1.2.1 The Polar Low

A polar low over the Barents Sea on 27 February 1987. The southern tip of Spitsbergen is visible at the top of the image with Norway in the lower right.

Nordeng and Rasmussen (1992) called this particular polar low "A Most Beautiful Polar Low." It shows the classic features of a true polar low:

The European Polar Lows Working Group (EPLWG) published the following definition of a polar low at their June 1994 meeting in Paris:

"The term 'polar mesoscale cyclone' is the generic term for all meso-alpha (200 - 2000 km) and meso-beta (20 - 200 km) cyclonic vortices poleward of the main polar front. The term 'polar low' should be used for intense maritime polar mesoscale cyclones with scales up to 1000 km with a near surface wind exceeding 15 m/sec (30 knots)."

A more up-to-date definition by Rasmussen and Turner (2003) states:

"A polar low is a small, but fairly intense maritime cyclone that forms poleward of the main baroclinic zone (the polar front or other major baroclinic zone). The horizontal scale of the polar low is approximately between 200 and 1000 kilometres and surface winds near or above gale force."

Rasmussen (1994) suggested classifying polar lows into primary and secondary types. The primary type are those initiated as the result of a cold core vortex. Those associated with all other vortices that fit the definition of a polar low belong in the secondary category.

The majority of polar lows seen in Canadian waters fall into the primary category. However, there are some vortices in which baroclinic processes appear more important than convection during the mature phase, and these would fall into the secondary category. Some of the vortices forming along the ice edge of southern Davis Strait appear to fall in this secondary category.

European cases are often a result of pre-existing baroclinic effects and are therefore of the secondary category. Occlusions from mature synoptic-scale lows are a favoured mechanism between the British Isles and southern Greenland, while primary polar lows are generally confined to the area north of Iceland across to the Barents Sea.

1.2.2 Examples of Polar Lows

Here are two examples of polar lows in the Labrador Sea and the Gulf of Alaska.

Examples of Polar Lows using NOAA-AVHRR IR (11µm)

The cloud pattern accompanying the Labrador Sea polar low is common for systems developing in the northwestern portions of the Labrador Sea. In their paper "Labrador Sea Polar Lows," Rasmussen et al. (1996) studied a number of Labrador Sea polar lows, all of which tended to form in the same general area and develop characteristic cloud patterns. The main features are the cloud bands northward and eastward from the centre. The eastward band or tail frequently marks a boundary between colder arctic air to the north and modified arctic air flowing eastward on the southern side of the system.

1.2.3 The Comma Cloud

The term "comma cloud" is used to describe a variety of comma-shaped cloud systems both over water and land. In this discussion it is used to describe comma-shaped cloud patterns poleward of the northernmost jet stream.

Relationship of comma cloud to major frontal cyclone
with overlay   hide overlay

Zick (1983) shows the typical relationship between a comma cloud and a major frontal cyclone. Here, the satellite imagery is of the northeast Pacific with the comma cloud positioned west of a main frontal zone that is visible over the Alaskan Panhandle, British Columbia, and Yukon

Mullen (1983) refers to an earlier report by Anderson (1969) in which he first pointed out that a cold air comma cloud is:

Businger and Reed (1989) add that the comma cloud typically begins as a region of enhanced convection and geographically is most likely to be observed over the western oceans in areas where cold air is being advected over warmer water surfaces.

Reed (1979) makes the following comments about comma clouds:

1.2.4 The Instant Occlusion

GOES-W IR 06 Jan 1981 showing an instant occlusion over the Gulf of Alaska
(click to view animation in separate browser window - 489 kb)

If the distance between the comma cloud and frontal wave becomes small enough, the two features may combine with the resultant cloud closely resembling that of a mature occluded cyclone (Mullen, 1983). This time series over the Gulf of Alaska at 1845 UTC shows a comma cloud in the cold air mass north of the main frontal band. Nine hours later, at 0545 UTC, there is an indication of a weak wave developing on the main front. The comma cloud then proceeds to merge with the main front over the next 24 hours.

This process represents explosive cyclogenesis on a scale much larger than polar low vortices.

1.2.5 Reverse-Shear Flow

Vortices and polar lows are often observed to form in an area of reverse shear. Reverse shear refers to the situation in which the storm motion is in the opposite direction to the thermal wind. This is unlike the situation that prevails with the comma cloud type, which propagates in the direction of thermal wind (Businger and Reed, 1989). Duncan (1978) was the first to identify the reverse-shear case and showed that baroclinic development is favoured in regions of reversed shear.

Comparison of reverse and forward shear flow in the development of polar lows.

In forward shear the colder air lies to the left of an observer positioned with his/her back to the wind. The thermal wind is positive and the actual wind increases with height. In a reverse-shear flow, warmer air lies to the left of an observer facing downwind. This produces a negative thermal wind, and the actual wind will decrease with height.

In both cases, the upward motion and comma-shaped cloud pattern are located where the thermal wind advects positive vorticity (i.e., down-shear of the trough). Because of the opposite relationships between thermal winds and steering winds, the cloud system lies ahead of the trough in the case of forward-shear and to its rear in the case of reverse-shear flow.

Reed and Duncan (1987) followed a series of disturbances that formed during a two-day period in January 1983. The disturbances were spaced about 500-600 km apart and moved southwestward at 10 knots. These lows were steered southwestward in conformity with the low-level winds. These winds were strongest near the surface where the thermal gradient was also strongest.

Norwegian Sea Reverse Shear Polar Lows, 30 Jan 1983
1000 hPa  850 hPa 700 hPa 500 hPa

From the orientation of the isotherms on the 1000-hPa and 850-hPa charts, it is apparent that the thermal wind is directed opposite to the low-level flow. Note that at these heights, the colder air lies to the right of an observer facing downwind, and the actual wind decreases with height.

 

Example of Reverse Shear Flow in the Davis Strait

Conditions for reverse-shear flow are not uncommon in the northern Labrador Sea and Davis Strait. When synoptic-scale lows pass south of Greenland they often develop a surface trough northward along the west Greenland coast. During the winter months this can produce a situation where the surface flow is closely oriented to the ice surface. This aids in the formation of a low-level baroclinic zone along the ice edge. Businger and Reed (1989) and Rasmussen (1994) state that these boundary-layer fronts, separating modified and unmodified air, appear to play a significant role in the development of baroclinic vortices. Although many remain weak circulations, others may develop into polar lows if they receive significant upper forcing.

A similar reverse-shear situation can be found at times north and east of Iceland. Here, arctic air flow at the surface is opposite to the direction of the thermal wind in the lower atmosphere. Any disturbance at the lower levels, including polar lows, tends to move slowly. If the pattern is persistent, then several shallow disturbances can develop over the course of a few days.

1.2.6 Shear Vortices and Lines of Enhanced Convection

A uniform flow of cold air is seldom sufficient and perhaps never sufficient by itself for development of true polar lows. Øakland (1989) says that for convection to be the energy source there must be spatial variations. The presence of a capping inversion is also necessary as it prevents deep convection from becoming widespread. The deep convection is then confined to or focused on the area immediately beneath the forcing mechanism.

This series of vortices over the Labrador Sea formed along a frontal zone. This frontal zone separates cold arctic air streaming southward behind a synoptic system southeast of Greenland and a warmer air mass which has moved around this low and lies in a surface trough extending from the low up the west Greenland coast.

This series of vortices over the Labrador Sea formed along a frontal zone. This frontal zone separates cold arctic air streaming southward behind a synoptic system southeast of Greenland from a warmer air mass that has moved around this low and lies in a surface trough extending from the low upward along the west Greenland coast.

Satellite IR imagery showing streamers of cumulus clouds as a result of cold air flowing over the Labrador Sea

This image shows cold air streaming across the Labrador Sea behind a major synoptic system that has moved east of Greenland. The change from shallow to deeper convection is obvious as the air moves further from the ice edge and the cloud pattern becomes open cellular. The flow appears fairly uniform. But an area of weak convergence and enhanced convection is evident downstream of the arrows. Interestingly, this area corresponds to the section of the Labrador Sea that experiences the highest frequency of polar low development. The cases by Rasmussen et al. (1996), Moore et al. (1996), and Mailhot et al. (1996) all formed in this general area.

Rasmussen et al. (1996) refer to a low-level jet that often develops because of the flow through Hudson Strait and extends over the Labrador Sea. This feature is often visible as streamers emanating from the adjoining capes. Small vortices are occasionally observed along the streamers.

Rasmussen (1996) also mentions the effect that the Torngat Mountains may play. Being a relatively high range, cold air over Labrador will have a tendency to flow around this feature. The flow south of the mountains is nearly westerly. The flow in the area to the north is northwesterly. The enhanced convection appears to be the result of this geographicaly induced convergence.

1.2.7 Vortices and Convergence Lines

Another area for the formation of vortices is along a boundary layer front formed due to the convergence of air streams with different over-water trajectories. For example, the eastward extending tail of the Labrador Sea polar low in this image lies along a frontal zone resulting from the convergence of two arctic air masses with different over-water trajectories.

NOAA AHHRR IR 20 March 1989 showing a polar low in the Labrador Sea

Northern European cases are often preceded by the presence of convergence lines or isolated baroclinic zones that give rise to an organized pattern of convection. Vortices can sometimes be seen at the boundary between air from different origins. Usually, a line of bright cloud tops is observed, as in this example.

References

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Duncan, C.N., 1978: Baroclinic instability in a reversed shear flow. Meteor. Mag. 107, 17-23

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Rasmussen, Erik A.and Anette Cederskov, 1994: Polar lows: A critical analysis. The Life Cycles of Extratropical Cyclones, Vol. 111. Proc. of an International Symposium, Bergan, Norway.

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Zick, 1983: Method and results of an analysis of comma cloud developments by means of vorticity fields from upper tropospheric satellite wind data. Meteor. Rundsch. 36, 69-84.