EUMeTrain: Case Study on development of two Waves

Meteorological Physical Background

Which physical theories do exist for explaining wave development generally; some theoretical background:
  1. Cyclogenesis according to the classical polar front theory
  2. Orographic lee cyclogenesis
  3. Cyclogenesis in connection with the left exit and the right entrance region of a jet streak
  4. The Hoskins theory based on PV and PV anomalies
Which of these theories can be applied to the two wave developments addressed in this case study?


Ad1. Cyclogenesis according to the classical polar front theory

The N-S oriented frontal cloud band where the first wave A develops is not a steady polar front but already an active, moving cold front. Nevertheless, in accordance with the polar front theory is the fact that a surface low is already existing which initiates a cyclonic rotation necessary for the cloud development and later the movement of cold against warm air and therefore the development of warm advection and cold advection maxima.

For wave B no similar surface features can be noticed. On the contrary, during a long path of the front and the wave bulge, a high pressure area is close to the surface.


Ad2. Orographic lee cyclogenesis

For wave A no distinct orographic influence can be noticed.

For wave B during a rather long phase in the beginning of the development lee cyclogenesis effects at the Alps cannot be excluded. This is especially true for the S-form of the wave bulge in the SE of the Alps (Slovenia, Croatia).


Ad3. Cyclogenesis in connection with the left exit and the right entrance region of a jet streak

Both waves A and B have distinct relations to jet streaks. Wave A during its first development phase close to the left exit region of a jet streak but then moves out of the critical area after some time.

For wave B this is a key process. The whole development is coupled to processes in the upper troposphere and a low development close to the surface is triggered from above. Wave B is already in the beginning phases related to a V-pattern of jet streaks which means that the development is in the right entrance region of the first and in the left exit region of the second jet streak. After a while the main development phase is clearly related to the left exit region of an increased jet streak.


Ad4. The Hoskins theory based on PV and PV anomalies

Especially for the last wave B such a process can indeed be stated. It is supported not only by relevant model fields like Potential Vorticity (PV) but also by the appearance in WV images, which show the downward protruding dry air and in the vertical cross sections of PV.