The grid values should not be considered as representing the weather conditions at the exact location of the grid point. 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:
Fig3.2-1: Comparison between NWP model output and observations ought ideally to follow a two-step procedure:
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 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).
Sub-grid variability (notably for rainfall but other parameters too) can be addressed by downscaling.
Downscaling converts:
New downscaling techniques are being developed accordingly (see for example the Point Rainfall product).