Case Study on Squall Line 2004

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