The classical polar front theory starts from a stationary frontal cloud band which separates two different air masses. Through a disturbance a cyclonic rotation starts at this polar front in low levels. This cyclonic development has of course influences on the temperature and temperature advection field and manifests itself in a cloud bulge of a wave. The circulation becomes more and more intensive, influencing from below the upper levels and leading to a very intensive wave bulge.
The classical occlusion process describes the cold front moving faster than the warm front and catching it after some time. As a consequence the warm air is lifted which is a charcteristic phenomenon of a fully developed occlusion stage.
In the case under discussion, this initial classical development cannot be followed. The cloud development starts in the cold air behind a frontal cloud band. Although there is a low center at the surface from the very beginning the development does not start from below and penetrates upwards but on the contrary there exists already a very intensive cyclonic development at high levels which influences also the lower levels.
The starting point of the concept of the Hoskins theory are PV anomalies, areas where stratospheric air penetrates downward into the troposphere. Because of the fact that the air becomes less stable in the troposphere and because of conservation of PV, the cyclonic rotation (vorticity) becomes much higher. This influences the lower layers and if there is a low level baroclinic zone, the sinking cyclonic rotation influences from above the lower layers and vice versa, the lower layers enhance the circulation in the upper layers. In the end cyclogenesis is forced intensively. This is for instance a typical process for a rapid cyclogenesis.
In the case study under consideration there are indeed some signs for such a development: there is from the very beginning a distinct PV anomaly which accompanies the system till to the end of the case study. A cyclonic influence among the different layers is very probable. However, one of the topics in the theory is a pre-existing baroclinic boundary along which the development takes place. It is not very sure if such a baroclinic zone is existing . The comma cloud clearly develops behind a W-E oriented frontal zone which decays quite soon. But the comma development appears also along a cloud mass which is very probably under WA and within a thickness ridge, but it is not evidently a baroclinic zone; in the vertical cross sections some indication for a very flat baroclinic zone could be derived.
The development of the cloud spiral is indeed coupled with a jet streak and appears in the left exit region under the influence of a PVA maximum. But the typical cloud features which develop in the left exit region of a jet streak are EC and commas and also (if an interaction with a front takes place) an enhancement of a front.
What cannot be explained with this theory alone is the intensive cyclonic development and the enhancement of the cloud band. Consequently, the effect of the PVA maximum in the left exit region of a jet streak indeed takes place but is not alone responsible for the cyclonic development; it seems to be coupled with the Hoskins theory which is the same as with a rapid cyclogenesis development.
Cold air features like EC and commas develop predominantly in unstable cold air under an enhanced upward motion due to a PVA maxium at high levels. The latter can be the consequence of a moving or deepening upper level trough or in the left exit region of a jet streak.
All of these conditions are fulfilled (except for instability). There is intensive PVA in the area of the cloud field from an intensive upper level trough as well as from (as discussed before) a left exit region. This can explain the development of a comma spiral but it cannot explain the intensification of the cloud spiral and the growth of the whole system.
Consequently several aspects of the cyclonic development in this case study can be explained by the theories mentioned above.
A CAD (Cold Air Development) describes a process where a comma is growing into a synoptic scale cloud spiral taking frontal character lateron.
From the sequence of satellite images such a process can be supported, but the model fields are much more in the direction of a rapid cyclogenesis. Especially because there is not such an intensive PV anomaly involved in a CAD process as it is in this case. Another phenomenon speaking against a mere CAD process is that the cloud develops at the boundary between CA and WA.