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

Within ecCharts users can view HRES PV at the 315K potential temperature level (example at Fig81.11.1).  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 more tropical / subtropical airmasses correspond to PV < 2. So by examining this PV field one can infer, approximately, where the upper level boundary between airmasses of polar and more tropical origins actually lies.

Occasionally there can be significant anomalies or variations in PV away from the norm, as illustrated on Fig81.11.2.  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.  Ordinarily such phenomena will not impact upon the 315K PV field available within ecCharts, although they may affect plots that users generate locally - e.g. showing the height of a PV=2 surface.


Fig8.1.11.1: ecCharts example showing HRES PV and mean sea level pressure.


 

Fig8.1.11.2: Cross-section of PV, at 41.5N, at T+24 = 00UTC 3 Jan 2019.   PV values are indicated on the right of the diagram (e.g. 2 PV units is shown by the red/yellow boundary). The dynamic tropopause is taken to lie on the PV=2 isopleth (in the red zone) which falls to about 550hPa near 10E within the axis of a cold trough.  Along vertical line A in the diagram, PV=2 PV units occurs at 3 locations, one for the tropopause, and two lower down related to a PV maximum generated by diabatic processes.  In contrast, along vertical line B, whilst there are also 3 levels where PV=2 PV units, two of these lie within the stratosphere and are attributable to standing wave activity.