Development of cold air features Enhanced Cumuli (EC) and Comma

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.

Different types of development of Commas are recognised:

Development of Commas within cold air

The classical development within cold air masses is the most common (Fig. 1). The development starts from Enhanced Cumulus (EC) cloudiness which is increasing and, under the influence of vorticity, finally forms to 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. There are different theories about the forcing circumstances leading to Comma development.


Fig 1. The Development of a Comma. From EC to Comma under influence of Vorticity

Baroclinic instability

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

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 there is a jet streak involved. (Reed, 1979). Many case studies of Commas made at ZAMG (more than 50) showed baroclinity in high levels which would support this theory.

Barotropic instability

Also barotropic instability should 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 be allowed to be broader than only 250 km in half-width which is unrealistically small. (Reed, 1979).

Case studies found in literature 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 a 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 character of the Comma phenomenon. The underlying assumption of this theory says 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 the 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. (Browning, 1993) This theoretical aspect is supported in the case studies by the fact that a very large number of Commas are connected to a PV anomaly represented by a very low height of the PV=2 surface, which corresponds to the transition between tropospheric and stratospheric air.

Further development

Commas, once developed, can be involved in or can be the starting point of other conceptual models such as Instant Occlusion or 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. 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 on the rear side of the upper level low or trough. On the contrary the innermost part of the Occlusion cloud spiral lies in an area with positive vorticity advection and sometimes with convective activity. Therefore this is an area where cloudiness can become enhanced.

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

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

References