Storm Catastrope Atlantic and Western Europe - Case Study 25

25 December 1999/06.00 UTC - Meteosat IR image; 25/06.00 - 26/00.00 UTC 3-hourly image loop	25 December 1999/06.00 UTC - Meteosat WV image; 25/06.00 - 26/00.00 UTC 3-hourly image loop

Summary of typical, yet important features in IR and WV imagery for the initial stage of Lothar:

A broad zonal frontal cloud band, highly structured and composed of cloud fibres at different heights above lower denser cloud fields, extends zonally across the Atlantic;
The Warm Front Band cloud does not differ much from the main frontal cloud band;
A "cloud head" exists from the very beginning: this is a lower cloud part penetrating below the zonal front band in direction of the cold air (see red arrow in left hand image). This is not yet a fully developed cloud head of "Rapid Cyclogenesis" but it does indicate possible areas for a Wave and a subsequent rapid development;
In the case of Lothar the frontal cloud decays immediately to the rear of the low cloud head and close to the rapid development (25/12.00 - 21.00 UTC);
The WV images show a Dark Stripe at the rear of and along the broad frontal cloud; in the beginning there appears to be a double structure, the more northern black line becomes a feature associated with a subsequent development ("Lothar successor" see below).
Such a situation can persist for some time until fast "Rapid Cyclogenesis" starts. This happened on 26 December from 00.00 UTC on.

26 December 1999/00.00 UTC - Meteosat IR image; 00.00 - 06.00 UTC half-hourly image loop	26 December 1999/00.00 UTC - Meteosat WV image; 00.00 - 06.00 UTC half-hourly image loop

Summary of typical and important features in IR and WV for the fast "Rapid Cyclogenesis" development of Lothar:

A local darkening (in WV and IR) of an area to the rear of or slightly upstream from the cloud head where the development is going to take place (26/00.00 - 01.00 UTC)
An intensive development of a Dark Stripe between the zonal cloud band and the cloud head within a very small time scale of several (about 6) hours in both: IR and WV (see region of red arrow);
The rapid and pronounced intensification of a cloud head (already from 25/18.00 UTC on);
A subsequent development of a pronounced cloud spiral comprising an increasing black area in the WV.

26 December 1999/03.00 UTC - Meteosat IR image; 03.00 - 12.00 UTC 3-hourly image loop	26 December 1999/03.00 UTC - Meteosat WV image; 03.00 - 12.00 UTC 3-hourly image loop

The further "rapid cyclogenesis" development shows:

The rapid development of a very intensive cloud spiral
Clouds close to the centre of the surface low (very small scale) can rotate several times around the centre, indicating very high wind speeds (similar to tropical storms);
A Comma - like but small to mesoscale line of cells often develops in the centre of the low.

25 December 1999/06.00 UTC - Meteosat IR image; 25/06.00 - 26/00.00 UTC 3-hourly image loop	25 December 1999/06.00 UTC - Meteosat WV image; 25/06.00 - 26/00.00 UTC 3-hourly image loop

Summary of typical and important features in IR and WV for the initial stage of Lothar successor:

"Lothar successor" can be seen from the very beginning in IR images as a substructure in the western part of the low cloud head belonging to Lothar and later as separate (cut off) cold air feature (25 December/06.00 - 15.00 UTC)
During the same period the WV image shows a pronounced Dark Stripe to the rear of the cloud structure of "Lothar successor"; this WV stripe is much more pronounced than the mere grey shade gradient to the rear of the cloud head belonging to Lothar;
At 18.00 UTC on 25 December the start of a dramatic change in the WV image can be seen: the area behind the cloud head belonging to Lothar darkens significantly compared to the still dark area behind the cloud head belonging to "Lothar successor"; this is the first indication of the Rapid Cyclogenesis going on within Lothar (see above);
From 18.00 UTC on 25 December a Wave feature develops in the frontal cloud band in the area of the cloud head belonging to "Lothar successor"; consequently, the bright cloud to the rear of the frontal cloud band is shifted northward and thus partly hides the lower cloud bulge.

Loop of 3-hourly images from 26 December 12.00 till 21.00 UTC shows the further development of "Lothar successor"

26 December 1999/03.00 UTC - Meteosat IR image; 03.00 - 12.00 UTC 3-hourly image loop	26 December 1999/03.00 UTC - Meteosat WV image; 03.00 - 12.00 UTC 3-hourly image loop

The following features can be summarised:

Although there are some features which are indicative of rapid development, no such development takes place.
Between 26 December 03.00 - 09.00 UTC there is a dissipation of the frontal cloud band close to the area of the cloud head of "Lothar successor" - a process which could also be observed with Lothar;
Between 26 December 06.00 and 09.00 UTC, there is a distinct darkening in the WV image, immediately behind the cloud head belonging to "Lothar successor"; this could be regarded as the start of "Rapid Cyclogenesis";
But: instead of a dissipation of cloudiness in the cloud head and a subsequent intensive spiral development, a rather weak spiral, typical of a Comma develops.
This spiral increases somewhat but remains at meso-scale; there are extensive cold air features behind.

Storm Catastrope Atlantic and Western Europe - Case Study 25 - 28 December 1999