Severe convective storm over Poland

By Piotr Struzik, Monika Pajek (IMGW) and Jochen Kerkmann (EUMETSAT)

2 July 2007 was a very hot day in Poland when temperatures reached 31°C. A severe weather event in Poland started with the creation of a large storm cell above the Sudety Mountains. It expanded rapidly, finally covering large part of the Polish territory. In Wroclaw the temperature decreased from 26 to 18°C within 15 minutes and wind speeds up to 90km/h were measured.

The most severe weather was recorded in south-western and central parts of Poland. In central Poland there were interesting cloud forms one being a well-marked roll (shelf) cloud accompanied with sudden strong winds and visibility drop.

As a result of this severe storm there was a large amount of damage. In a period of less then one hour, 40mm of rainfall (half the monthly average) was measured close to Wroclaw. In many places damage was caused by hail, lightning (fires) and flash-flooding. Many trees and several electrical and telecommunications masts were broken.

In the southern Wielkopolska region two large fires occurred — one in a forest and the other in a farm building where 27 pigs were choked to death by smoke. Hundreds of houses and fields were inundated with flood water. Agricultural losses were especially large because this event occurred near harvest time. In Wroclaw many streets were completely under water (in some cases up to half meter deep), as were the hospital and city court buildings.

The satellite images below show the mesoscale cloud system over Poland at 15:00, 17:45 and 18:00 UTC, respectively. The most interesting 'warning' features in these images are:

• the cold-ring (IR10.8 image at 15:00 UTC) or cold-U shape (IR10.8 image at 18:00 UTC) of the convective storm, which are typical satellite indicators of severe convective storms;
• the rapid growth of the convective system, covering about 50% of Poland in the evening hours;
• the overshooting tops and gravity waves on the anvil top seen in the HRV images;
• the large areas of positive BTD WV6.2–IR10.8 and WV7.3–IR10.8.

In the IR10.8 image of 17:30 UTC, the area of positive BTD WV6.2–IR10.8 pixels is marked with a red colour. As can be seen from the added scatterplot IR10.8 versus BTD WV6.2–IR10.8, the colder the cloud top the larger the BTD. Also, as seen in the lower right image, the BTD WV7.3–IR10.8 reaches smaller positive values than the BTD WV6.2–IR10.8.

Both factors suggest the existence of a relatively warm, moist layer above the cold cloud top such that the radiation emitted from the cold tops gets 'warmed up' while passing through the warm moisture layer (enhanced by the slant path through this layer; Poland has a satellite viewing angle of about 60°). It is not known whether the moisture above the storm is self-produced by the storm (eg by the cloud top gravity wave breaking mechanism as described by Wang (2007) or if a large-scale moist layer existed in the lower stratosphere before the storms started.

Additional content

Met-8 channel 09 (IR10.8) animation (rapid scans) (12:00–18:40 UTC)
Met-8 channel 12 (HRV) animation (rapid scans) (14:30–16:00 UTC)
NOAA-15 AVHRR RGB composite (15:05 UTC)
Met-9 HRV image with lightning strokes (16:15 UTC)
Surface chart 12:00 UTC (Credit: KNMI)
Surface chart at 18:00 UTC (Credit: KNMI)