EUMeTrain: VERTICAL CROSS SECTIONS

Concept of potential vorticity (PV)

In everyday synoptic work potential vorticity has come up as very important for the diagnosis and forecast in case of cyclogenesis. It is important for studying the appearance and evolution of dynamical structures at synoptic scale due to the following important properties:

  • conservation for adiabatic frictionless motion

  • specific climate distribution in the atmosphere

  • invertibility

    • To learn more about potential vorticity, its theory and applications you can go to SATMANU manual, where in chapter BASICS you can find a subchapter dealing with Potential vorticity.

    Important about potential vorticity is that it plays a crucial role in the generation of vorticity in cyclogenesis. Due to the fact that PV is conserved, some significant features related to synoptic scale systems can be followed in space and time. The most important of these is the lowering of the dynamical tropopause, also called an upper level PV anomaly. In the vertical cross section it looks like this:

    In mid-upper troposphere PV has values

    0.5 to 1.5 PVU

     

    In the stratosphere the value of PV is

    PV > 3 PVU

    due to strong increase of stability.

     

    The PV discontinuity around the tropopause and its conservation property allows to define the surface of

    PV=1.5 PVU

    as the DYNAMICAL TROPOPAUSE. It separates the troposphere (with weak PV) from the stratosphere (with strong PV).

     

    PV anomalies

    A PV anomaly is produced by the intrusion of stratospheric air into the upper troposphere.  An upper level PV anomaly, advected down to middle troposphere is called: tropopause dynamic anomaly or folding of the dynamical tropopause.

    Due to PV conservation, the anomaly leads to deformations in vertical distribution of potential temperature and vorticity.

    In a baroclinic flow increasing with height, the intrusion of PV anomaly in the troposphere produces a vertical motion:

    The deformation of the isentropes imposes ascending motion ahead of the anomaly and subsiding motion behind it.
    Above you have seen what the PV anomalies look in the theory and in sketches. In the synoptic practice PV anomalies are clearly seen in the cross-sections, but also in water vapour satellite images. The reason for this is that the air protruding from the stratospfere is much drier than the troposphere air, and can be seen as dark area in the water vapour images.
    Follow this link to see the examples from the "real world".