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Appearance in Satellite Data

Arctic Fronts form in the Arctic region, and move southwards in southerly flows. When they reach Northern Europe, they have usually travelled over an open sea, and convective cloudiness has developed. The appearance of an Arctic Cold Fronts is then, essentially, that of a shallow Cold Front.

Arctic Cold Fronts are usually so far north that Meteosat images alone are inadequate to recognize them. Also, the following conceptual models may look like Arctic Cold Fronts: polar Cold Front, Polar Low and Comma. The final check is best made using a loop of AVHRR images with the help of numerical model parameter fields.


As stated in the Common Remarks, Arctic Cold Fronts can be classified into two types:

  1. Baroclinic fronts
    These fronts resemble polar cold fronts, but are usually not so extensive. The frontal cloudiness becomes more convective with time.
  2. Ice/sea boundary fronts
    These fronts form over the ice/sea boundary and move southwards with the basic flow. There is only an isolated Cold Front. Often this type is so shallow and weak that it can not be detected in Meteosat water vapour images.

Examples of both types are given below.

 

Appearance in Meteosat images

  • light grey to white band in IR and VIS images
  • light grey to grey band in WV images
23 March 2009/12.00 UTC - Meteosat 9 IR10.8 image. The location of the Arctic Front is shown in the images with a frontal line.
23 March 2009/12.00 UTC - Meteosat 9 WV6.2 image
23 March 2009/12.00 UTC - Meteosat 9 VIS0.6 image
23 March 2009/11.29 UTC - NOAA 18 VIS0.6; VIS0.9; IR10.9 RGB image

Another example of an Arctic front over land is show below. The Arctic air mass over the north-western part of Russia can be seen in extremely cold surface temperatures over land , but - unlike for sea cases - the convective cells are absent. Most of the cloudiness associated with the Arctic front is upper level thin Cirrus clouds denoting the place of the baroclinic zone.

05 February 2009/09.00 UTC - Meteosat 9 IR10.8 image. The location of the Arctic Front is shown in the images with a frontal line.
05 February 2009/09.00 UTC - Meteosat 9 WV6.2 image
05 February 2009/09.00 UTC - Meteosat 9 VIS 0.6 image
05 February 2009/09.00 UTC - Meteosat 9 HRV + HRV + IR 10.9 RGB image

Meteorological Physical Background

An Arctic Cold Front is a boundary between arctic and polar air masses. An Arctic air mass forms, when the air above a snow/ice covered surface cools down due to very low solar heating and strong heat emission from the surface. Consequently, it is only found above the polar ice areas. The Arctic Cold Fronts come to Northern Europe only during the winter season, from October to March.

It can be stated that the air mass is arctic, when the 850 hPa temperature is -18°C or lower. The corresponding threshold value for Θe at 850 hPa depends slightly on the humidity of the air mass, but is usually around -4°C. The temperature at 500 hPa is -40°C or lower.

Using this threshold, the image below shows the mean position of the arctic air mass in January.

The mean location of Arctic air mass in January (1971-2000); blue: temperature range 254K to 256K

We can clearly see that the effect of warm sea currents (particularly the Gulf Stream) pushes the arctic air on an average near to the 75th northern latitude, whereas over the cold continental area in North America and Siberia the Arctic air mass extends even to 50thnorthern latitude.

As stated in Common Remarks, the Arctic Cold Fronts are classified into two categories:

  1. Baroclinic front
    • A Wave with a Warm and a Cold Front forms in a thermal gradient zone accompanied by a weak arctic jet found at 700-500 hPa. It can develop into an Occlusion, or disappear.
    • The systems come to Northern Europe within a cold flow, therefore, the Warm Front is very weak and disappears quickly.
    • A Back Bent Occlusion often forms in mature systems.
    • Most arctic fronts are of this type.
    • The difference between Polar and Arctic Cold Fronts:
      • Arctic frontal zones are restricted to the lower troposphere
      • In arctic situations the tropopause is higher than the top of the front, and often difficult to detect
      • There is no jet at 300 hPa over Arctic Fronts
      • With Arctic Fronts the only precipitation is in the form of snow
      • Arctic systems occlude faster
  2. Ice/sea boundary front
    • The front is formed over the boundary between the relatively warm sea and cold ice and is advected south with the basic flow.
    • There is only a Cold Front present.
    • Small scale convective cells form immediately behind the front.

Key Parameters

  • Θe 850 hPa
    The air mass behind an Arctic Cold Front is -4°C (269K) or colder.
  • Temperature Advection 850 hPa
    The Arctic Cold Front is partly or entirely within cold advection.
  • Height Contours 700 hPa
    There is a trough on the top of the arctic frontal zone.
  • PVA 700 hPa
    PVA connected to the upper trough

Θe 850 hPa

23 March 2009/12.00 UTC - Meteosat VIS 0.6 image; magenta: θe 850 hPa. The location of the Arctic Front is shown.

 

Temperature Advection 850 hPa

23 March 2009/12.00 UTC - Meteosat VIS 0.6 image; red dashed: temperature advection - CA 850 hPa, red solid: temperature advection - WA 850 hPa
arctic_front

 

Height Contours 700 hPa

23 March 2009/12.00 UTC - Meteosat VIS 0.6 image; red: height contours 700 hPa

 

PVA 700 hPa

23 March 2009/12.00 UTC - Meteosat VIS 0.6 image; green: PVA 700 hPa

Typical Appearance In Vertical Cross Sections

  • Isentropes
    • A shallow frontal zone in the lower troposphere
    • The tropopause often unclear
  • Temperature advection
    Cold advection within and behind the frontal zone
  • Divergence
    Convergence within the frontal zone and divergence above it
  • Vertical Motion (Omega)
    Ascending motion ahead of the frontal zone and descending motion behind it
23 March 2009/11.29 UTC NOAA 18 VIS0.6; VIS0.9; IR10.9 RGB image; position of vertical cross section indicated

 

Temperature advection

23 March 2009/12 UTC - Vertical cross section; black: isentropes (θe), red thick: temperature advection - WA, red thin: temperature advection - CA

 

Divergence

23 March 2009/12 UTC - Vertical cross section; black: isentropes (θe), magenta thick: convergence, magenta thin: divergence
arctic_front
arctic_front

 

Vertical Motion (Omega)

23 March 2009/12 UTC - Vertical cross section; black: isentropes (θe), cyan thick: vertical motion (omega) - upward motion, cyan thin: vertical motion (omega) - downward motion
arctic_front

Weather Events

The precipitation connected with Arctic Cold Fronts is in the form of snow.

Parameter Description
Precipitation
  • Snowfall
  • Snow showers
Temperature
  • Usually falls after the passage of the front
  • Rise after landfall
Wind (incl. gusts)
  • Veering of the wind at the front passage
  • Gusty winds behind the front
Other relevant information
  • There can be fog ahead of the front near the ice/sea border

23 March 2009/12.00 UTC - Meteosat 9 VIS0.6 image - Weather events
arctic_front

References

General Satellite Meteorology

  • BADER M. J., FORBES G. S., GRANT J. R., LILLEY R. B. E. and WATERS A. J. (1995): Images in weather forecasting - A practical guide for interpreting satellite and radar imagery; Cambridge University Press

Specific Satellite Meteorology

  • AMAP (Arctic Monitoring and Assessment Program) 1998 Oslo, Norway
  • Comet web site http://meted.ucar.edu/
  • PELKONEN A (2002): Arktinen Rintama, Helsingin Yliopisto
  • PELKONEN, A (2004): European forecaster newsletter, Volume, 10, http://www.euroforecaster.org/newsletter10/finland.pdf