Radar images (PI-composite and marker for hail warning): time sequence

18th August 2004: 12UTC
Over the western parts of France the international radar composit (horizontal resolution: 4 km, altitude: 2 km) exhibits a band with radar reflectivty up to the green level (Zr-relation at DWD: 8 mm/h). However, we have to be careful to derive directly the precipitation amount from that colours due to discrepancies of definitions between the DWD and the weather services of France, Belgium and the Netherlands.

18th August 2004: 13UTC
The band of high reflectivity moved to the East without sigificant change.

18th August 2004: 14UTC
At 14 UTC the band of heavier precipitation moved east and the radar colours brightened in the south.

18th August 2004: 15UTC
At 15 UTC the line of higher radar values extended from Brussels (“B”) to southwest France. Over Belgium the DWD evaluation software derived two warning markers for hail. We can see red and blue areas (35, 150 mm/h, respectively) over Belgium. This is a clear indication for the formation of the squall line. Belgium and the DWD use different colour definitions so there is room for error in the way in which the automated process derives intensities.

18th August 2004: 16UTC
The typical shape with a chain of warning markers formed at the leading edge to the middle of the developing squall line with high radar reflectivity in relation to prevailing stratiform rain at the rear. A new cell developed over East Netherlands. This is typical for the beginning of acceleration of a squall line. At the leading edge new convection began and merged with the succeeding squall line.

18th August 2004: 17UTC
Hail warnings appeared as the squall line moved across western Germany. (The blue areas over the Netherlands are probably due to the other definitions, as mentioned earlier.)

18th August 2004: 18UTC
In the meanwhile the chain of warning markers achieved a length of about 500 km. The highest intensity was in its northern part. From the radar reflectivity values precipitation amounts of up to 35 mm/h can be derived (The blue areas over the Netherlands were ignored.)

18th August 2004: 19UTC
The squall line broadened and the chain of warning markers for hail is still near its centre over west Germany. Reflectivities up to level 5 (red, 35 mm/h) were detected. Further to the south a broad band of reflectivity values (mainly up to the green level, 8 mm/h) can be found. In the eastern parts we see more structure (convection). In the western parts more homogeneous patterns consistent with stratiform rain (less than 8 mm/h). If we compare the precipitation amounts with the corresponding reports (see special link) it becomes clear that the observation network is not of sufficiently high resolution to obtain representative values of precipitation.

18th August 2004: 20UTC
The image shows a broadened squall line with many warning markers at the leading edge. New cells developed here and merged with the succeeding cloud band. This is a typical process for a squall line. West of the warning markers we find bigger areas with level 2 and 3 (cyan, violet approximately 1 mm/h), occasionally level 4 (green, 8 mm/h). (The red and blue areas over Belgium are probably not relevant.)

18th August 2004: 21UTC
The squall line moved quickly eastwards. At leading edge a long chain of warning markers with some gaps can be seen over west Germany. West of it a broad band with rain amounts between 1 and 8 mm/h was detected.

18th August 2004: 22UTC
This image shows a chain like structure extending from northwest to southwest Germany. A few warning markers are evaluated. Further to the west a big smoother area of rain can be recognised. The squall line is still holding its shape but it is now weakening.

18th August 2004: 23UTC
The whole system continued weakening. Four warning markers are derived for the eastern North Sea. A squall line structure is visible over Central Germany.

19th August 2004: 00UTC
No warning marker are derived. Reflectivity levels 3, occasionally 4 (about 3 and 8 mm/h, respectively) were detected. The precipitation amounts at the rear decreased to about 1 mm/h.