The Hoskins theory based on PV and PV anomalies

As PV anomalies and baroclinic zones can benefit to the development of synoptic scale cloudfeatures it can provide a good example to the Hoskins theory. This theory states that an upper level PV anomaly, with its associated lowered tropopause, overrunning a low level baroclinic zone induces a cyclonic circulation within the upper levels of the troposphere. In the Figure 1. below this phenomena is graphically represented. As a result of the deepening of the troposphere because of the existence of a postive PV anomaly air is drawn into the column. Alternatively, the sea pressure sinks and due to the coriolis force a cyclonic rotation exists.


Fig 1. Graphical representation of a positive PV anomaly overrunning a baroclinic zone

The idea behind this theory goes according to the recognition of PV-anomalies. These upper-level anomalies that cause the tropopause to sink are characterised in the WV imagery as dark shaded areas. These characteristics of an upper-level PV anomaly can be examined in more detail and a discrimination in positive and negative upper-level PV anomalies can be made.

In practise the lowering of the tropopause is almost always picked up by the WV-imagery (dark stripes and darker areas), but also by the modelfields. The height of PV=1 is found at some stages at 500 hPa, where it normally would reside at 0 to 100 hPa. Positive values for the PV-anomaly and therefor a clear indicator to the above described Hoskins theory.

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