies suggest that cloud radiative feedbacks may affect the melting and freezing of the sea-ice. Despite this importance, regional climate models have difficulty capturing the spatial and temporal evolution of mixed-phase clouds, which may introduce biases into the surface radiative budget. It has been shown in past studies that aerosol impacts, through heterogeneous ice nucleation, substantially alters mixed-phase cloud lifetime such that incorrect aerosol-cloud interactions may be the reason for the poor performance of at least some regional Arctic climate models. However, the method by which ice habit is parameterized in most numerical models also introduces a significant degree of uncertainty into the model predictions. In this study, we examine the influence that ice habit parameterizations have on the partitioning of cloud mass between liquid and ice in mixed-phase Arctic clouds. We show that one current method of parameterizing ice mass, fallspeed, and vapor growth leads to a wide range of possible distributions of liquid and ice in mixed-phase Arctic clouds. Our studies also show that different habits cause different sensitivities of mixed-phase Arctic clouds to heterogeneous ice nucleation. This result helps to reconcile differences in model sensitivities that have been reported in the literature. Finally, our studies show that ice habit parameterizations can have an impact on the surface radiative budget that is potentially as large as that produced by aerosol-cloud interactions. Methods for improving ice habit prediction in Arctic cloud models will also be presented.