COMMA - KEY PARAMETERS

by ZAMG

500 hPa and 1000 hPa pressure heights show a marked trough, sometimes even an isolated low. The Comma is situated at the leading edge 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).

	20 January 1999/06.00 UTC - Meteosat IR image; magenta: height contours 1000 hPa, cyan: height contours 500 hPa

Temperature advection: Since Commas are cold air features they are often situated at least partly in cold advection. Commas which are completely in cold advection show a higher propability for further development into Instant Occlusion and Cold Air Development (see Instant Occlusion and Cold Air Development Commas are characterised by a quite unstable airmass. 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 airmass extends from the surface to about 800 - 700 hPa (see Typical appearance in vertical cross section), a layer which is probably too low to be fully resolved by the Showalter index (which starts at 850 hPa).

	20 January 1999/06.00 UTC - Meteosat IR image; red: temperature advection 700 hPa, yellow: Showalter index

The relative vorticity is always positive, i.e. cyclonic. The maximum can be found at the rear edge of the cloud spiral. Both components (curvature and shear vorticity) are positive. Curvature vorticity dominates in the majority of the 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).

	20 January 1999/06.00 UTC - Meteosat IR image; cyan: vorticity 500 hPa

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.

	20 January 1999/06.00 UTC - Meteosat IR image; red: vorticity advection 500 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.

	20 January 1999/06.00 UTC - Meteosat IR image; yellow: isotachs 300 hPa

Another very characteristic property is the potential vorticity. The height of PV=2 units shows a maximum at the rear side of the cloud spiral which is equivalent to a lowering of the tropopause there to are 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 at the Comma a characteristic maximum can be observed (lowest level in troposphere).

	20 January 1999/06.00 UTC - Meteosat IR image; cyan: height of PV=2 units