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• systematic errors reflect the limitations of the NWP model’s ability to simulate the physical and dynamic properties of the system;
• non-systematic errors reflect synoptic phase and intensity errors (as indicated by the left hand green arrow in Fig3Fig32.2.1A).

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 due to location of the model NWP output not being representative of the location, height and aspect of the observation, and also to sub-grid scale variability.

Fig3Fig32.2.1A:  The comparison between NWP model output and observations ought ideally to follow a two-step procedure: first from grid point average to observation area average.  The systematic errors are then due to model shortcomings; the non-systematic stem from synoptic phase and intensity errors.  In the next step, the systematic errors between observation average and point observation result from station representativeness (i.e. the location, height and aspect of the observation) and the non-systematic from sub-grid scale variability.

Fig3Fig32.2.2B:  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 Fig3Fig32.2.1 A are here combined into one.

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