Page History

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Note: HRES and Ensemble Control Forecast (ex-HRES) are scientifically, structurally and computationally identical.  With effect from Cy49r1, Ensemble Control Forecast (ex-HRES) 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.

Overview

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.

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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). 

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Note: HRES and Ensemble Control Forecast (ex-HRES) are scientifically, structurally and computationally identical.  With effect from Cy49r1, Ensemble Control Forecast (ex-HRES) 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.

(FUG Associated with Cy49r1)