ARCTIC COLD FRONT

by FMI


UNDER CONSTRUCTION


Arctic Fronts occur frequently over the Arctic Sea, sometimes over Scandinavia and even as far south as the North Sea. An Arctic front separates the cold polar air mass from the even colder air mass of arctic origin.

The Arctic Front is typically a shallow tropospheric feature, with the strongest baroclinicity observed most often in a lowest layer 2 - 3 km above the surface. In the middle and upper troposphere baroclinicity is weak or absent. Normally only a quite extensive cold upper level low pressure area or trough can be found above the Arctic Front. The front usually forms west or north of the primary polar front depression (deep within the polar air mass), often in connection with an elongated Occlusion tail (sometimes referred to as a Bent-Back Occlusion) north or west of the low centre.

The Arctic Front often experiences gradual changes in its intensity as it moves over open sea areas. The strong temperature gradient in the lowest layers at early stages weakens significantly, when heat flux from the sea warms the lower body of the arctic air mass (this can happen also over warm land areas in spring and early summer, though the heat reservoir of soil is much smaller compared to the sea). The pressure trough connected to the front can be followed over a longer period, even though the temperature contrasts across the front weaken. The distribution of convective activity also helps to pinpoint the location of the Arctic Front, as there normally is quite intensive convective cloudiness on the colder side of the front compared to the less intensive convection on the warmer side.

Strong gradients in potential temperature at the 850 hPa level as well as in the 500/1000 hPa thickness field are the most useful parameters when identifying the Arctic Front. In contrast to this the use of fields at higher levels (for instance 500 hPa temperature field or 300 hPa wind field) does not help in detecting the location of the Arctic Front.

The Arctic Front will not normally develop into an Occlusion at a later stage (though this is not straightforward; see the example in the Nordic case study). Sometimes an Arctic Front can change to a polar front when moving southwards but in most cases it will dissappear gradually.

An example of the appearance of the Arctic Front in a satellite image is presented in the Nordic case study of 29 - 30 January 1998.

Weather accompanying the passage of the Arctic Front over open sea shows gusty winds and heavy snowfall mainly due to intensive convection. Over land in wintertime the convection and hence the snowfall is remarkably weaker, even totally absent, the only effect being the remarkable drop in temperature and/or dewpoint.

29 January 1998/01.44 UTC - NOAA CH4 image; red: location of surface low indicated

Weather Events

The weather situation differs over sea from above land: due to relative high sea surface temperature strong convection is quite common above sea.

Over Sea

Parameter Description
Precipitation
  • Moderate to heavy showery snowfall
Temperature
  • Falls rapidly after the passage of the surface front
Wind (incl. gusts)
  • Strong gusts are possible
  • Veering of the wind at the front passage
Other relevant information
  • Cloudiness, quite often convective, in lowest 2 - 3 km of troposphere
  • Convection causes locally big differences in precipitation intensities.
  • Risk of moderate to severe icing and turbulence
  • Poor visibility during heavy snowfall

Over Land

Parameter Description
Precipitation
  • Slight to moderate showery snowfall
Temperature
  • Falls rapidly after the passage of the surface front
Wind (incl. gusts)
  • Strong gusts are possible
  • Veering of the wind at the front passage
Other relevant information
  • Cloudiness, quite often convective, in lowest 2 - 3 km of troposphere
  • In wintertime quite often locally no precipitation (lack of convection)
  • Risk of moderate to severe icing and turbulence
  • Poor visibility during snowfall

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