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Fig3.2.1:  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  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.

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Non-systematic synoptic errors can be dampened by different ensemble approaches (e.g. medium range ENSensemble, probability considerations, forecast error growth).  However, sub-grid variability (notably for rainfall but other parameters too) can be addressed through downscaling.  Downscaling converts the grid box area average probability density functions from the raw ENS into "point rainfall probability density functions" for points within each grid box.

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