Numerical simulations of lead-generated clouds and their effect on large-scale surface fluxes

Due to 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. Due to their relatively small scales, these quasi-linear openings in the pack ice can not be explicitly resolved by large-scale models. Despite this fact, these small-scale features may indeed 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 from the snow and ice surfaces. Thus, the total fluxes associated with leads can be of the same magnitude as those through the snow and ice pack.

In addition, the convective plumes emanating from leads have been observed to contribute to cloud development under certain conditions. As seen at the Surface Heat Budget of the Arctic Ocean (SHEBA) site, the presence of clouds can profoundly impact the energy balance at the surface through radiative effects. Depending upon lead size and ambient atmospheric conditions, the convective plumes, and associated cloud development, may penetrate to varying depths. For very large leads under relatively calm conditions, plumes may penetrate to heights of greater than one kilometer. For smaller leads, or under more stable or windier conditions, the convective plume may be confined to a relatively shallow depth on the order of one hundred meters or less.

In an attempt to better understand the effects the enhanced small-scale surface fluxes can have upon the large-scale, a two-dimensional cloud resolving model is employed. Numerous 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 (approximately 10 K temperature increase in the lowest 250 m of the atmosphere). In a simulation containing a lead of a size similar to one observed in the vicinity of the SHEBA camp, a low level ice cloud was generated, which propagated at least 50 kilometers downwind. Surface fluxes of both heat and moisture were seen to be strongly affected over the snow and ice pack downwind of the lead, in large part due to radiative effects of the cloud. It is notable that similar cloud features were also observed at the SHEBA site near the times when active leads were in the vicinity, though it has not been established that these clouds were in fact lead-generated.