COMMA

written by ZAMG

UNDER CONSTRUCTION

A comma is a very prominent feature developing and existing in cold air. It has a lot in common with enhanced cumuli cloudiness (compare also chapter ENHANCED CUMULUS AND CELLULAR COLD AIR CLOUDINESS

A comma is a small cloud spiral within the cold air cloudiness at the rear side of fronts, and consists of white (i.e. cold) cloud cells partly overlaid by cirrus shields. Sometimes a comma head, which is the enhanced cloud part, and a comma tail, which consists of much smoother and warmer cloud tops, can be discriminated.

There are different theories for its development:

Commas develop under a baroclinic situation.
Commas are mainly convective phenomena, but result from interaction between small and mesoscale CISK (conditional instability of the second kind) processes: an interaction and iteration between small scale cumulus convection occurring within a pre-existing mesoscale convergence line. This CISK theory has been developed for commas in tropical areas.

Both types can be observed with commas. The baroclinic type is thought to appear closer to the rear side of frontal systems where baroclinicity still has high values. Such commas develop often in Nordic areas and are called Polar lows. The convective type develops more away from the main frontal system in the cold air and very often is configured from an EC cloud area forming a more and more distinct spiral form.

Typical model parameters for the detection and explanation of the comma conceptual model are upper level parameters like vorticity and vorticity advection. Within a maximum of vorticity advection an enhanced cumulus area develops which then increasingly takes the form of a spiral structure under the influence of cyclonic vorticity.

Another frequently observed but completely different development is the splitting from a highly advanced occlusion spiral. In this case the innermost part of the cloud spiral is torn off and remains as a separate comma structure. Commas with such a development background differ from the classical conditions as they exist under warm advection, which is a remnant of the occlusion process.

The synoptic importance of commas lies on the one hand in the strong weather activity with showers and thunderstorms but also in the various possibilities for further development:

a rapid development of the whole system may lead to an occlusion-like cloud spiral, a process which is called cold air development (compare also chapter COLD AIR DEVELOPMENT CD )
an interaction of a mesoscale comma with the cold front cloud band at the rear side of the upper level trough may lead to a special type of occlusion called instant occlusion (compare also chapter INSTANT OCCLUSION IO )

Left: 10 June 1998/06 UTC - IR image; lines: cyan: height contours at 500 hPa, green: equivalent thickness 500/850 hPa
Right: 10 June 1998/06 UTC - IR image; lines: green: positive vorticity advection (PVA) at 500 hPa, red: temperature advection 500/1000 hPa

The satellite image shows a comma above the southern part of England. The comma has a structured appearance which indicates embedded convective cells.
The comma is situated in a trough, immediately in front of the trough axis, of the height field at 500 hPa and the equivalent thickness (left image).
Cold advection and positive vorticity advection can be found within the area of the comma (right image).

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