Idealized dry quasi 2-D mesoscale simulations of cold-air outbreaks over the marginal sea ice zone with fine and coarse resolution

A nonhydrostatic model (NH3D) is used for idealized dry quasi 2-D simulations of Arctic cold-air outbreaks using horizontal grid spacings between 1.25 and 60 km. Despite the idealized setup, the model results agree well with observations over Fram Strait. It is shown that an important characteristic of the flow regime during cold-air outbreaks is an ice-breeze jet (IBJ) with a maximum wind speed exceeding often the large-scale geostrophic wind speed. According to the present simulations, which agree very well with those of another nonhydrostatic mesoscale model (METRAS), the occurrence, strength, and horizontal extent L of this jet depend strongly on the external forcing and especially on the direction of the large-scale geostrophic wind relative to the orientation of the ice edge. The latter dependency is explained by the effects of the thermally induced geostrophic wind over open water and Coriolis force. It is found that coarse-resolution runs underestimate the strength of the jet. This underestimation has important consequences to the surface fluxes of heat and momentum, which are also underestimated by about 10-15% on average over the region between the ice edge and 120-180km downstream. Our results suggest that a grid spacing of about L/7 is required (about 10-30km) to simulate the IBJ strength with an accuracy of at least 10%. Thus, the results of large-scale models as well might contain uncertainties with regard to the simulated IBJ strength which would influence the energy budget in a large region along the marginal sea ice zones.