Summary
An old frontal cloud band was recognized in the IR 10.8 imagery, this was identified as a pre-existing baroclinic
zone. In the WV 6.2 imagery very clearly an upper level feature can be seen, which was marked by dark shades
in the WV-signal,indicating low tropopause heights. This appeared to be related to heigh values of PV,
indicating a PV anomaly. An intensive cyclone south of Iceland developed due to the interaction of the
upper-level
PV anomaly and the
low-level baroclinic zone, according to Hoskins theory. This cyclone had an
associated long-lasting strong wind-field in the NW and NE quadrant of the cyclone. The region of this
windfield can be denoted as rather atypical.
In the sequences it was very clearly seen that a process of rapid cyclogenesis was going on. The
typical characteristics
of this cyclogenesis events were:
- Existence of an upper level PV-anomaly, related to a pronounced upper level trough.
- Existence of a low-level baroclinic zone.
- Interaction between item 1 and 2.
- Appearance of a wave-structure in the frontal cloud band.
- Appearance of a dry intrusion, related to the upper-PV anomaly.
- Development of a strong surface cyclone with a strong surface windfield.
Atypical aspects of this cyclogenesis event were.
- The upper level PV-anomaly and related upper level trough are stationary during the cyclogenesis process.
- The surface cyclone shows hardly no displacement and even retrogrades at the end.
With use of vertical cross sections the vertical structure of the PV-anomaly became very clear: In time a
downward protrusion of stratospheric air was detected. As the cyclogenesis proces went on, also in the lower levels
a PV anomaly can be seen. It is known that tight low-low level baroclic zones can be represented in terms of PV
as well. The two anomalies seems to interact and giving reason to the strong development.
The stationary character of the system can be explained by the vorticity fields. In a nearly East-West cross
section, the PVA maximum is located almost in the same area as the vorticity maximum. Therefore eastward
propagation of the trough is hardly possible. So, the PVA maximum is indicating not the movement of the trough,
but the deepening of the trough. According to the classification of conceptual models (SATMANU) this case can be
denoted as a mixture of Rapid Cyclogenesis and Upper Level Low.