TWO POLAR LOWS AFFECTING DENMARK* By ERIK A. RASMUSSEN’ and PETER D. AAKJWR’ ‘Geophysical Institute, University of Copenhagen

2Danish Meteorological Institute

NE of the basic characteristics of polar lows is that their horizontal extent is relatively 0 small. They may be rather intense, and they form in airmasses poleward of the main baroclinic zone (the polar front). With this ‘definition’ in mind the past two decades of research have shown that there exists a variety of types, or a ‘spectrum’, of polar lows, and that these form occasionally in a very complex way. The spectrum ranges from ‘real’ polar lows forming well within the cold airmass, where convection is a crucial factor in the forma- tion, to baroclinic polar lows including the so-called comma clouds.

The present paper is based on two earlier papers in Vejret: Rasmussen and Aakjzer (1985) and Rasmussen et al. (1988). In the first paper we have discussed the formation of a ‘baroclinic polar low’ and in the second a frequent mechanism for the initial formation of polar lows in north-westem Europe.

Fig. I N O A A ~ visible satellite image or 0641 GMTOn 29 March 1985, showing the baroclinicpolar low (black arrow) over southern sfweden. The track of thesnow le by the olarlow overJutland

main baroclinic zone is indicated by the dash-dotted line. (Courtesy of Danish Meteorological Institute.)

is shown by the white arrow. The northern limit of a deck of hig R P -level c ouds belonging to the

*This article was first published in Vejret, 1989, and is reproduced with the permission of the Danish Meteorological Society.

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A BAROCLINIC POLAR LOW

The ‘Easter storm’ discussed by Rasmussen and Aakjaer (1985) was formed from a rather insignificant disturbance and developed into an intense small-scale low during its passage over Denmark early on 29 March 1985. When the disturbance passed Copenhagen the winds were of gale force, the visibility near zero due to heavy snowfall and blowing snow, and thunder and lightning were observed. A satellite image from that time (Fig. 1) shows that although the horizontal scale of the disturbance was small it had nevertheless a cloud structure similar to that of a large baroclinic wave. Well defined fronts could be analysed on the surface chart for 0 6 0 0 G ~ ~ , i.e. the time when the centre of the system was observed in the Copenhagen region (Fig. 2), and there is no doubt that from the very start the system was of a baroclinic nature. On the other hand, the small scale, the intensity of the low, and the fact that it formed well into the polar airmass warrant that it should be defined as a polar low, i.e. a baroclinic polar low.

Although such baroclinic polar lows are relatively common in the North Sea region, there have been very few studies of their formation and structure.

A satellite imagery-based interpretation The initial formation of the low can be traced back to the region near Iceland on the

day before, i.e. on 28 March 1985. The surface chart for OOOOGMT (Fig. 3) shows a ‘classical’ frontogeneticzone stretching from southern Greenland to Ireland. The diffluent flow around Iceland can be observed on the satellite image (Fig. 4). It is clearly seen how Iceland blocks the northerly flow and that a cyclonic circulation has formed in the lee of the island. Such lee cyclones south of Iceland are quite common in a general north to north-easterly flow, and

Fig. 2 Su acechart(mbar) for0600G~~on29March 1985showingthepolarlowcentredjustsouth-

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east ojfCopenhagen

Fig. 3 Surface chart for oo00 GMT on 28 March 1985. The incipient disturbance south of Iceland is shown by an arrow. (Courtesy of the Meteorological Ofice.)

Fig. 4 NOAA-Y infra-red satellite image a? 0331 GMT on 28 March 1985 (courtesy of University of Dundee)

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Fig. 5 850mbar wet-bulb potential temperature at WGMT on 28 March 1985 (courtesy of the Meteorological Ofice)

in this case a cyclonic circulation was already observed in the region on the 26th. After midnight, between the 27th and 28th, the low begins to move southeast.

On the satellite image (Fig. 4) the cloud streets (shown by arrows) indicate that the low- level air in the region around Iceland originates in the pack-ice near the coast of Greenland. Theadvectionofairfrom thisverycoldregion towards thesouthcontributes totheformation of a strong low-level baroclinic zone as seen in Fig. 5 which shows the wet-bulb potential temperature distribution at 850mbar at O~OOGMT on 28 March.

The satellite image of the incipient low in Fig. 6 shows some interesting wave patterns in the clouds, which look very like a horse’s mane. Striated clouds are quite often observed on satellite images, but the physical cause of the striation is not always clear. In the present case the striations seem to be caused by gravity-wave activity. The waves were observed on two more images on 28 March for 0505 and 0 7 0 5 ~ ~ ~ . From the pictures we can estimate the wavelength to be 15-20km with up to 40 waves in a wave train of a total length of 50& 8OOkm. The wave pattern moves with an overall speed of 18-20ms-’, which is comparable with the speed of the low, and from the temperature calibration the cloud tops can be estimated at -40 to -45°C. From a sounding it is seen that this corresponds to a height of 4.5-5.Okm. Using the model by Lalas and Einaudi (1976) and radiosonde data from the Faeroe Isles from 1200Gm on 28 March 1985, Aakjax (1987) concluded that the waves seen on the three satellite images most probably were gravity waves formed below an upper-level jet around a critical level of maximum wind shear. The height of the critical level was in good agreement with the observed height of the clouds and the computed wave lengths (around 15 km) were in good accordance with the observed ones.

Around noon on the 28th the disturbance was situated a little west of Scotland. A satellite image from that time (not reproduced) shows that the cloud system looks rather disorganised. Six hours later, however, a satellite image (Fig. 7) shows that the cloud deck has become much more organised and now resembles what has been called a ‘baroclinic leaf (Weldon 1979). Weldon describes a baroclinic leaf as ‘a cloud system associated with frontogenesis aloft within a westerly wind field‘, and points out that this cloud pattern is associated with the ‘frontogenetic’ phase of development, whereas the ‘comma pattern’ is the ‘cyclogenetic’ phase. (There is some discrepancy among meteorologists about the term ‘comma’ or ‘comma cloud’. Originally the term was coined for cloud fields associated with regions of positive vorticity advection ahead of upper-level troughs of a horizontal scale of about 1OOOkm. In recent years the tern has been used in a more indiscriminate way for phenomena ranging from the mesoscale up to and including synoptic-scale cyclones. In this

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Fig. 6 N O A A ~ infra-red satellite image for 0 8 1 8 6 ~ ~ on 28 March 1985. The ‘horse manes’ are indicated by arrows. (Courtesy of University of Dundee.)

Fig. 7 N O A A - ~ infra-red satellite ima e for 1 8 4 2 6 ~ on 28 March 198s. The concave art of the baroclinic lea is situated justsout8of Norway. The ‘slot’ is indicated by an arrow. (Eourtesy of University o/Dundee.)

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Fig. 8 NOAA-9 infra-red satellite ima e for O3ZJGMOn 29 March 1985. The black arrow shows the comma head and the open arrow t ie comma tail. (Courtesy of Mktkorologie Nationale-Lannion, France.)

article we will, following Weldon, use the term for a cloud structure which loosely resembles a comma with a ‘stratocumulus-shaped‘, relatively well defined, back border.) The leaf as seeninFig. 7isinitstransitionphasefrom ‘leaf to ‘comma’. Theconcave part ofthenorthern border to the west is very ragged and a ‘slot’ has begun to form in the region of deep cold clouds. As the ‘tail’ consisting of the ragged clouds to the west narrows and the slot develops, a comma begins to form. At this time, according to Weldon, the surface centre consolidates and the pressure begins to drop.

As the evolution of the pattern continues, the convex northern part of the leaf becomes the ‘head’ of the comma (Fig. 8). The tail on Fig. 8 is partly hidden by high-level cirrus belonging to the main baroclinic zone. At this time cyclogenesis at the surface is well under way and three hours later at 0600Gm a well developed small-scale surface low had formed (Fig. 2). The corresponding satellite image (Fig. 1) shows a well defined small-scale comma, which finally, as seen in Fig. 9, goes through an occlusion process and rolls up.

A theoretical approach The discussion above has been based on the appearance of the cloud pattern and

its evolution as seen on satellite imagery. The discussion can be supplemented by some dynamical considerations based on a paper by Blumen (1979). Blumen extended Eady’s well known theory for baroclinic waves in a basic flow with constant static stability to a slightly more complex basic state where the troposphere can be divided into two layers, a lower, relatively unstable layer, and an upper, more stable layer. The radiosonde ascent

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Fig. 9 NOAA-9 uisiblesatelliteimage or1128Gmon29 March 1985showingtheocclusionphaseof the baroclinic polar low. Jutlanlis indicated by a thin black arrow and the occluded polar low by a fat arrow. (Courtesy of Danish Meteorological Institute.)

from Stornoway in Scotland at 1200GMT on 28 March (not shown), which is representative of the environment of the disturbance, shows that the stratification corresponds to that envisaged by Blumen, and that Blumen’s second condition, a unidirectional wind which increases with height, is also fulfilled.

The growth rate curves in the form of e-folding times (the time taken for the pressure perturbation togrow by afactor of2.718)forwave solutions to thistwo-layer systemgenerally have the appearance of a curve with two maxima as shown in Fig. 10. The left branch of the curve, with a most preferred wavelength around 4000km. corresponds to the ‘normal’ one-layer Eady solution, whereas the right branch, with a maximum at a much shorter wavelength, is a feature caused by the extra degree of freedom in the vertical. It is the waves with the short wavelengths that we may identify with the cloud structure seen in Fig. 7.

The phase of intense development At OOOOGMT on 29 March when the disturbance was approaching Jutland it still had a

leaf-cloud structure. In the next few hours, however, a remarkable development took place which transformed the disturbance into an intense small-scale cyclone.

The centre of the low passed Copenhagen early in the morning around 0 6 0 0 ~ ~ ~ on 29 March. The barograph trace from Kastrup Airport, Copenhagen, shown in Fig. 11, resembles the barograph curves known from tropical storms. The low, however, although of small scale, had all the characteristics of a baroclinic low. Note, for example, the very sharp cold front (Fig. 2) which had formed in only three hours from 0300 to MOOGMT, and the characteristic cinus shield in Fig. 1. Note also the band of snow over Jutland left by the small cyclone/polar low (Fig. 1).

The reason for the extraordinary development over Denmark is not clear. The release of deep convection in the potentially unstable air may have contributed to the intensification of the baroclinic development (thunderstorms were observed at many places during the passage of the low). It is well known that cumulus heating can act co-operatively within a

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I I I I I I 4400 a m 1600 1100 s w wave-

-7ii length Fig. 10 e-folding time (decreasing upwards) as a function of wavelength for unstable waves in

Blumen’s model (see text for details)

ETORSDAG~ I FREDAG

Fig. 11 Barogra h trace for Kastrup Airport, Copenhagen showing thepassage of the polar low on 29 March 19&

baroclinic system resulting in significantly increased growth rates (Mak 1982; Sardie and Warner 1983), and recently Craig and Cho (1988; 1989) have considered the interaction of baroclinic and convective processes for polar lows. If the heating rate due to convection is not too great, they find that the baroclinic wave grows in the usual way. However, since the effect of the convective heating corresponds to a reduction in the static stability, the wave will grow faster and have a shorter wavelength. If the heating, described through a heating parameter determined by the moisture content of the subcloud layer and by the static stability of the environment, is larger than the value of a certain threshold parameter, the disturbance will be ‘cIsK*-dominated‘ with a low-level cyclone beneath an upper-level

*Conditional instability of the second kind.

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anticyclone. According to the analysis by Craig and Cho the so-called ‘real’ polar lows (of the type observed for example in the Bear Island region) tend to be asK-dominated while the comma clouds tend to be primarily baroclinic. Craig and Cho also point out that in rapidly changing airmasses, where polar lows often form, it is conceiveable that a polar low can change character over time. An example of such a transformation where a convection- dominated system changes to a baroclinic wave will be briefly discussed in the next section.

Another alternative explanation to the rapid development is that the low-level devel- opment was triggered by an upper-level region of divergence. The upper-level wind field has been analysed by Christian Zick of the Freie Universitat, Berlin, by tracking high-level clouds from Meteosat images. The result of such a wind analysis from a ‘high’ level between 400 and 500mbar for 0 3 0 0 ~ ~ ~ on 29 March is shown in Fig. 12. The wind field at this time corresponds at least qualitatively to what we would expect. However, the wind field analysis prior to the onset of the strong development is very ‘noisy’, and no firm conclusions regarding the upper-level divergence, vorticity, etc. can be drawn.

The polar low crossed Danish territory very quickly. At noon the same day (29 March) this remarkable disturbance, which for a short time brought back the winter and caused temporary traffic chaos in many places in Denmark, was far away over the eastern Baltic (Fig. 9).

Fig. I2 ‘High-level’ wind analysis for 0 3 0 0 ~ ~ ~ on 29 March 1985, based on cloud-track winds

OCCLUSIONS AND POLAR LOWS

After several years of research it is now known that polar lows form through many mechanisms. One common way of formation, at least near Scandinavia, takes place within the occlusion of a large-scale, extratropical low. The polar low in this case forms in the interior part of the cold vortex, which is the end product of the occlusion process. An actual case of such a polar low development from March 1988 will be briefly illustrated below. Apart from illustrating a basic type of initial formation of a polar low, the case is a good example of how a polar low may change its character from a convective-type system to a disturbance of a baroclinic nature.

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The initial formation of the polar low The initial formation of the polar low ‘P, is illustrated in Figs. 13 and 14. The surface

chart (Fig. 13) shows the polar low over the Norwegian Sea embedded in the large-scale circulation associated with an occluding extratropical cyclone. Fig. 14 shows the cor- responding satellite image. The cloud band belonging to the occluded frontal system is clearly seen (indicated by the open arrows), and the centre of the polar low is situated in a region of deep convection.

One day later the polar low ‘P, had moved south along the Norwegian coast, and at 1 4 2 7 G ~ ~ on 13 March it was situated north-west of Bergen (Fig. 15). The satellite image shows several interesting features. Note that two other disturbances indicated by ‘A’ and ‘B’ can be observed simultaneously with ‘P and that the three disturbances form what may be called a wave train. The open arrows in Fig. 15 indicate an arctic front. The arctic front separates an outbreak of fresh, shallow, arctic air from modified maritime air further south. The term ‘arctic front’ should be used only for this type of front heading shallow arctic airmasses. Note also the widespread cloud deck which has formed on the south-eastem flank of polar low ‘P,, over the high mountains in the southern part of Norway.

Animportant featureclearlydiscemibleon the thicknessmap for 1200GMTon 13 March (not shown) is the existence of a warm core which has formed around the polar low in the middle of the cold airmass. The warm core is clearly discernible in the lCW-500mbar thickness field as well as in the thermal winds and is centred around the surface position of the low. The question of whether some polar lows have a warm core structure similar to tropical systems was for a long time rather controversial. Several cases (such as the present) have shown conclusively, however, that (some) polar lows do have a warm core.

Fig. 13 Surface chart for 1200GMTon 12 March 1988. The polar low over the Norwegian Sea is shown by a strai ht arrow and Denmark by a curved arrow. (Reproducedfrom the European Meteorological bulletin.)

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Fig. 14 NOAA-9 visible satellite image for 1438 GMTOn 12 March 1988. The cloud band belonging to the occlusion is shown by the open arrows and the centre of the polar low by the thin arrow. (Courtesy of Danish Meteorological Institute.)

The baroclinic phase After crossing southern Norway, the polar low changed its character and was trans-

formed into a more baroclinic-like disturbance. This phase is illustrated in Fig. 16 which shows a satellite image for 0712GMT on 14 March 1988. The centre of the original low ‘L1’ is still seen (indicated by the ‘fat’ arrow) but, in addition, a wave-like disturbance has formed as indicated on the figure. On the corresponding surface chart for O~OOGMT (not shown) both the original low and a ‘new’ one ‘L2’ formed around the apex of the developing wave. ‘L2’ never became very intense, but developed a typical baroclinic cyclone-like structure including a warm and a cold front, which could be followed on the weather radar at Kastrup Airport. (Images showing the development and movement of these fronts were reproduced in Rasmussen et al. (1988).)

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Fig. 15 NOAA-9 visible satellite ima e for 1 4 2 7 ~ ~ ~ o n 13 March 1988. The centre of the polar low is shown by the black arrow, w h k the open arrows indicate theposition of an arctic front heading an outbreak of a shallow arctic mass. ‘A’ and ‘B’ indicate two other perturbations in the cold airmass. The wides read cloud deck from Iceland (upper left corner) to eastern Europe belongs to the polar front. [Courtesy of Danish Meteorological Institute.)

NUMERICAL FORECASTS

Present numerical models have difficulties handling small-scale systems such as polar lows, especially when convection is involved. As discussed earlier, the rapid development of the baroclinic disturbance over Denmark early on 29 March 1985 was probably a result of widespread convection. It is thus not surprising that even a sophisticated numerical model, such as the Meteorological Office fine-mesh model, did not forecast the observed development. It is a little surprising, however, that while the model, based on data for OOOOGMT on 28 March, predicted a weak disturbance over Denmark at 06oOGm the fol- lowing day (the 29th), the forecast for the same time, but based on data for 1200GMT on the Bth , showed even less development.

The first forecast which ‘caught the development’ was based on data for OOOOGMT on the 29th, i.e. too late to be of any practical use, and even in this case the model only indicated the development of a weak trough in the pressure field.

Concerning the 1988 case the Meteorological Office fine-mesh model produced a much better forecast even 36 hours ahead with position as well as strength over Denmark very close to the verifymg observations. Whether this very positive result was a result of an

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Fig. 16 NOAA-10, uisiblesatellite image for0712 GMTOn 14 March 1988showing the polar low over Denmark. The centre of the original ‘convective disturbance’ is shown by the short fat arrow while the lon arrow indicates the centre of the newly formed baroclinic wave disturbance. The coldfront befon ing to the wave is indicated by the black triangles. The open arrows show the upper-level ou&w. (Courtesy of Danhh Meteorological Institute.)

improvement of the model or a consequence of the fact that this particular system was rather weak and at this phase of its development definitely of a baroclinic nature is hard to say. Also the limited area model from the Norwegian Meteorological Institute produced a very good forecast for this polar low development.

REFERENCES

Aakjaer, P. D. (1987) Polar lows over Denmark and their simulation by a numerical forecast model.

Blumen, W . (1979) On short-wave baroclinic instability. J. Atmos.Sci., 36, pp. 1925-1933 Craig, G. and Cho H.-R. (1988) Cumulus convection and CISK in the extratropical atmosphere.

Part I: Polar lows and comma clouds. J. Atmos.Sci., 45, pp. 2622-2640 (1989) Baroclinic instability and CISK as the driving mechanisms for polar lows and

comma clouds. In: Twitchell, P. F., Rasmussen, E. A. and Rasmussen, K. L. (Eds.) Polar and arctic lows, A. Deepak Publishing, Hampton, Virginia, pp. 131-140, 420

Lalas, L. and Einaudi, F. (1976) On the characteristics of gravity waves generated by atmospheric shear layers. J. Atmos.Sci., 33, pp. 1248-1259

Mak, M. (1982) On moist quasi-geostrophic baroclinic instability. J. Atmos.Sci.. 39, pp. 2028- 2037

Rasmussen, E. and Aakjaer, P. (1985) The Easter storm, a baroclinic polar low. (In Danish.) Vejret. 7, pp. 3-16

Rasmussen, E., Aakjaer, P. and Steffensen, M. (1988) Occlusions and polar lows. (In Danish.) Vejret, 10, pp. 23-40

Sardie, J. M. and Warner, T. T. (1983) On the mechanism for the development of polar lows. 1. Atmos.Sci., 40, pp. 869-881

Weldon, R. (1979) Satellite training course noies. Part IV Cloud patterns and the upper air wind field. Application Division National Environmental Satellite Service, NOAA

Ph D. thesis, The Danish Meteorological Institute, Copenhagen, 150 pp.

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