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ecCharts documentation gives a clear and comprehensive description of options and practical use of ecCharts.

Comparing fields for consistency

Simulated visible imagery

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Simulated imagery

These products display cloud-related fields from the model in a format that is very familiar to forecasters and that they are used to interpreting.  They can easily be compared to actual satellite imagery.

Simulated visible imagery

These show simulations of the upward flux of visible radiation (as would be detected by a weather satellite).   This is derived from the model representation of reflectances derived from underlying model forecast cloud tops and surfaces.  The brightness reflectances can then be used to produce pictures equivalent to the visible images available from geostationary satellites.

Note:

• The area of coverage includes high latitudes but every pixel is assumed to be an overhead (nadir) view.
• The solar illumination is always assumed to be directly overhead.  This means that cloud structures can still be seen at times and locations even when in reality it is dark.
• Unlike on real visible images, shadowing from clouds is never represented.
• The displayed cloud reflectance is a function of the total column cloud liquid water.

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•  When this is modelled too low the low clouds do not stand out well.

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•  The reflectance can be close to the reflectance from the surface and then clouds can’t be distinguished from terrain.

Simulated infra-red imagery

These show simulation of the upward flux of infra-red radiation (as would be detected by a weather satellite).  This is derived from the model representation of temperatures from underlying model forecast cloud tops and surfaces.  The brightness temperatures can then be used to produce pictures equivalent to the infra-red images available from geostationary satellites.

Note:

• The area of coverage includes high latitudes and every pixel is assumed to be an overhead (nadir) view. 

Simulated water vapour imagery

These show simulation of the upward flux of radiation (as would be detected by a weather satellite) derived from a radiative transfer model dealing with emission from model forecasts of water vapour through a vertical column.

The model radiation value can then be used to produce pictures equivalent to the water-vapour images available from geostationary satellites.

Note:

• The area of coverage includes high latitudes and every pixel is assumed to be an overhead (nadir) view. 

Comparing fields for consistency

Ideally, simulated water vapour images can be compared with water vapour imagery to allow an assessment of any departure of the analysed and forecast fields from reality.  The structure of the IFS model atmosphere may also be explored by comparing the water vapour imagery with the potential vorticity fields.  Currently satellite imagery is not available on ecCharts and comparison of observed and simulated imagery cannot be done directly.

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 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 purpleof 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.

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