Accumulation of Orographic Prediction Errors

We can assess the accuracy of any proposed prediction by considering the orography of the reference and potential sites, before the use of WAsP. The following illustrate this, assuming that the two sites considered have a high correlation between their measured wind speeds.

• Gentle Terrains
  Used within its operational limits involving no more than smooth and low hill slopes at both sites, both WAsP procedure errors E1 and E2 are small. As the terrain between the reference and potential sites would likely be similar in this situation, both procedures errors would tend to be cancelled each other. The accuracy of the WAsP estimation under these conditions will be high.
  
• Similar sites in rugged terrains
  To study situations outside the performance limits of WAsP, let's consider similar sites in rugged terrain where the two procedure errors are significant but similar in magnitude: E1 ≈ E2. The overall prediction is then expected to be still accurate, as the two errors should tend to cancel each other. Of course, such a situation occurs for the self-prediction at any type of site. But this situation also occurs for two neighbouring sites in rugged terrain, which have similar orography.
  
• From rugged to smooth sites
  If we consider two very different sites such as a rugged reference site and a less rugged or flat potential site, the procedure errors are different, E1 > E2, and the overall prediction will be largely under-estimated with a significant negative error.
  
• From smooth to rugged sites
  In the other way, if we consider a smooth or less rugged reference site and a rugged potential site, we will have E1 < E2. This time, the prediction will be over-estimated with a significant positive error.

To summarise, we can say that: 

•  The overall prediction error is equal to the difference between the individual WAsP Application and Analysis errors.
•  These two errors largely depend on the degree of flow separation at the site considered.
•  The degree of flow separation depends on the site ruggedness.
•  A possible performance indicator can be the difference between the site ruggedness of the reference and potential sites.

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