A NUMERICAL CASE-STUDY OF A POLAR LOW IN THE LABRADOR SEA

A mesoscale (15 km) version of the Canadian regional finite-element mo del 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 associat ed 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 th e vortex and deep convection nearby during the mature stage. The simul ated structure of the polar low agrees quite well with observed featur es. Based on the detailed mesoscale model outputs, the evolution of th e Labrador Sea polar low is discussed at the initiation and mature sta ges. The polar low developed under a combination of baroclinic and con vective processes. At an early stage, baroclinic development takes pla ce in conditions of reversed shear flow, marked by low-level baroclini city near the ice edge and a mobile upper-level short wave. Rapid modi fication of the Arctic boundary layer by strong surface heat fluxes is similar to that observed in other areas. Sensitivity experiments indi cate that the mutual interaction between the upper-level potential vor ticity anomaly and the low-level baroclinicity at the Arctic front, fa vored by the deep convective boundary layer, appears to trigger the po lar low. At the onset of the mature stage, the approach of a cold air dome favors the outbreak of deep convection, in agreement with satelli te imagery. As shown by sensitivity experiments, latent heat release f rom organized convection contributes to the major part of the rapid de epening of the polar low in its mature stage, and sea surface evaporat ion is the primary feeding mechanism for condensation processes. The s tructure of the polar low is characterized by a warm core due to the c ombined effects of warm air seclusion and diabatic heating. Comparison s with previously studied polar lows developing in similar conditions of reversed shear at other locations are discussed.