Polar depression

A polar depression is a system atmospheric basic pressure of short duration of life, of low diameter but very intense which one finds in water Arctique S and the Antarctic S, in general beyond the polar face.

Characteristics

They received several names: cloud in comma, Mésocyclone (or more correctly Cyclone of Méso-scale), polar swirl of méso-scale, Arctic hurricane, Arctic depression and cyclone of cold air. One will hold this term with the systems giving at least wind forces of blows of winds (35 node S or 17 m/s) and of a variable diameter between 100 and 400 km.

the polar depressions are formed in less than 12 noon when very cold air in altitude invades the edge between the ices and the open sea. They will tend to be dissipated quickly by friction and loss of their source of heat of surface while entering on the coast. This leaves them only one day or two of life in general.

Their development explosive is especially due to the convection which occurs then and which slackens many Latent heat. Orage S can even be overlapping in these systems. Probes released by plane showed that they have a " heart chaud" similar to a tropical Cyclone like a hurricane and the satellite photographs show a standard eye in the same way.

They can give winds of more than 100 km/h close to their center in a zone equivalent to the wall to the eye of a hurricane. They give snow in relatively small quantity but the winds will raise it in Poudrerie, strongly reducing the visibility and giving conditions of blizzard.

Difficulties of description

These systems are very compact and pass in little inhabited areas, it is very difficult to locate them by the conventional reports/ratios of boats and terrestrial weather stations. For example, a polar depression holds very easily inside the Hudson Bay in the north of the Canada and it is very possible not to have any index of the coastal stations on its presence. The sailors of the Arctic seas often announced the passage of intense storms come from nowhere even after the development from telecommunications which could have informed them such conditions weather. They can thus have an significant impact but very localized.

Since the satellite advent of the S weather in the years 1960, their location became current. They are rather frequent in extreme cases of the ices in the Mer of Norway, the Mer of Barents, the Mer of Japan, the Gulf of Alaska, the Hudson Bay and the Détroit of Davis. They disappear during complete freezing of the Banquise. In the southern hemisphere, these systems are generally less strong because the Gradient of temperature is marked less there.

One notices on the photographs several characteristic owners of which arm of clouds in spirals around the center, of the clouds convectifs and an eye without clouds for most intense. Those more in the south have a configuration of the clouds in comma like a mature depression of the average latitudes.

Forecast

The forecast of the development and the trajectory of these depressions is very difficult. Indeed, as they have a reduced scale and are strongly influenced by the convection, the digital model of forecast need a fine resolution to take account of their dynamics. As in the case of the tropical cyclones, they will also tend to move with a circulation on méso-scale which they will have helped to create.

The current operational models, like the Canadian GEM (Weather service of Canada) and the American NAM (NOAA), have resolutions of about 15 to 20 km between each point of grid. They can generate cyclonic systems where polar depressions are found but generally the pressures are much less low than reality and they tend to move them too quickly. The improvement of the resolution and the introduction of a better parameterization of the convection should help in the future.

Polar depression

Characteristics

Difficulties of description

Forecast

See also