Extreme Case of Convection over Austria on 23 July 2009

Radar blips

The analysis of the collected sample of radar data revealed some prominent pictures as shown below.

• Radar data + wind gusts: wind gusts are displayed when the wind speed is exceeding 60 km/h.
• Radar data + risk of hail: the risk of hail is linked to the reflectivity of the clouds. Numerous research projects showed that hail can be expected at reflectivities about 46 dB (Waldvogel-criteria).

To discriminate the different reflectivities there is a legend given to the right. At 17:30 UTC the first cell complex is drifting through Lower Austria heading northwards. The second cell complex is already approaching Upper Austria and contains a great threat of hail. The following day many daily papers in Germany reported big hailstones and severe damage in Bavaria.

At 19:00 UTC the second cell complex has developed and increased its extension as well as its intensity. Several severe wind gusts can be detected in the west parts of Upper Austria. They are resulting from down bursts as well as the gust front.

With a decreasing south component of the upper level flow the cell complex is heading eastwards. At 20 UTC the gust front is directly in front of Vienna and shows high wind speed in a broad area. In the red marked zone (down to the right) there is high risk of hail. The analysis of radar data can only show the risk of hail, but not the size of possible hail. Unfortunately the radar ray cannot discriminate a package of lots of small hailstones compared to the same package with several big hailstones.

Within the next few minutes the weather situation in Vienna is getting tense. At 20:15 UTC a respectable wind speed of 137 km/h is measured in the north-west of Vienna.

With increasing wind speed the cell complex is rushing eastwards. In the north of Vienna many wine-growing districts faces enormous damage due to hail.

At 20:45 UTC centre of the cell complex has passed Vienna and is situated in Burgenland still showing high intensity. Summing up this second cell complex caused enormous financial damage in all Austrian provinces except Carinthia.