The marginal ice zone (MIZ) is that region within which more or less total sea-ice coverage gives way to a generally ice-free ocean. From the geophys- ical standpoint, the significance of the MIZ largely derives from dramatic changes in atmosphere/ocean interaction as one traverses this region. Sur- face properties can vary considerably in the MIZ providing sharp gradients in surface/atmosphere interactions. For example, snow-covered sea ice has a relatively high albedo and large surface roughness in comparison to open water. More important is the stark contrast in surface latent and sensible heat fluxes from ice-covered to open ocean.
Some effort has been made to numerically simulate atmosphere-ice- ocean interactions in the MIZ. Much of the work to date has focused on de- velopment of thermal internal boundary layers in off-ice flow (wind blow- ing from ice to ocean), momentum effects due to spatially-varying surface drag coefficients, and the effects of varying large-scale wind direction on lo- cal circulations. The modeling approaches have varied from slab-symmetric steady-state boundary layer models to more sophisticated primitive equa- tion (PE) models.
In the work presented here, we use the Regional Atmospheric Modeling System (RAMS) to simulate two observed cases of off-ice flow. One case, from REFLEX-II, is a cold-air outbreak and has extremely strong surface thermal forcing. The second case, which has received a fair amount of atten- tion, is weakly forced from the surface. While these cases have been studied with other numerical models, our work is, to the best of our knowledge, the first to consider in detail the role of clouds and precipitation in the develop- ment of the convective boundary layer resulting from off-ice flow.
We have conducted, with essentially the same model, simulations at a va- riety of scales, varying from the cloud resolving model (CRM) scale (80m delta-x, 40m delta-z) which resolves the dominant turbulent eddies, to the regional forecast model scale (~10km delta-x). We used a variety of cloud microphysical schemes ranging from dry (no cloud) to a bin-resolving mixed-phase parameterizations. These schemes work in concert with a two- stream radiation model to produce self-consistent simulations coupling ra- diative forcing, dynamics, precipitation/evaporation and microphysical properties.
Not surprisingly, the presence of clouds was found to have a significant impact on the dynamical evolution of the boundary layer, particularly at the CRM scale and for the case where the surface thermal forcing is weaker. In this presentation, we summarize our results for the different cases and at dif- ferent scales and also contrast this with previous work. We also attempt to present some general guidelines as to the level of numerical sophistication necessary to achieve credible simulations in the parameter space commonly found over the high-latitude oceans. We hope this work will benefit our- selves and others who intend to simulate SHEBA and REFLEX-III cases in the near future.