This chapter presents the IR-enhanced images (see legend).
By clicking through the time steps the movement and development of the cloud structures can be observed. We will investigate the cloud top temperatures and will relate them to a selection of images PV plus WV and images of airmass-RGB.
This image reveals a frontal cloud band from Scandinavia to Scotland with cloud top temperatures of partly less than -60°C (violet areas)
During the last 3 hours the frontal cloud band moved fast to the Southeast and intensified over the North Sea and Scotland/England (increasing area with cloud top temperatures below -60°C).
The frontal cloud band over England and the North Sea continued moving to the Southeast.
The frontal cloud band moved further to the Southeast. We recognise an area with enhanced cold air convection east and north of Scotland.The cloud top temperatures decreased to -45°C.
Whereas the top temperatures of the cold front increased (about 55°N/5°E) the succeeding comma exhibits now lower cloud tops (partly less than -45°C). The distance between comma and the rear edge of the cold front became smaller.
The increase of the cloud top temperatures are due to decending dry stratospheric air yielding the development of a split front. When we consider WV and PV=2 we see high PV at the rear edge of the frontal clouds. The
air mass RGB shows that the rear edge of the frontal clouds was superimposed by stratospheric air.
At 15 UTC, the area of intense cold air convection is very near to the rear site of the frontal clouds (about 55°N, 6°E). The corresponding air mass RGB shows a superimposing of stratospheric air in this area (combination of WV and PV not available).
The comma and the frontal clouds are now closer together. The image reveals high cloud top temperatures (more than -30°C) over Belgium and South Netherlands. The reason for that is the descending dry stratospheric air ( WV and PV=2 and air mass RGB).
We recognise the process "instant occlusion" (Dutch-German border): Comma and preceeding cold front were merging. From the air mass RGB high PV values in this region can be assumed (combination of WV and PV not available).
We clearly recognise the surface low's centre and that the comma and the frontal clouds merged. The image of WV and PV=2 shows high PV values near-by the centre of the surface low that correspondens also to the air mass RGB image. The rapid cylogenesis already started.
This image reveals a big cloud spiral with centre near the German-Dutch border. Over the North Sea we recognise a compact and stratiform cloud field with cloud top temperatures between -50 and -55°C (dark blue). This field intensified after the process of instant occlusion and moved to the South.
This cloud field extended and covered also parts of the Netherlands. Cloud top temperatures of about -55°C indicate intense precipitation.
The clouds of the occlusion extended at 09 UTC from the North Sea over the Netherlands to West Germany The cloud top temperatures did not change significantly.
We recognise that the situation became more critically. Between the English and Dutch coast clouds with top temperatures of -55 to -60°C developed.
During the last 3 hours clouds with low top temperatures moved over the Netherlands and we see a new bigger field approaching from the Northwest.
During the last 3 hours the situation did not change significantly. The Netherlands and Belgium were influenced by high extending clouds.
This image reveals that areas of high extending clouds moved to the South. Over the Netherlands and West Germany only some small areas of low cloud top temperatures (-50 to -55°C) can be seen. The most critical phase came to an end.
The entire system moved to the Southeast and weakened (cloud top temperatures above -55°C).