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Appearance in Satellite Data

A Comma is a very prominent cloud feature developing and existing in cold air. There are several similar phenomena within cold air: One of them is Enhanced Cumulus cloudiness (see Enhanced Cumulus ). Also a ring of vortices, crescent, oval, solid cloudiness, multiple deep or shallow bands, single deep or shallow bands or swirls in cumulus streets can all be found in cold air.

In order to distinguish the Comma from other features in cold air, the Comma cloudiness is defined as a small to meso-scale cloud spiral consisting of white (i.e. cold) cloud cells partly overlaid by cirrus shields. In most cases the strongest convection can be found in the Comma tail, but sometimes enhanced convection can also be found in the Comma head. In a few cases the Comma head and tail are separated by a narrow cloud-free area or only connected by low clouds (although this may not be apparent during the whole life time of the system). The scale of a Comma lies between 200 and 1000 km, i.e. much smaller than a fully developed depression or a cyclone.

Larger Commas can be a sign of a development process called Cold Air Development (see Cold Air Development ), where a Comma increases in size and finally gains some frontal characteristics.

07 December 2004/15.00 UTC - Meteosat 8 IR 10.8 image
07 December 2004/15.00 UTC - Meteosat 8 WV 6.2 image
07 December 2004/15.00 UTC - Meteosat 8 RGB image (0.6, 1.6 and 10.8)
07 December 2004/18.00 UTC - Meteosat 8 RGB image (3.9, 10.8 and 12.0)

Appearance in AVHRR imagery

  • Images from all channels (below) show more details of the cloud structure when compared to Meteosat 8 images.
  • RGB-combinations of channels (below left, second row) provide a quick overview of thin and thick high level cloudiness.
  • Channel manipulation (eg. subtraction with thresholding, third row, right) highlights the cloud pattern and physical features.

Concerning the Comma cloud pattern, high clouds are the only features to look at in AVHRR satellite images. The characteristics of such cloud are shown in the table below.

17 February 2000/04.35 UTC - NOAA RGB image (channel 3, 4 and 5)
17 February 2000/04.35 UTC - NOAA CH5 image

In the images above, there is a northwesterly flow over the North Sea. A Cold Front from Iceland to Scandinavia is slowly moving to the east. During passage of the Comma cloud heavy snow showers are reported in The Netherlands and in Germany. The Comma feature was already seen over the British Isles (following images), but the Comma shape is still recognizable and looks like a Cold Air Development (CAD). The fuzzy outlines (turquoise in image above left) indicate thin cold cloudiness and are the result of strong outflow at higher levels.

16 February 2000 /14.45 UTC - NOAA RGB image (channel 3, 4 and 5)
16 February 2000 /14.45 UTC - NOAA RGB image (channel 1, 2 and 4)

In the RGB-images above, the most important cloudiness of the Commas appear to be white: cold, thick, high level clouds. The borders are rather fuzzy. The Commas are located at the left exit of several jetstreaks and some jet fibres are seen over Ireland (in these images, the jet core is too far to the west to be seen properly).

16 February 2000/14.45 UTC - NOAA CH2 image
16 February 2000/14.45 UTC - NOAA CH1 minus CH3B-

In the image above left, the fuzzy edges of the Comma cloudiness are quite remarkable. In above right, the outlines of the Commas are classical examples. Radar images and surface reports from E. England and Scotland indicate heavy rain, hail and snowshowers and are in agreement with the features shown in the image above right.

Meteorological Physical Background

Commas are quite frequent phenomena and whilst they can occur during the whole year they are slightly more common in winter time. Commas are often found over the Atlantic and Western Europe, but are rare in Central and Eastern Europe as they tend to weaken and dissolve over the continent. The usual areas for the development of Commas are higher latitudes which are also typical areas for the development of polar lows. Commas have also been observed on several occasions in the Mediterranean.

The typical horizontal scale of Commas is 200 to 1000 km and their duration can range from several hours to two days.

Here are different types of Comma development:

  • Within cold air either
    • as further development of Enhanced Cumulus cloudiness or
    • as Polar Lows
  • As a cut off from a mature and well developed Occlusion spiral
  • There are a few other developments from Cold Air Development (CAD) and Baroclinic Boundaries. They are regarded as exceptions and are not addressed further here.

Development of Commas within cold air

The classical development within cold air masses is the most common. A typical life cycle is shown below using a sequence of satellite images (interval one hour).

The development starts from Enhanced Cumulus cloudiness (see Enhanced Cumulus ) which is increasing and, under the influence of vorticity, and finally forms into the typical Comma spiral shape with a Comma head and a Comma tail. This happens mostly in the cold air behind a frontal cloud band under the influence of vorticity and positive vorticity advection.

25 April 2005/00.00 UTC - Meteosat 8 IR 10.8 image; red: vorticity advection 500 hPa
25 April 2005/03.00 UTC - Meteosat 8 IR 10.8 image
25 April 2005/06.00 UTC - Meteosat 8 IR 10.8 image

There are different theories relating to the forcing mechanisms leading to Comma development.

Baroclinic instability

One theory focuses on baroclinic instability. Although baroclinic instability is regarded as a larger scale phenomenon, it can also be used for explaining smaller systems.

Baroclinic waves with a wave length of about 3000 to 4000 km usually show a rapid rate of growth. If parameters such as vertical wind shear (large horizontal temperature gradient) and static stability are taken into account, disturbances with much shorter wave length can also grow rapidly if a jet streak is involved. (For more details see Reed, 1979)

Many case studies of Commas made at ZAMG (more than 50) showed baroclinity at high levels which would support this theory. One example is shown below.

25 April 2005/06.00 UTC - Meteosat 8 IR 10.8 image; position of vertical cross section indicated
25 April 2005/06.00 UTC - Vertical cross section; black: isentropes (ThetaE), blue: relative humidity, orange thin: IR pixel values, orange thick: WV pixel values

The image shows a Comma feature extending west of Ireland, 53N25W. In the vertical cross section (right) a frontal zone at middle levels (500 - 700 hPa) can be detected.

Barotropic instability

Barotropic instability should also be taken into account when considering a Comma formation. Commas develop on the left side of the polar jet axis. The problem with this theory is the scale: The wavelength of maximum instability is about four times the half-width of the jet. If a Comma (Polar Low) is assumed to have a wavelength of 1000 km the jet would not normally be broader than 250 km in half-width, which is unrealistically small. (For more details see Reed, 1979).

Case studies found in the literature, as well as the ones studied at ZAMG, support this theory with respect to the location of the Comma on the left (cyclonic) side of the jet.

So both theories can explain some features of Comma development but are not totally satisfactory.

CISK mechanism

Another theory is Conditional Instability of the Second Kind (CISK) which focuses on the unstable convective characteristic of the Comma phenomenon. The underlying assumption of this theory states that cumulonimbus convection alone cannot maintain the low-level convergence necessary for the development of clouds. Therefore large scale convergence is also essential. If an air parcel rises in a conditionally unstable, unsaturated air mass, it will give rise to cumulonimbus convection because of the release of latent heat. The vertical distribution of the wind field (low level convergence and high level divergence) results in a convergence of moisture in the boundary layer and an increase of cyclonic vorticity. As a consequence vertical velocity will increase at the top of the Ekman layer. Now a positive feed-back loop is established which amplifies convection as well as the large-scale convergence. This theory was developed for tropical cyclones as the atmosphere normally is (conditionally) unstable at lower latitudes. In several aspects this process is also true for other regions, e.g. if cold polar air streams over a relative warm sea surface.

This theory is supported by those cases where a Comma develops from an EC area far behind the frontal cloud band.

Potential vorticity

Another interesting point of view is the role of potential vorticity (PV). If there is an upper tropospheric PV maximum rising motion occurs at the leading edge - assuming the PV maximum is sinking. There is also a region of reduced static stability there. If a major incursion of high PV overruns a region of moist air and strong baroclinity it leads to cyclogenesis, especially if the induced upward motion ahead of the PV anomaly becomes moist and diabatic effects are involved. (For more details see Browning, 1993)

This theory is supported in the case studies by the fact that a very large number of Commas are associated with a PV anomaly, represented by a very low height of the PV=2 surface, which corresponds to the transition between tropospheric and stratospheric air.

07 December 2004/18.00 UTC - Meteosat 8 IR 10.8 image; magenta: Height of PV=2 units

The case from 07 December 2004 shows a lowering of the tropopause just behind the Comma to around 500 hPa. In the Comma itself a relatively high tropopause can be found.

Further development

Commas, once developed, can be involved in or can be the starting point of other conceptual models such as Instant Occlusion (see Instant Occlusion ) or Cold Air Development (see Cold Air Development ).

Development of Commas from Occlusion spirals

If an Occlusion is accompanied by a very intensive, pronounced cloud spiral the innermost part can become separated, i.e. the connection between the Occlusion spiral and the rest of the cloud spiral becomes an area of only shallow cloudiness and finally disappears so that the remaining Comma cloud becomes a separate feature. Sometimes the innermost part of the Occlusion cloud band disappears and a Comma develops in the cloud-free area. Nevertheless its origin is the Occlusion spiral.

The area where this separation happens is the north-western part of the Occlusion cloud band which disappears as a result of negative vorticity advection and divergence in lower levels. This happens to the rear of the upper level low or trough. In contrast, the innermost part of the Occlusion cloud spiral lies in an area with positive vorticity advection and sometimes within an area of convective activity. Therefore, this is an area where cloudiness can become enhanced.

A Comma which is generated by an Occlusion is shown by the following sequence of satellite images (interval six hours). Such a development is no less frequent than the development of Commas within cold air mentioned above.

07 December 2004/06.00 UTC - Meteosat 8 WV 6.2 image
07 December 2004/12.00 UTC - Meteosat 8 WV 6.2 image
07 December 2004/18.00 UTC - Meteosat 8 WV 6.2 image
08 December 2004/00.00 UTC - Meteosat 8 WV 6.2 image

Relative streams and Commas

From the viewpoint of conveyor belt theory, Commas usually lie within one single air mass, the dry intrusion.

Relative streams related to Commas show a characteristic pattern, namely, an isolated cyclonic circulation around the cloud spiral with upward motion within the cloudiness and descending motion behind. This circulation can be observed at all levels above the unstable air mass, up to the tropopause and does not change much between different levels. Only vertical motion is decreasing with height. In the case of Commas without specific circulation, a pronounced cyclonic curvature of the streamlines can usually be found.

29 March 1999/06.00 UTC - Meteosat IR image; magenta: relative streams 282K - system velocity: 247° 16 m/s, yellow: isobars 282K
29 March 1999/06.00 UTC - Meteosat IR image; magenta: relative streams 300K - system velocity: 247° 16 m/s, yellow: isobars 300K

Key Parameters

  • Height Contours 500 and 1000 hPa:
    show a marked trough, sometimes even an isolated low. The Comma is situated at the leading edge of or near the trough line. Commas are situated at the cyclonic side of the polar jet. There may also be a left exit region of a jet streak (see below).
  • Temperature advection:
    Since Commas are cold air features they are mainly situated in cold advection. Commas which are also partly in Warm Advection have higher baroclinic instability and show a higher probability for further development into Instant Occlusion and Cold Air Development (see Instant Occlusion and Cold Air Development )
  • Instability indices:
    Commas are characterised by a rather unstable air mass. The typical range of the Showalter index with Commas is 0 to (max) 9, but mostly around 3. But strong instability is rarely seen in the Showalter field even though there can be heavy precipitation. The unstable air mass extends from the surface to about 800 - 700 hPa (see Typical appearance in vertical cross sections), a layer which is probably too low to be fully resolved by the Showalter index (which starts at 850 hPa). The Boyden index shows values that exceed 95.
  • Relative vorticity at 500 hPa:
    is always positive, i.e. cyclonic. The maximum can be found to the rear of the cloud spiral. Both components (curvature and shear vorticity) are positive. Curvature vorticity dominates in the majority of cases, but shear vorticity is also important, especially at 300 hPa and if the Comma is situated in the left exit region of a jet streak (see further below).
  • PVA at 500 hPa:
    has its maximum above the Comma cloud or sometimes shifted to the south. At a level of 300 hPa the maximum is not so pronounced although absolute values are not smaller. There is also a correlation between the two levels: If there is no or only a weak maximum at 500 hPa the probability of a pronounced maximum at 300 hPa is quite small; a left exit region of a jet streak can lead to higher values at 300 hPa.
  • Isotachs at 300 hPa:
    Cyclonic vorticity and PVA at 300 hPa can also be the reason for the position of a Comma in a left exit region of a jet streak. But, in these situations shear vorticity dominates curvature vorticity and the Comma spiral structures are rather weak.
  • Potential vorticity:
    The height of PV=2 units shows a maximum to the rear of the cloud spiral which is equivalent to a lowering of the tropopause there to around 400 hPa. This is significantly lower than at the corresponding frontal cloud bands. Behind a Cold Front a low height of the PV can be observed in a rather large area parallel to the front, but for the Comma a characteristic maximum can be observed (lowest level in the troposphere).

Height Contours 1000 hPa/Surface Pressure

25 April 2005/00.00 UTC - Meteosat 8 IR 10.8 image; magenta: Surface Pressure

Height Contours 500 hPa/Surface Pressure

25 April 2005/00.00 UTC - Meteosat 8 IR 10.8 image; magenta: height contours 500 hPa

Temperature Advection

25 April 2005/00.00 UTC - Meteosat 8 IR 10.8 image; red: Temperature Advection 1000-500 hPa

Boyden Index

25 April 2005/00.00 UTC - Meteosat 8 IR 10.8 image; red: Boyden Index

Relative Vorticity 500 hPa

24 April 2005/18.00 UTC - Meteosat 8 IR 10.8 image; cyan: relative vorticity 500 hPa

Positive Vorticity Advection 500 hPa

25 April 2005/00.00 UTC - Meteosat 8 IR 10.8 image; red: positive vorticity advection 500 hPa

Isotachs 300 hPa

24 April 2005/18.00 UTC - Meteosat 8 IR 10.8 image; yellow: Isotachs 300 hPa

Height of PV=2 units/Height of tropopause

25 April 2005/00.00 UTC - Meteosat 8 IR 10.8 image; magenta: Height of PV=2 units

Typical Appearance In Vertical Cross Sections

  • ThetaW:
    One of the most characteristic properties of Commas is the instability in lower and middle levels. The unstable air mass is indicated by the distribution of the isentropes up to 800 - 700 hPa. In most cases no frontal character can be seen in the lower troposphere, but in upper levels (above 500 or 600 hPa) sometimes one can see a downward inclination of the isentropes leading to a higher gradient. The peak belonging to the Comma feature under consideration is the one in the right half of the cross section (52.5N/24W). The more western peaks belong to convective areas in the centre of the Comma spiral.
  • Relative Vorticity:
    Commas exist in areas of positive relative vorticity throughout the whole vertical cross section. The maximum lies to the rear at about 400 hPa (never below 500 hPa and not higher than 300 hPa).
  • Vorticity Advection (PVA/NVA):
    Vertical cross sections show PVA above the cloudy area with its maximum in the upper troposphere, in advance of the corresponding vorticity maximum. There can be both PVA and NVA behind and in front of the Comma.
  • Omega:
    Commas are often associated with upward motion throughout the whole atmosphere.
  • Temperature Advection:
    The profile of temperature advection usually shows cold advection in the lower levels trough out the whole area of the Comma. In middle and upper-levels some warm advection can be found over the cloudy area, and cold advection behind the Comma.
  • Divergence:
    The vertical distribution of divergence shows convergence in lower levels often decreasing with height; that can result in divergence in upper levels. This is consistent with upwards motion (see above). Convergence is generally found near the cloudy area but may sometimes be present in the surrounding areas. Convergence is occurring in this case in a thick layer from the surface up to 700 hPa.

As shown below the appearance of a Comma in a vertical cross section is best described by a cross section taken south of the Comma head.

25 April 2005/06.00 UTC - Meteosat 8 IR 10.8 image; position of vertical cross section indicated

ThetaW

25 April 2005/06.00 UTC - Vertical cross section; black: isentropes (ThetaE), orange thin: IR pixel values, orange thick: WV pixel values

Relative Vorticity

25 April 2005/06.00 UTC - Vertical cross section; black: isentropes (ThetaE), blue: relative vorticity, orange thin: IR pixel values, orange thick: WV pixel values

Vorticity Advection (PVA/NVA)

25 April 2005/06.00 UTC - Vertical cross section; black: isentropes (ThetaE), green thick: vorticity advection - PVA, green thin: vorticity advection - NVA, orange thin: IR pixel values, orange thick: WV pixel values

Vertical Motion (Omega)

25 April 2005/06.00 UTC - Vertical cross section; black: isentropes (ThetaE), cyan thick: vertical motion (omega) - upward motion, cyan thin: vertical motion (omega) - downward motion, orange thin: IR pixel values, orange thick: WV pixel values

Temperature Advection

25 April 2005/06.00 UTC - Vertical cross section; black: isentropes (ThetaE), red thick: temperature advection - WA, red thin: temperature advection - CA, orange thin: IR pixel values, orange thick: WV pixel values

Divergence

25 April 2005/06.00 UTC - Vertical cross section; black: isentropes (ThetaE), magenta thin: divergence, magenta thick: convergence, orange thin: IR pixel values, orange thick: WV pixel values

Weather Events

Commas can be split into a head and a tail, whereby the head clouds are layered cloudiness with dynamical precipitation and the tail clouds are more convective cloudiness with showery precipitation.

Parameter Description
Precipitation (incl. thunder)
  • Moderate to heavy precipitation, rain or snow, in Comma head
  • In Comma tail more showery precitation
  • In Comma tail quite often hail and thunderstorms are observed.
Temperature
  • Surface temperature and sea surface temperature are forcing features of convection.
Wind (incl. gusts)
  • In surface trough strong winds can be expected.
  • Ahead of the Comma head strong gusts are common.
  • In and near showers also strong gusts can be observed.
Other relevant information
  • Risk of moderate to severe icing and turbulence
  • Poor visibility during heavy snowfall
  • Heavy snow fall can cause hazardous weather for society
  • Different behaviour over sea and land

3 April 1999/06.00 UTC - Meteosat IR image; weather events (green: rain and showers, blue: drizzle, cyan: snow, red: thunderstorm, black: no precipitation)

In the example from 13 April 1999/06.00 UTC the Comma head extending from N. Ireland to Scotland is accompanied by reports of rain and snow, whilst the Comma tail over central England has fewer reports of precipitation.

References

General Meteorology and Basics

  • FORBES, LOTTES (1985): Classification of Mesoscale Vortices in Polar Airstreams and the Influence of the Large-scale Environment on their Evolutions, Tellus, 37A, 132 - 155
  • RASMUSSEN (1979): The Polar Low as an Extratropical CISK Disturbance, Quart. J. Royal Meteor. Soc., 105, 531-549
  • REED (1979): Cyclogenesis in Polar Air Streams, Monthly Weather Review, 107, 38-52
  • TURNER, LACHLAN-COPE, THOMAS (1993): A Comparison of Arctic and Antarctic Mesoscale Vortices, J. Geophysical Research, 98, D7, 13019-13034

General Satellite Meteorology

  • CARLETON, CARPENTER (1989): Satellite climatology of Polar Lows and Broadscale Climatic Associations for the Southern Hemisphere, Int. J. Climatology, 10 (3), 219-246
  • CLAUD ET AL (1993): Satellite Observations of a Polar Low over the Norwegian Sea by Special Sensor Microwave Imager, Geosat, and TIOS-N Operational Vertical Sounder, J. Geophysical Research, 98, C8, 14487-14506

Specific Satellite Meteorology

  • BROWNING (1993): Evolution of a Mesoscale Upper Tropospheric Vorticity Maximum and Comma Cloud from a Cloud-free Two-dimensional Potential Vorticity Anomaly, Quar. J. Meteor. Soc., 119, 513, 883-906
  • CRAIG (1992): A Study of Two Cases of Comma-Cloud Cyclogenesis Using a Semigeostrophic Model, Monthly Weather Review, 2942-2961
  • REED (1979): A Case study of Comma Cloud Development, Monthly Weather Review, 114, 1681-1695
  • REED (1979): A Further Case study of Comma Cloud Development, Monthly Weather Review, 114, 1696 -17