EUMeTrain: Case Study on development of two Waves

Summary

For both wave developments A and B the same three development phases can be seperated:
  1. First appearance of wave: From the first impression of a cloud bulge to the formation of a distinct cloud bulge
  2. Intensification of wave: Intensification and better structure of a wave bulge
  3. Development of occlusion spiral: Transition from the wave bulge to an occlusion spiral and further intensification.

The physical background represented by relevant numerical parameters is different for the two waves A and B in the different phases.


1. First appearance of wave

Wave A shows a pronounced surface trough in the development area of the wave bulge and a distinct upper level trough at the rear. A maximum of PVA in high levels indicates the deepening and the area is situated in the left exit region of a W-E oriented jet streak and under influence of PVA at 300 hPa.

Wave B develops in the area of a surface ridge and the main upper level feature is a distinct SW stream parallel to the cloud band with a very small and weak indication of an upper level trough.


2. Intensification of wave

For the second phase wave A shows the same features as in the first phase but all of them are now even more distinct! Especially the surface and upper level trough have further deepened.

Wave B still does not show any signal for a low near to the surface. However, in the upper levels some features become more distinct: an upper level trough intensifies, the wave bulge is connected to a PVA maximum in the V pattern but with a clear weight on the left exit region and the WV image shows a very dark area as well as thunderstorm developments. Both features are indicative for a growing PV anomaly. This distinct development can be noticed before an occlusion stage!


3. Development of occlusion spiral

For wave A all relevant parameters still intensify (deepen) like the surface and upper level height. On the other hand there is no longer any intensification by a PVA maximum at 500 and 300 hPa. At this stage which can be called an occlusion stage, typically a PV anomaly develops rapidly.

For wave B the processes in the upper levels which developed clearly in the phase before deepen also in this phase but influence the lower levels in so far that a low develops in the lower levels as well as a warm advection maximum. The PV anomaly develops also in this occlusion stage.