# Convergence Lines in Denmark and Sweden - 10 November 2007

## Print Version

## Chapter V: Weather events

### Table of Contents

- Chapter V: Weather events
- Weather events
- Side effect

### Weather events

In the previous sections the answers were found for the several reports of snow showers over the Sont and Baltic Sea (fig 3). For the evolution of the weather events it is interesting to have a closer look at the 12.00utc synoptic surface chart in fig 21. Only a few reports of present weather are found in the area: distant showers at the exit of the Sont, actual hail showers just off the northwest coast of Poland (see the red arrows) In fact these two weather reports are betraying a line of showers which is very good depicted in fig 23: the image of the 12.00utc weatherradar, supplied by the Danish Meteorological Institute DMI. It is obvious the line of showers still exists with smaller parts of higher intensity well displayed by the changes in the yellow colour (the darker the more intense).

Figure 21: Synoptic chart 10 November 2007 1200UTC

It lools like an intensification of the convergence line due to coastal convergence, relative high surface temperature (+ 6 °C outside showers, falling rapidly to +10 only in showers), the arrival of the coldest upper air and the associated convergence - divergence pattern. With the synoptic information and the satellite information (see fig 24) the radar information is completing the overall picture on the weather events that took place in this case.

Figure 22: Radar image 10 November 2007 1200UTC,issued by DMI

### Side effect

At the same time the third and most southerly located red arrow in fig 21 tracks our attention to the showers in the Berlin area at a long distance from the sea. In fact this is the first sign of a developing and remarkable line of showers with a number of thunderstorm reports in this area well depicted in the synoptic weather chart, presented in fig 24: like a summer squall line.

Figure 23: Synoptic chart 10 November 2007 1400UTC

For explaining this side effect the evolution in the later developments in the upper air (since 0600UTC), the rising surface temperature and the Boyden index are of specific interest. In the satellite image of fig 4 we recognize the developing cloudstreet downwind the island of Fünen in the west part of Denmark. While reaching and passing the northern coast of Germany the individual cells expand into a well pronounced line of showers over land at time of the maximum temperature. This development becomes clear by comparing fig 4 and fig 24, the NOAA HRPT image of 11.26UTC. The intensifying convergence line is widening downstream, individual CB-cells are well pronounced displayed. It is obvious the western boundary of the coldest upper air (-35°C at 500 hPa, fig 19), at time of the image just overhead, plays an important role together with the high surface temperature of + 6°C just in front of the arrival of the first showers. These "high temperatures" (autumn!), the extreme low upper air and the presence of the left exit region of a strong northwesterly Jetstreak just above (fig 20), triggered the intensification of the original cloud street into a squall line with active TS, at best depicted in the satellite image in fig 24. Notice again the drop in the surface temperature to +10 °C or even lower: passing "cold weather showers" probably with intense snow, sleet or soft hail.

Figure 24: NOAA HRPT 10 November 2007 1126 UTC © B.J.Burton

Some changing aspects in this weather situation merge in the midday radiosonde sounding of Berlin (fig 25). A surface temperature of +6°C and ascending upper air (not shown) are enough for convection to overcome the inversion layer at 3000ft reaching finally a height of about 4500m or 530 hPa with development of so called low topped CB - activity.

Figure 25: Radiosonde sounding 10393 (Berlin) 10 November 2007 1200UTC

The Boyden-index is often used as an objective parameter for the probability of thunderstorm in summer. With the index above 95, this day around 96 in the Berlin area and over the Baltic Sea, thunderstorm are likely to occur with other parameters favourable as well, see fig 26 below). In this case Boyden look as useful as it is in forecasting summer storms.

Figure 26: Boyden forecast HR 10 November 2007 1200UTC