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Two-Dimensional Cloud Resolving Modeling of Arctic Leads Based Upon Mid-Winter SHEBA Conditions |
Department of Meteorology, University of Utah
mazulauf@atmos.met.utah.edu
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 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.
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 a kilometer. For smaller leads, or under more stable or windier conditions, the convective plume may be confined to a relatively shallow depth 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). Additionally, both synthetic aperture radar (SAR) and IR satellite observations, as well as surface IR measurements from the ferry aircraft, provide an estimate of the the distribution of lead sizes, orientation, and number. In a simulation containing a 3.2 km lead under idealized SHEBA conditions, a surface based ice cloud was observed to propagate at least 50 kilometers downwind. 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 they are in fact related.
