Introduction
This case study is about a typical formation of a squall line moving from France to Central Germany. Middle Europe was influenced by subtropical air coming from the Southwest. A short-wave trough induced intense convection over Eastern France / Benelux and Western Germany. Due to more stable air over over East Germany the squall line weakened weakened once it passed Central Germany.
Gusts of up to Beaufort 9 (flat areas) and 11 (mountaineous regions) and precipitation of about 30 mm over Eastern France and and Western Germany were observed.
Figure 1: Meteosat 8 IR10.8: 18th August 2004: 12 UTC. The squall line extending from Eastern England to the French Atlantic Coast is in its development phase (CTT up -55°C, blue areas)
Figure 1: Meteosat 8 IR10.8: 18th August 2004: 21 UTC. The squall line just after its mature state (CTT up to -65°C, violet areas). It moved at up to 100 km/h and is now positioned over the Benelux-states and Germany.
The aim of the study is:
- Follow the development of the squall line using satellite imagery (different channels, colour tables)
- Investigation of the dynamic background by considering the basic observations / measurements and numerical model parameters
In order to describe the relevant processes near the surface we consider:
- Surface pressure and tendency
- Pseudopotential temperatures
- 10m-wind
- Radio-soundings
To get better insight into the structure in addition to satellite images we investigate:
- Radar images with hail markers
- Lightning detection
For the relevant dynamic processes in higher levels the following data were used:
- Geopotential 300 hPa
- Voriticity advection 300 hPa
- Vertical motion 500 hPa in combination with KO-index
- Radio-soundings
Preconditions likely for a squall line are:
- Very warm and humid air, potentially unstable
- Convergence zones near the surface in connection with an upper air trough and a zone of lower surface pressure
- Frontal zone nearby
- Increasing wind speed with height (vertical wind shear)
Assuming that the preconditions mentioned above are fulfilled - the typical development stages of a squall line are:
- Weak convergence and pressure falls at the surface (intensifying with time)
- Sometimes water vapour imagery indicates humidity identified in the imagery as a "Medium grey" tone although no clouds developed in this area
- First line-orientiated convective cells
- Formation of squall line by merging of different types of convective cells (single, multi, sometimes super)
- Although narrow these features can extend for hundreds of kilometres
- At the leading edge of the squall lines downdraughts from the convective cells produce a cold air outflow. This outflow collides with the warm air producing new convective cells
- To the rear of the squall line surface pressure increases and moderate sometimes severe rain occurs within the colder air
- "Structured" patterns are observed in radar and satellite images on the leading edge with smooth structures to the rear of the squall line