Interpretation of Potential Vorticity Charts

The height of the dynamic tropopause is commonly taken to be the level at which potential vorticity (PV) equals 2.0 PV units.  PV is normally below 2.0 PV units In the troposphere and is relatively uniform.  PV is very much higher in the stratosphere due to the increased stability.

Within ecCharts users can view HRES PV at the 315K potential temperature level (example at Fig8.1.13.1).  Values above 2.0 PV units (i.e. the stratosphere) are shaded while lower values (i.e. the troposphere) are unshaded.  Generally on these charts polar airmasses correspond to PV > 2 whilst tropical or subtropical airmasses correspond to PV < 2.  So the edge of the shaded region approximates to the boundary aloft between these airmasses.

Occasionally there can be significant anomalies or variations in PV away from the norm, as illustrated on Fig8.1.13.2.  For example, major anomalies can occur where folds in the tropopause form, or in the lower atmosphere in response to diabatic heating.  Standing waves can also induce substantial anomalies - even within the stratosphere.  Usually these will not impact upon the 315K PV field shown on ecCharts.  However, they may affect plots that users generate locally (e.g. showing the height of a PV=2 surface).

Fig8.1.13.1: ecCharts example showing HRES PV at 315K potential temperature with mean sea level pressure.

Fig8.1.13.2: Cross-section of PV, at 41.5N, at T+24 = 00UTC 3 Jan 2019.   PV values are indicated on the right of the diagram.  The dynamic tropopause lies on the PV=2 isopleth (on the red/yellow boundary).  In the diagram this falls almost to 500hPa near 10E in the axis of a cold trough.  On vertical section A , PV=2 PV units occurs at the tropopause near 250hPa and twice near 850hPa related to a PV maximum generated by diabatic processes.   On vertical section B, PV=2 PV units occurs at the tropopause near 450hPa and twice in the stratosphere associated with standing wave activity.