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The grid values should not be considered as representing the weather conditions at the exact location of the grid point, but .  They should be considered as a time-space average within a two- or three-dimensional grid box.  The discrepancy between the forecast grid-point value and the verifying observed average value can be both systematic and non-systematic:

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When the NWP model output is compared with point observations (as commonly happens in verification), additional systematic and non-systematic errors are introduced.  This is These are due to location of the model NWP output not being representative of the location, height and aspect of the observation, and also .  Also errors are due to sub-grid scale variability.

Fig3.2-1:  The comparison Comparison between NWP model output and observations ought ideally to follow a two-step procedure:

• first

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• step: compare grid point average to observation area average.  The systematic errors are then due to model shortcomings; the non-systematic errors stem from synoptic phase and intensity errors.

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• second step: compare the systematic errors between observation average and point observation

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• .  The systematic errors come from station representativeness (i.e. the location, height and aspect of the observation) and the non-systematic errors from sub-grid scale variability.

Fig3.2-2:  In reality, the comparison between NWP and observations must for simplicity bypass the area average stage.  This results in the systematic and non-systematic errors arising from distinctly different sources.  The effects related to the two green arrows in Fig3.2-1 are here combined into one.

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