A Numerical Study of an Extreme Cold-Air Outbreak over the Labrador Sea: Sea Ice, Air–Sea Interaction, and Development of Polar Lows

Abstract

In this paper, the ability of the MM5 mesoscale forecast model to simulate the air?sea interaction, boundary layer development, and mesoscale structure associated with a cold-air outbreak over the Labrador Sea is investigated. The case chosen was one for which research aircraft data and satellite imagery are available for validation. The default surface-layer parameterization included in the model is shown to grossly overestimate the magnitude of the air?sea interaction resulting in forecasts of boundary layer growth and mesoscale development that differ substantially from observations. It is also shown that a representation of the inhomogeneities in sea-ice cover results in a significant improvement in simulations of the air?sea interaction, boundary layer development, and mesoscale structure both within the marginal ice zone and downstream over the open ocean. Finally, the mesoscale cyclones or polar lows observed in the wake of the cold-air outbreak are shown to be coupled to the evolution of an upper-tropospheric potential vorticity 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 the model, was unable to correctly simulate the development and track of this anomaly and this ultimately led to an incorrect forecast of the polar lows? finite-amplitude behavior.