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The simulated water vapour image is derived from a radiative transfer model applied to the IFS atmosphere.  As the distribution of moisture is governed by ascent, areas that are coloured light grey by convention (=low temperature emissions) commonly tally with regions of current or past ascent, and regions coloured dark grey commonly tally with current or past descent.  The potential vorticity (PV=2) pattern is essentially governed by the dynamics of the upper flow and variations induce ascent or descent at around the level of the tropopause (conventionally taken as indicated by 2 PV units; higher values represent stratospheric air as shown here by shades of purple).  In the vicinity of developing depressions it would be expected that descending air (here shown by stratospheric air at 300hPa) would match the confinement of moisture to lower levels.  Comparing the detail of these patterns with satellite imagery can provide a powerful method of identifying discrepancies between the modelled atmosphere and reality.

Fig10.2.3A: ecChart DT 00Z 8 August 2017 T+00 VT 00Z 8 August 2017 from HRES showing the simulated water vapour image and 300hPa potential vorticity.  These can be compared with WV satellite imagery to highlight discrepancies between the modelled atmosphere and reality.  Stratospheric air (PV=2 taken as near the tropopause, greater PV values (shown purple)) imply stratospheric air has descended to 300hPa.  Descent is also implied by the darker areas of the simulated water vapour image.  Comparing WV imagery with simulated imagery and PV charts can provide a powerful method of identifying discrepancies between the modelled atmosphere and reality.

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