Print Version

Table of Contents

Appearance in Satellite Data

Polar Lows are small-scale cyclones in the arctic or polar air mass with some similarity to tropical hurricanes. They occur during the wintertime (October until April) far north of the polar front. Because of their northern position, their small scale and their appearance in the dark winter season, AVHHR-NOAA IR images are often the best way to detect Polar Lows. At latitudes lower than 70 degrees north, Meteosat IR and WV images are also useful for detecting such systems.
This Conceptual Model is based on existing literature and the investigations of 25 cases, most of them in the winter seasons of 1999-2000 and 2000-2001.
Three different phases in the life cycle of a Polar Low can be distinguished: the initial/development phase, the mature phase and the decaying phase.

  • A developing Polar Low has the strucutre of a cyclonic curl in a polar or arctic airmass far away from the main polar frontal zone. Upstream of the cloud band, Cold Air Cloudiness (CAC) with relatively low tops (dark in IR) is present. The inner part of the curl shows sharp cloud edges while the outer part is often capped by cirrus cloud.
  • The cloud structure of a mature Polar Low is often a pronounced vortex with a (partly) cloud free eye. This vortex consists of Cbs. The deepest Cbs with the brightest cloud tops in the IR images are located around the eye.
    A cirrus shield often partly covers small-scale Polar Lows.
    Clouds further away from the centre tend to be less bright which means lower cloud tops, caused by descending air motion.
  • A decaying Polar Low is indicated by the disappearance of the cirrus cloud and the eye feature, lower cloud tops (less bright in IR images) and the disruption of the vortex. Most Polar Lows will decay as a result of landfall.

Developing Phase
Mature Phase
Decaying Phase

22 February 2001/21.00 UTC - Meteosat IR image; Developing Polar Low near the Faroe Islands; 22/21.00 UTC - 23/12.00 UTC 3-hourly image loop; developing - mature phase
23 February 2001/09.00 UTC - Meteosat IR image; Mature Polar Low south east of Scotland
22 February 2001/21.00 UTC - Meteosat WV image; Developing Polar Low near the Faroe Islands; 22/21.00 UTC - 23/12.00 UTC 3-hourly image loop; developing - mature phase
23 February 2001/09.00 UTC - Meteosat WV image; Mature Polar Low south east of Scotland

If the scale of a Polar Low is not too small and the position is not too far to the north, it is even possible to trace Polar Lows with Meteosat images. In the images above, a Polar Low is developing just east of the Faroe Islands. The first IR images shows the development of a Polar Low in a band of cloudiness while a developed vortex is already present in the water vapour image. In the morning of 23 February the Polar Low reaches its mature phase over Scotland. The IR image shows a lot of Cbs encircling the center. In the water vapour image the vortex is still present.

Appearance in AVHRR imagery

As Polar Lows are phenomena which frequently occur in the dark winter season, the use of combined VIS-IR images is normally not possible, however, single channel NOAA IR images (e.g. Ch 4) are most appropriate for the detection of Polar Lows.

26 February 1987/04.28 UTC - NOAA CH4 image; Developing polar low north of Norway
27 February 1987/04.18 UTC - NOAA CH4 image; Mature polar low north of Norway
27 February 1987/12.32 UTC - NOAA CH4 image; Decaying polar low over Norway

The satellite images above show the three phases in the life cycles of a typical Polar Low. The development starts on an old occluded front extending from northwest Finland to the Southeast of Svalbard. A clear curl develops at the most northern part of the cloud band. The inner part of the curl shows sharp cloud edges while the outer part is blurred. West and north of the cloud band, Cold Air Cloudiness (CAC) is present.
A few hours later a mature Polar Low is positioned just north of Norway with a clear eye and numerous Cbs encircling the center. An area of clouds with low tops, as a result of descending air, surrounds these Cbs.
Finally, having passed across the north coast of Norway, the Polar Low starts to lose its well-organized structure. In the last satellite image, only some unstructured Enhanced Cumuli are left over north Norway and Sweden.

Meteorological Physical Background

A Polar Low is a meso-scale cyclone with a warm core only existing in a cold air mass at quite a distance from the polar front. They occur in the winter period between October and May. In its mature phase the surface winds are near or above gale force.

The reason why a Polar Low develops within a small baroclinic disturbance in a potentially unstable environment in northern regions can be explained with the Rossby radius of deformation: R~N0H/f. In this equation, f is the coriolis parameter, N0 is the stability parameter, H is the scale height and R is the minimum scale of a system to be dynamically stable. The saturated-adiabatic lapse rate (small N0), the northern position (large f) and very cold air mass (small H), result in an R being significantly smaller in a Polar Low environment than in an environment of an extra tropical cyclone.

In the lifecycle of a Polar Low three different phases can be distinguished: the initial or developing phase, the mature phase and the decaying phase. In the developing or initial phase baroclinicity and upper level triggering by positive vorticity advection (PVA) and potential vorticity (PV) play an important role. In the mature phase convection is often the driving force.

Developing/initial phase

Baroclinicity

In most cases a Polar Low develops on a secondary shallow baroclinic zone in a polar or arctic air mass far away from the polar front. This baroclinic zone can have different origins. The zone could be a border between air from ice fields and maritime polar air (Arctic Front), or the remnants of an Occlusion. The enhanced cloudband, indicating the baroclinic zone, is a result of both positive vorticity advection and advection of warm air. Normally this zone can be visualised in the potential equivalent temperature (ThetaE) contours at 850 hPa. The potential wet bulb temperature (ThetaW) shows a similar pattern.
Very cold air overlays the baroclinic zone, resulting in a (potential) unstable atmosphere.

02 March 2001/06.00 UTC - NOAA Ch4 image; magenta: ThetaW 850 hPa

In the example above a polar low is developing west of Scotland in a gradient zone of ThetaW.

Curvature Vorticity Advection

A Polar Low normally develops within a surface trough and in front of an upper trough within the cold air mass behind a major depression or Cold Front. Positive vorticity advection (PVA) plays an important role in the spin up of the low. The upper level trough overruns the band of enhanced cloudiness and causes this PVA. In many cases this process happens at a trough of a large scale decaying Upper Level Low (ULL) which is often already accompanied by a low at the surface. Therefore curvature vorticity advection also plays a dominant role.

05 February 2001/18.00 UTC - Meteosat IR image; green: positive vorticity advection (PVA), cyan: height contours 500 hPa

Potential Vorticity

Another triggering mechanism, although closely related to vorticity advection, is the advection of potential vorticity (PV). The baroclinic band can be regarded as a low level PV anomaly. If an upper level anomaly of high potential vorticity values overruns the baroclinic band or low level PV anomaly, both upper and lower level PV maximum will reinforce each other and start a spin-up process. This spin-up process, however, will only be possible if the cyclonic flow induced by the upper level PV anomaly can penetrate sufficiently deep down into the troposphere (see Introduction chapter - Additional parameters and helpful tools for the diagnosis of cloudiness: Potential vorticity ).

Potential vorticity triggering a Polar Low
24 December 1995/02.17 NOAA CH4 image
24 December 1995/12.12 NOAA CH4 image
25 December 1995/07.32 NOAA CH4 image
26 December 1995/01.55 NOAA CH4 image

An example of a Polar Low induced by high PV values occurred during Christmas 1995. On 24 December 06.00 UTC a baroclinic band (not shown in the sequence of the above images) was positioned between Scotland and Norway with cold arctic air to the north and polar air in the south. Before the development started, a weak cyclone with a low level PV maximum was already present just north of Scotland. During the development period an upper level maximum of PV high in the troposphere shifted southward causing positive PV advection above the weak cyclone. Interaction between the two maxima took place triggering the formation of a Polar Low. On 25 December 06.00 UTC the Polar Low reached its mature state. At this moment the upper level PV maximum passed the centre of the polar low moving to the south. Eighteen hours later the polar low passed the coastline of Germany and started to decay.

Mature phase

Conditional Instability of the Second Kind (CISK)

As a result of an unstable or potentially unstable atmosphere, deep convection starts in the developing phase of a Polar Low. This convection is fed by strong latent and sensible heat fluxes resulting from a large difference between air and sea surface temperature and due to high wind speed.
The latent heat release from convection and the Ekman pumping due to a low level cyclonic circulation result, according to the CISK-theory (see Comma ), in a positive feedback mechanism resulting in a deepening of the Polar Low.

Warm core/eye formation

A striking feature in the mature stage of a Polar Low is the formation of a warm core. In some cases (see Appearance in Satellite Data) the inner part of the warm core is cloudless resembling a hurricane eye.
There are two main mechanisms responsible for the formation of a warm core.

  1. Relatively warm air is transported to the centre of the Polar Low and finally cut off from the main flow.
  2. A warm core can be formed by convection. Enhanced surface winds cause enhanced latent and sensible heat fluxes. This heat will be transported aloft by convection. As already shown, the redistribution of heat by convection is important in the intensification of a Polar Low.

Very low surface pressures at the centre, sinking air and high windspeed in the surrounding wall of cloud all chracterize the eye of a tropical hurricane. Although satellite images sometimes show eyes or eye-like features in Polar Low formations, so far there is no evidence of exceptionally low surface pressure values or extremely high windspeed around the central region.
To evaluate the potential for eye formation in a Polar Low, it is necessary to consider the radiosonde ascent. If for example an individual updraft is followed by descending upper level air in the potential eye region, an eye will only form if the potential wet bulb temperature (ThetaW) of the descending air is considerably higher than the ThetaE of the air involved in the updraft.
A warm eye would form if high ThetaW surface air, after ascending in the eye wall, would start to subside in the (interior) eye region (figure below). Even in this case, however, only a relatively small increase in the mean temperature of the column could be expected in the eye of a Polar Low. This is because of the low moisture content of the cold air resulting in relatively small temperature changes as result of the release of latent heat. Therefore Polar Lows only form eyes under special conditions.

polar_low

Decaying phase

Polar Lows usually start to decay after landfall or "icefall". Their central surface pressure starts to increase and their strong wind fields disappear. Three effects are mainly responsible for this decay:

  • Reduction in evaporation
    An important source of energy is the evaporation of seawater. After landfall this source disappears resulting in a decay of the Polar Low.
  • Reduction in sensible heat flux
    During wintertime the land surface normally is colder than the sea surface. Therefore the sensible heat flux will be reduced after landfall of a Polar Low.
  • Increase in surface roughness
    The roughness of the land surface is greater than the roughness of the sea surface. Therefore, when a Polar Low reaches land, this increase in roughness results in enhanced surface convergence. The net inflow results in increased surface pressure, assuming a weaker adjustment of upper level divergence.

Polar Lows associated with strong baroclinicity are not necessarily dependent on energy sources such as sensible and latent heat fluxes. Therefore, these Polar Lows may not necessarily decay after landfall. Baroclinic Polar Lows normally start to decay when negative dynamic forcing mechanisms like cold air advection or negative vorticity advection start to play a dominant role.

Special phenomenon: Reverse-shear Polar Lows

Some Polar Lows are called reverse-shear Polar Lows. The wind at the steering level is light and generally opposite to the thermal wind. The Polar Low is located where the thermal wind advects positive vorticity.

polar_low

Key Parameters

In this chapter statistical analyses of most of the key parameters are included. These analyses are based on approximately 25 cases. Each graphic shows the average, standard deviation and total distribution of a key parameter in the developing (dev.), mature (mat.) and decaying (dec.) phase.

  • 1000 hPa and 500 hPa topography
    A Polar Low normally starts to develop in the synoptic situation where there is a surface trough ahead of a trough at 500 hPa height contours. In its mature phase a closed circular isobaric pattern is often present, although if the background flow is too strong, no closed circular isobaric pattern will develop.
  • Surface wind speed (> 27 knots/14 m/s or >= 7 bft)
    By definition Polar Lows are accompanied by fields of strong winds with velocities exceeding 27kt/14 m/s. The strongest winds are normally to be found at the position where the relative motion of the polar low is in the same direction as the actual wind.
  • Positive Relative Vorticity Advection (PVA) at 500 hPa
    A maximum in PVA at 500 hPa is often superimposed over the area of an initial Polar Low ahead of a trough at 500 hPa. The statistical analysis shows the highest value of PVA normally occurs in the developing phase and gradually decreases in the following phases. Finally in the decaying phase the value is almost zero or may even be negative.
  • Potential Vorticity
    A maximum of PV is sometimes an important feature in the development of a Polar Low. The maximum value of PV at a suitable isentropic level is normally larger than 5 PVU and covers a small area. During the developing phase this area is situated upstream of the developing Polar Low.
  • Temperature at 500 hPa (T500 < -40°C)
    Deep convection is indicative of a Polar Low. This deep convection is caused by very cold arctic air overrunning relatively warm sea. At 500 hPa this air more or less keeps its original temperature. From the statistical analysis and the literature a temperature of -40°C or less at 500 hPa (T500=< 40°C) is a good threshold value for the triggering of Polar Low development. An other (even better) parameter for the indication of deep convection is the temperature difference between the sea surface temperature and the temperature at 500hPa. The threshold value for this is 44°C.
  • Instability index - Boyden
    From statistical analysis it follows that a useful stability parameter here is the Boyden index. Although this index is not developed for high latitudes and the winter season almost all cases show a Boyden index higher than 94 in the region of a (potential) Polar Low. This threshold value indicates a high chance of heavy showers.
  • Vertical Motion (Omega) 850 hPa
    Because of unstable conditions and upper level forcing mechanisms, vigorous ascending motion could develop. As a result of a low tropopause, the ascending motion is present in a relatively shallow layer, making omega at 850 hPa more indicative than omega at 500 hPa.
    During the development phase the enhanced cloud band is characterized by a negative omega with a relative minimum at the position of the developing Polar Low. This minimum expands and finally, in the mature stage, a cut-off minimum is present surrounded by a band of descending air with positive omega values.

1000 hPa and 500 hPa topography

Initial phase:

23 February 2001/00.00 UTC - Meteosat IR image; magenta: surface isobars, blue: height contours 500 hPa

Mature phase:

23 February 2001/05.54 UTC - NOAA CH4 image; magenta: surface isobars, blue: height contours 500 hPa

Surface wind speed (> 27 knots/14 m/s or >= 7 bft)

23 February 2001/05.54 UTC - NOAA CH4 image; magenta: surface isotachs in knots. A mature polar low north of Scotland

Positive vorticity advection (PVA) at 500 hPa

05 February 2001/18.00 UTC - Meteosat IR image; green: positive vorticity advection (PVA) 500 hPa, blue: height contours 500 hPa. A developing Polar Low west of Norway
Statistical analysis PVA 500 hPa

Potential Vorticity

23 February 2001/05.54 UTC - NOAA CH4 image; magenta: potential vorticity 303K > 2 PV units. A developing Polar Low just north-east of Scotland

Temperature at 500 hPa (T500 < -40°C)

23 February 2001/12.00 UTC - Meteosat IR image; red: temperature at 500hPa. A mature Polar Low east of Scotland
Statistical analysis temperature 500 hPa

Vertical Motion (Omega) 850 hPa

05 February 2001/18.00 UTC - Meteosat IR image; yellow: vertical motion (omega) 850 hPa. Developing Polar Low west of Norway
06 February 2001/06.00 UTC - Meteosat IR image; yellow: vertical motion (omega) 850 hPa. Mature Polar Low west of Norway
Statistical analysis vertical motion (Omega) 850 hPa

Typical Appearance In Vertical Cross Sections

Although the Polar Low is a small-scale system, the typical polar environment and main dynamic forcing mechanisms can be observed in a vertical cross section.

19 March 2001/16.50 UTC - NOAA CH4 image; position of vertical cross section indicated. A developing polar low south west of Svalbard

  • ThetaW
    During the developing phase a baroclinc zone in the potential wet bulb temperature (ThetaW) pattern is visible. Upstream of the Polar Low a shallow layer of cold air is pouring out over the sea.
    This air is warmed up gradually by the relatively warm seawater, resulting in higher ThetaW values.
  • Vorticity advection
    One of the main dynamical mechanisms in the initial or developing phase is Positive Vorticity Advection (PVA) caused by a moving upper level through. To trigger a Polar Low, a PVA maximum has to be superimposed upon the baroclinic zone.
  • Potential vorticity
    Potential Vorticity (PV) is an important parameter in the develoment phase of a Polar Low. In this stage an upper level PV maximum is normally just upstream of a Polar Low.
    This PV maximum is situated at mid levels of the troposphere but has transferred down from the stratosphere.
    A PV maximum can also be found in the gradient zone of the very stable layer of ThetaW near the surface over or near icefields.
  • Vertical Motion (Omega)
    Upper level mechanisms result in negative omega. Negative omega above the Polar Low centre does not cover a deep layer because of a low tropopause and a stable upper layer.

ThetaW

19 March 2001/18.00 UTC - Vertical cross section; black: isentropes (ThetaW)

Vorticity advection

19 March 2001/18.00 UTC - Vertical cross section; green: vorticity advection - PVA, black: isentropes (ThetaW)

Potential Vorticity

19 March 2001/18.00 UTC - Vertical cross section; green: potential vorticity, black: isentropes (ThetaW)

Vertical Motion (Omega)

19 March 2001/18.00 UTC - Vertical cross section; cyan: vertical motion (omega), black: isentropes (ThetaW)

Mature Phase

20 March 2001/23.30 UTC - Meteosat IR image; position of vertical cross section indicated

Vertical Motion (Omega)

In the mature phase, a deep layer of negative omega is surrounded by areas of positive omega. Although the ascending air motions are sometimes situated in an area of PVA and some warm advection, this negative omega can not be completely explained by these mechanisms alone. Convective instability is playing an important role too. The descending air motion to the rear of a Polar Low is often enhanced by cold air advection. Because of the relative small resolution of most operational models compared to the scale of a Polar Low, descending air motion at the centre of a Polar Low is difficult or impossible to see in a vertical cross section.

21 March 2001/00.00 UTC - Vertical cross section; cyan: vertical motion (omega), black: isentropes (ThetaW)

Weather Events

Polar Lows are mesoscale weather systems. During their life cycle they produce severe weather with strong surface winds and locally heavy precipitation. Due to the very cold environment of a Polar Low most of the precipitation is falling as snow. During landfall convective Polar Lows only produce significant precipitation in the coastal area.

Some Polar Lows show a small-scale baroclinic zone resulting in a more frontal type of precipitation. These Polar Lows will shift farther inland producing hazardous weather conditions with heavy snowfall and strong wind gusts.


Parameter Description
Precipitation In developing phase: widespread light to moderated showers
In mature phase: heavy (snow) showers in most cases
Temperature Around freezing point but in centre slightly warmer. Strong decrease in temperature directly after passage of the centre.
Wind Mean wind > 27 kts/14 m/s or >=7 bft. Strong windgusts. Max wind occurs where relative motion of the Polar Low is in same direction as the wind direction.
Other relevant information Thunderstoms or even small tornadoes are possible.
The heaviest showers are located close to the centre. Occasionally these showers are thunderstorms with strong wind gusts. Even some small tornadoes are observed.
High risk of icing over sea in case of low sea surface temperatures. Blizzard conditions over land.


21 March 2001/06.00 UTC - Meteosat IR image; Weather events in a Polar Low near the west coast of Norway

Icing on ships

Normally Polar Lows reach their mature stage over sea. Therefore ships have to be aware of the hazardous weather conditions. One of the risks small boats have to be aware of is icing in location with very low sea surface temperatures (mostly north of 70 degrees latitude). Because of the strong winds and the low air temperature, spray may freeze on the superstructure of a boat. The weight of accumulated ice may eventually become a considerable hazard.
The degree of icing depends both on temperature and wind speed, as shown in the figure below.

Degree of icing with a wind force of 8-9 bft. (34 kt/17 m/s - 47 kt/24 m/s)

References

General Meteorology and Basics

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

  • AAKJAER, P. D. (1992): Tellus 44a, p155-172 - Polar lows affecting Denmark; Munksgaard Int. Publ. LTD, Copenhagen.
  • BLIER W. R., J. REED (1986) Monthly weather review. - Vol. 114, no. 9, p1696-1708 further study of comma cloud development in the eastern Pacific
  • BUSINGER S., REED R.J. (1985) Tellus Vol. 73A, no 5 p419-432 - The synoptic climatology of polar low outbreaks; Munksgaard Int. Publ. LTD, Copenhagen.
  • DELDEN A. van (1996) - Meteorologica (1) - Leven van een polar low; Nederlandse Vereniging van BeroepsMeteorologen, Wageningen
  • NIELSEN N.W. (1997) Journal of geophysical Research-Atmophere An early-autumn polar low formation over the Norwegian Sea; Amer Geophysical Union, Washington.
  • RASMUSSEN E. A. (1989) Polar and arctic lows p47-80 - A comparative stud y of tropical cyclones and polar lows; Hampton DEEPAK Publishing
  • RASMUSSEN E. A. (1992): Tellus 44a, (2) p81-99 - A most beautiful polar low - A case-study of a polar low development in the Bear-Island region; Munksgaard Int. Publ. LTD, Copenhagen.