Synoptic Situation: Satellite images and derived numerical parameters
Basic parameters like surface pressure (height of 1000 hPa) and upper level height (here at 500 hPa) do not reveal the whole physical background of a situation and can therefore not explain details seen in the cloud features of satellite images as well as in the different conceptual models.
Parameters and parameter combinations which are helpful in the diagnosis of cyclonic developments in addition to the basic parameter of the surface are:
Thickness, temperature advection and a front parameter like the Thermal Front Parameter (TFP) for a manifestation of the frontal situation; Upper level parameters like height in an upper level, positive vorticity advection (PVA) in upper levels and especially in the jet level as an indication for the potential of a further development of cyclogenesis. The same is true for Potential Vorticity (PV) which is a parameter indicating the interaction of high and low levels and release of cyclogenesis.
In the next chapter some of these parameters will be discussed in detail.
TFP + Relative Topography + Temperature Advection (frontal characteristics)
In a classical frontal situation of a cold front one would expect a zone of high thickness gradient, the zeroline of TA as well as a Thermal Front Parameter (TFP) all of them situated within the cloud band. In case of an occluded front one would expect the cloud spiral within a ridge of the thickness, while the zeroline of TA and the TFP again accompany the cloud band.
In this chapter the parameters thickness, TFP and TA are discussed together with the cloud features in a 6-hourly sequence.
Isotachs and PVA 300
Postive Vorticity Advection at high levels has different influences on frontal cloud bands as well as on cold air cloud features.
As a PVA maximum at high levels (theoretically an increase of PVA with height) is a sign for upward motion, it can be used as a sign for cloud intensification and mostly also development of convective features. Those effects can be found in front of a moving upper level trough which can be followed for instance in the 500 hPa level. The most important situation however is the existence of a PVA maximum in the left exit region of a jet streak usually looked at in the 300 hPa level. The latter configuration can be called something like a “hot spot” for the development of cold air features or the enhancement of cloudiness within fronts.
In the next chapter the parameters isotachs and PVA at 300 hPa are discussed together with the cloud features in a 6-hourly sequence.
Height of PV = 1 unit (Height of tropopause)
Another parameter which gives insight into cyclonic developments is Potential Vorticity (PV). Maxima of PV indicate stratospheric air protruding downward. This is a process which might enhance cyclogenesis.
In the following chapter the fields of height of PV=1 unit are displayed in a 6-hourly sequence for 500 hPa.
Summary of the investigations in this chapterThe most important explanations that come from the additional derived parameters are the following:
- The W-E oriented cloud band in the first phase of the development shows weak frontal conditions; after some time a meridional distribution of the TA maxima develops.
- The synoptic-scale cloud spiral under consideration gets frontal characteristics during its development, however in the relevant parameters it looks like an occluded front; a classical occlusion process however is not supported from the kind of development as well as from the appearance of cloud systems in the satellite image
- The cloud feature from which the synoptic scale spiral develops during the next two days is a cold air feature in the north of the frontal cloud band where the high levels of the troposhere play an important role:
- In common with similar situation is
- The correspondence between the cloud field and a PVA maximum in upper levels, especially in the left exit region of a jet streak
- Deviating from similar situations is the fact that this cloud part is:
- already in the beginning phases connected with a WA maximum in the leading part and – as usual – strong CA behind
- already in the beginning phase connected to a PV anomaly immediately behind the cloud feature
- During the phases of development the relations to all the parameters mentioned here is kept; intensification of cloudiness is under the influence of PVA maxima in upper levels.