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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 Fig3.2-1).
• systematic and non-systematic errors occur when the NWP output is verified against point observations.  The NWP output may not be representative of the location, height, aspect of the observation or capture sub-grid scale variability.

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

• first 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.
• second step: compare the systematic errors between observation average and point observation.  The  The systematic errors come from station representativeness station representativeness (i.e. the location, height and aspect of the observation) and the .  The non-systematic errors come from sub-grid scale variability.

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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.

Systematic errors maybe due to model deficiencies and/or observational representativeness.  These can be partly corrected by statistical means (e.g. model output statistics statistics MOS).  A series of forecasts also helps with dealing with uncertainty.

Non-systematic synoptic errors can be dampened by different ensemble approaches (e.g. medium range ensemble, probability considerations, forecast error growth).  However, sub

Sub-grid variability (notably for rainfall but other parameters too) can be addressed through by downscaling.  Downscaling converts the

Downscaling converts:

•  the average grid box

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• probability density functions from

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• raw ENS into
• "point rainfall probability density functions" for points within

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• the grid box.

New downscaling techniques are being developed accordingly (see for example the Point Rainfall product).