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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.

Computation of PV

The package tests the computed structure of the atmosphere at each grid point to identify geopotential heights where potential temperature equals 315K.  At each of these the value of PV is computed.  This process starts from the lowest layers and progresses upwards and the greatest altitude is selected (i.e. NOT the first value from the lowest layers). 

In this way the large values of PV generated by diabatic process in the lower troposphere are not indicated.  However, as the package continues to test further upwards, it may find further levels where potential temperature equals 315K and then computes the associated PV.  Thus some anomalously low values can be identified, which at higher altitudes will over-write the more representative value from a lower altitude and cause anomalies to be plotted on the PV charts.   

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Fig8.1.11-1: ecCharts example showing Ensemble Control Forecast forecast PV and mean sea level pressure.  

Note: HRES and Ensemble Control Forecast are scientifically, structurally and computationally identical.  With effect from Cy49r1, Ensemble Control Forecast output is equivalent to HRES output shown in the diagrams.   At the time of the diagrams, HRES had resolution of 9km and ensemble members had a resolution of 18km.

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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.  

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