Synoptic Situation: Satellite images and derived numerical parameters
Basic parameters like surface pressure (height of 1000 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 the different conceptual models.
Parameters and parameter combinations which are helpful in the diagnosis of waves in addition to the basic parameter of the surface pressure (height 1000 hPa) are:
- Upper level parameters like height
- Positive vorticity advection (PVA) in upper levels and especially at jet level.
- Temperature advection at mid level of the troposphere (in this investigation 700 hPa is chosen) indicates with maxima of warm advection (WA) and cold advection (CA) the ongoing cyclonic rotation.
- Potential vorticity (PV) is a parameter that indicates the interaction of high and low levels; it characterises stratospheric air.
The next chapters will look into the two waves of the case study and will compare the actual situation with the expected typical one.
H500 + PVA
The upper level height field and PVA in the same levels are an indication for areas where it becomes more cyclonic and where there is additional upward motion. The typical configuration is an intensifying and approaching upper level trough and a PVA maximum which is localised over the southern part of the S-shape of the wave bulge.
In the following chapter the fields of upper level height and vorticity advection are displayed in a 6-hourly sequence for the 500 hPa level.
Temperature Advection
Temperature advection reflects also a cyclonic rotation which leads to a pattern of a WA maximum within the cloud bulge and a CA maximum in the cold air behind the S-shape of the wave bulge.
In the following chapter the fields of temperature advection (representative for 700 hPa) are displayed in a 6-hourly sequence for 700 hPa.
Jets (Isotachs) and PVA at 300 hPa
The relation of the a wave bulge to a jet streak is of much importance for the potential development. It is a different situation if a jet streak is parallel to the rear side of the frontal cloud band or if it crosses the frontal cloud band. In the first situation, the wave very typically develops in the right entrance region of a jet streak which is a rather long term process. The PVA maxima usually are than not so intensive. In the second case, the wave develops in the left exit region of a jet streak which usually is a very rapid development with high values for PVA.
In the following chapter the fields of isotachs and PVA at 300hPa are displayed in a 6-hourly sequence.
Height of PV = 1 unit (stratospheric air) and PV anomalies
Another parameter which contributes to a wave diagnosis is Potential Vorticity (PV). Maxima of PV indicate stratospheric air protruding downward. This is a process which might enhance cyclogenesis and consequently benefits the development of the Wave.
In the following chapter the fields of height of PV=1 unit are displayed in a 6-hourly sequence.
Summary of the investigations in this chapter
There are many indications for wave processes in the relevant derived model parameter fields for both waves A and B. But there are also differences between the two types which may help explain the different developments.
The first wave A develops very smoothly. The main features are a surface and also an intensive upper level trough which moves across the cloud band after some time; both phenomena do exist from the very beginning and stay the same during the whole developing phase into the occlusion spiral. A development step which could be interpreted as a fast development of a spiral structure turns out to be the mergance of the wave, the frontal cloud band and a comma in the cold air.
A more intensive development takes place at the second wave B. It is a development released and triggered by processes in upper levels which influence after some time also the lower levels which after some time show a very classical distribution. In contrary to the first wave A there is not only no surface low existing in the beginning, there is also no upper level trough from the very beginning. It develops only later. But the main processes are the release of cyclogenesis through an approaching PV anomaly. Enhancements in the left exit region of a jet streak go into the same direction.