Two-Dimensional Cloud Resolving Modeling of Arctic Leads Based upon Mid-Winter Sheba Conditions

Because of the extreme temperature differences between the air and the sea surface during the Arctic winter, leads can be a significant source of heat and moisture for the Arctic atmosphere. Unfortunately, the relatively small scales of these quasi-linear openings in the pack ice can not be explicitly resolved by large-scale models. This is problematic as these small-scale features may in fact have significant impacts upon the large-scale atmosphere. For example, while leads typically account for only 1 - 2% of the surface area of the Arctic, the surface fluxes of heat and moisture from them may be in excess of two orders of magnitude greater than those through the ice and snow surface. Thus, the total fluxes associated with leads can be of the same magnitude as those through the ice and snow surfaces.

Observations taken at the Surface Heat Budget of the Arctic Ocean (SHEBA) site indicate that clouds can profoundly impact the surface energy balance through radiative effects. Under certain conditions, convective plumes emanating from leads have been observed to contribute to cloud development. Depending upon lead size and ambient atmospheric conditions, the convective plumes and associated clouds may penetrate to varying depths. For example, the plumes from large leads which form in relatively quiescent conditions may penetrate to depths of over a kilometer while smaller leads tend to produce relatively shallow plumes on the order of one to two hundred meters. The ambient stability and wind profiles are additional factors that tend to modulate plume depth.

We employ a two-dimensional cloud resolving model (CRM) in an attempt to better understand the effects that enhanced small-scale surface fluxes have upon the large-scale atmosphere. Numerous types of observations from the SHEBA project have been used as the basis for an idealized clear-sky mid-winter case. Under these conditions an extremely stable surface layer is observed (i.e., a 10 K increase in temperature over the lowest 250 m). When simulating a 3.2 km wide lead under these idealized conditions, a surface based ice cloud was seen to propagate over 25 kilometers downwind. Similar cloud features have also been observed at the SHEBA site near active leads, though the source of these cloud features has not yet been established.