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 the case of a positive PV anomaly, the isentropes are
characterised by higher values than in the surrounding areas
(indicating colder air). The height of the tropopause has a local
minimum. The corresponding pressure and wind field shows an area with
low pressure and a cyclonic circulation. The cyclonic circulation is a
result of the coriolis force. As the pressure at the surface drops, so
will the upward motion increase, which draws air in hat begins to
rotate due to the coriolis force. At the northern hemisphere this force
is directed cyclonically.
- In the case of a negative PV anomaly, the isentropes are
characterised by lower values than in the surrounding areas (indicating
warmer air). The height of the tropopause has a local maximum. The
corresponding pressure and wind field shows an area with high pressure
and an anticyclonic circulation.
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.
References
- Manual of Synoptic Satellite Meteorology
- Hoskins B. J., McIntyre M. E. and Robertson A. L. W.
(1985): On the use and significance of isentropic potential vorticity
maps; Quart. J. R. Meteor. Soc., Vol. 111, p. 877 - 946