A numerical study of an extreme cold-air outbreak over the Labrador Sea: Sea ice, air-sea interaction, and development of polar lows

In this paper, the ability of the MM5 mesoscale forecast model to simulate the air-sea interaction, boundary layer development, and mesoscale structur e associated with a cold-air outbreak over the Labrador Sea is investigated . The case chosen was one for which research aircraft data and satellite im agery are available for validation. The default surface-layer parameterizat ion included in the model is shown to grossly overestimate the magnitude of the air-sea interaction resulting in forecasts of boundary layer growth an d mesoscale development that differ substantially from observations. It is also shown that a representation of the inhomogeneities in sea-ice cover re sults in a significant improvement in simulations of the air-sea interactio n, boundary layer development, and mesoscale structure both within the marg inal ice zone and downstream over the open ocean. Finally, the mesoscale cy clones or polar lows observed in the wake of the cold-air outbreak are show n to be coupled to the evolution of an upper-tropospheric potential vortici ty anomaly that was advected over the region. The model simulations suggest that MM5, most probably due to inaccuracies in and the limited resolution of the analyzed fields that supply the initial and boundary conditions to t he model, was unable to correctly simulate the development and track of thi s anomaly and this ultimately led to an incorrect forecast of the polar low s' finite-amplitude behavior.