Summary
The case from 18 – 19 August 2004 is an example of a classical squall line:
- During the 18th August a frontal cloud pattern moved from Western France to Central Germany.
- From noon until the evening this cloud pattern transformed into a squall line due to positive curvature vorticity advection induced by a short wave trough.
- From the afternoon to the evening a mesoscale low with negative surface pressure tendencies developed east of the squall line. A mesoscale high with rising surface pressure west of this produced an isallobaric west wind. In combination with the increasing temperature contrast between the front and rear of the squall line the velocity of the wind reached 100 km/h.
- At the leading edge of the squall line the radio-soundings showed increasing potential instability due to warm air advection between the surface and 850 hPa and negligible advection above. The wind speed at 700 hPa was 60 knots, corresponding well to gusts up to Beaufort 9 (flat areas) and Beaufort 11 (mountainous areas).
- During the afternoon pseudopotential temperature at 2 m exhibited values of more than 70°C (risk of severe thunderstorms).
- The NWP-field of the equivalent potential temperature in 850 hPa gave hints of intense precipitation. Precipitation amounts of up to 30 mm/3h were reported.
During its maximum intensity the squall line showed the typical structure of:
- A band of high radar reflectivities with hail warning markers, intense lightning activity and rough banded patterns in the IR and HRV (with occasional overshooting tops) at the leading edge of the trough and a predominantly homogeneous structure between the centre and the rear
- A well developed confluence zone with mesoscale lows at the frontal part with negative surface pressure tendencies and a mesoscale high with rising surface pressure at the rear.