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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

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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 IPV 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 IPV 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 IPVPV.  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 IPV PV charts.   

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

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