A numerical case study of a polar low in the Labrador Sea

A mesoscale (15 km) version of the Canadian regional finite-element model is used to study a polar low that developed in the Labrador Sea on 11 January 1989, in the wake of an intense cold air outbreak associated with a major synoptic-scale system located to the east of Greenland. The rapid evolution of the polar low is well revealed from satellite imagery showing a complex structure with strong surface winds near the vortex and deep convection nearby during the mature stage. The simulated structure of the polar low agrees quite well with observed features. Based on the detailed mesoscale model outputs, the evolution of the Labrador Sea polar low is discussed at the initiation and mature stages. The polar low developed under a combination of baroclinic and convective processes. At an early stage, baroclinic development takes place in conditions of reversed shear flow, marked by low-level baroclinicity near the ice edge and a mobile upper-level short wave. Rapid modification of the Arctic boundary layer by strong surface heat fluxes is similar to that observed in other areas. Sensitivity experiments indicate that the mutual interaction between the upper-level potential vorticity anomaly and the low-level baroclinicity at the Arctic front, favoured by the deep convective boundary layer, appears to trigger the polar low. At the onset of the mature stage, the approach of a cold air dome favours the outbreak of deep convection, in agreement with satellite imagery. As shown by sensitivity experiments, latent heat release from organized convection contributes to the major part of the rapid deepening of the polar low in its mature stage, and sea surface evaporation is the primary feeding mechanism for condensation processes. The structure of the polar low is characterized by a warm core due to the combined effects of warm air seclusion and diabatic heating. Comparisons with previously studied polar lows developing in similar conditions of reversed shear at other locations are discussed.