Labrador Sea Polar Lows.

Rasmussen E., Claud C., Purdom J.F.

The Global Atmosphere-Ocean System 4, 275-333.

Recently a southern genesis region for polar lows was found over the Labrador Sea. Practically all polar lows which develop in this region show the same basic characteristics which justify the use of the generie term "Labrador Sea polar lows". Because of the comparatively low latitude of the genesis region, 55N-60N, the Labrador Sea polar low developments can be followed in great detail, both temporally and spatially using geostationary satellite data. The formation of a number of Labrador Sea polar lows are discussed. The discussion is mainly based on satellite and routine meteorological observations, but some results from a recent successful numerical simulation of a Labrador Sea polar low are included as well. The Labrador Sea polar lows always seem to start their development within a limited geographical region around 58N-58W. In this connection a number of factors which may influence the formation of the lows are discussed, including the effect of a wind channelling through the Hudson Strait, the presence of strong low-level baroclinic zones in the region, and strong latent and sensible heat fluxes over the sea. Also a possible effect of the Greenland ice cap with its steep topography at the eastern boundary of the region of interest is discussed. The Labrador Sea polar lows are invariably observed at the western part of an approximately west-east or northwest-southeast oriented cloud line or band along which also several minor vortices usually can be seen. The possible role of barotropic instability associated with horizontal shear across these eloud lines for the formation of the Labrador Sea polar lows is discussed. "PV-thinking" seems a fruitful approach for the understanding of the Labrador Sea polar low developments, and the potentia] of this approach is briefly discussed. A likely scenarium for the initial formation of a Labrador Sea polar low close to the coastal ice edge is the penetration to low levels of the upper-level circulation associated with a strong PV-anomaly. Following, or simultaneously with the formation of the low-level circulation, the characteristic cloud band is formed. The numerical simulation illustrates that baroclinic instability is of major importance for the development of the polar low. On the other hand the small-scale vortices which develop along the cloud band extending from the polar low are most likely caused by barotropic instability (or mixed barotropic-eonveetive instability). Finally the occurence of polar lows in the Labrador Sea different from the type focused on in this paper, and the similarity between the Labrador Sea polar lows and polar lows in other regions is briefly discussed.

KEY WORDS: Polar lows, Labrador Sea polar lows, satellite data, upper-level vortex, numerical simulations, cloud lines, instability of cloud lines, baroclinic instability, barotropie instability, heat fluxes, terrain effects, PV-anomalies

Last update : 2011/01/25

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