Summary
The case study describes the merging process of a comma-like cloud feature with a cold front cloud band. The process took place in an area of cold air without a warm front being present. An occlusion stadium is reached without a prior formation of a wave.
Following conclusions can be summarised:
- After the merging process the cloud spiral was connected to a rapidly deepening surface low. This surface low remained almost stationary over a long period of time.
- The merging of the comma and the frontal cloud band took place in an area of strong positive vorticity advection which is linked to the left exit region of the jet streak.
- The PV anomaly matches a dry slot in WV images. Typically for a rapid cyclogenesis, two PV maxima could be recognised. The first one was in higher levels in the rear side of the cold front. The second one could be found in a baroclinic zone and in lower levels further to the Southeast.
- The development fits very well to the conceptual model of an instant occlusion.
- The rapid cyclogenesis in combination with an instant occlusion was probably crucial for this severe weather situation.